Adding upstream version 6.6.15.upstream/6.6.15

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 2104 insertions, 0 deletions

diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
new file mode 100644
index 0000000000..65e000ca33
--- /dev/null
+++ b/kernel/rcu/tasks.h
@@ -0,0 +1,2104 @@
+/* SPDX-License-Identifier: GPL-2.0+ */
+/*
+ * Task-based RCU implementations.
+ *
+ * Copyright (C) 2020 Paul E. McKenney
+ */
+
+#ifdef CONFIG_TASKS_RCU_GENERIC
+#include "rcu_segcblist.h"
+
+////////////////////////////////////////////////////////////////////////
+//
+// Generic data structures.
+
+struct rcu_tasks;
+typedef void (*rcu_tasks_gp_func_t)(struct rcu_tasks *rtp);
+typedef void (*pregp_func_t)(struct list_head *hop);
+typedef void (*pertask_func_t)(struct task_struct *t, struct list_head *hop);
+typedef void (*postscan_func_t)(struct list_head *hop);
+typedef void (*holdouts_func_t)(struct list_head *hop, bool ndrpt, bool *frptp);
+typedef void (*postgp_func_t)(struct rcu_tasks *rtp);
+
+/**
+ * struct rcu_tasks_percpu - Per-CPU component of definition for a Tasks-RCU-like mechanism.
+ * @cblist: Callback list.
+ * @lock: Lock protecting per-CPU callback list.
+ * @rtp_jiffies: Jiffies counter value for statistics.
+ * @lazy_timer: Timer to unlazify callbacks.
+ * @urgent_gp: Number of additional non-lazy grace periods.
+ * @rtp_n_lock_retries: Rough lock-contention statistic.
+ * @rtp_work: Work queue for invoking callbacks.
+ * @rtp_irq_work: IRQ work queue for deferred wakeups.
+ * @barrier_q_head: RCU callback for barrier operation.
+ * @rtp_blkd_tasks: List of tasks blocked as readers.
+ * @cpu: CPU number corresponding to this entry.
+ * @rtpp: Pointer to the rcu_tasks structure.
+ */
+struct rcu_tasks_percpu {
+	struct rcu_segcblist cblist;
+	raw_spinlock_t __private lock;
+	unsigned long rtp_jiffies;
+	unsigned long rtp_n_lock_retries;
+	struct timer_list lazy_timer;
+	unsigned int urgent_gp;
+	struct work_struct rtp_work;
+	struct irq_work rtp_irq_work;
+	struct rcu_head barrier_q_head;
+	struct list_head rtp_blkd_tasks;
+	int cpu;
+	struct rcu_tasks *rtpp;
+};
+
+/**
+ * struct rcu_tasks - Definition for a Tasks-RCU-like mechanism.
+ * @cbs_wait: RCU wait allowing a new callback to get kthread's attention.
+ * @cbs_gbl_lock: Lock protecting callback list.
+ * @tasks_gp_mutex: Mutex protecting grace period, needed during mid-boot dead zone.
+ * @gp_func: This flavor's grace-period-wait function.
+ * @gp_state: Grace period's most recent state transition (debugging).
+ * @gp_sleep: Per-grace-period sleep to prevent CPU-bound looping.
+ * @init_fract: Initial backoff sleep interval.
+ * @gp_jiffies: Time of last @gp_state transition.
+ * @gp_start: Most recent grace-period start in jiffies.
+ * @tasks_gp_seq: Number of grace periods completed since boot.
+ * @n_ipis: Number of IPIs sent to encourage grace periods to end.
+ * @n_ipis_fails: Number of IPI-send failures.
+ * @kthread_ptr: This flavor's grace-period/callback-invocation kthread.
+ * @lazy_jiffies: Number of jiffies to allow callbacks to be lazy.
+ * @pregp_func: This flavor's pre-grace-period function (optional).
+ * @pertask_func: This flavor's per-task scan function (optional).
+ * @postscan_func: This flavor's post-task scan function (optional).
+ * @holdouts_func: This flavor's holdout-list scan function (optional).
+ * @postgp_func: This flavor's post-grace-period function (optional).
+ * @call_func: This flavor's call_rcu()-equivalent function.
+ * @rtpcpu: This flavor's rcu_tasks_percpu structure.
+ * @percpu_enqueue_shift: Shift down CPU ID this much when enqueuing callbacks.
+ * @percpu_enqueue_lim: Number of per-CPU callback queues in use for enqueuing.
+ * @percpu_dequeue_lim: Number of per-CPU callback queues in use for dequeuing.
+ * @percpu_dequeue_gpseq: RCU grace-period number to propagate enqueue limit to dequeuers.
+ * @barrier_q_mutex: Serialize barrier operations.
+ * @barrier_q_count: Number of queues being waited on.
+ * @barrier_q_completion: Barrier wait/wakeup mechanism.
+ * @barrier_q_seq: Sequence number for barrier operations.
+ * @name: This flavor's textual name.
+ * @kname: This flavor's kthread name.
+ */
+struct rcu_tasks {
+	struct rcuwait cbs_wait;
+	raw_spinlock_t cbs_gbl_lock;
+	struct mutex tasks_gp_mutex;
+	int gp_state;
+	int gp_sleep;
+	int init_fract;
+	unsigned long gp_jiffies;
+	unsigned long gp_start;
+	unsigned long tasks_gp_seq;
+	unsigned long n_ipis;
+	unsigned long n_ipis_fails;
+	struct task_struct *kthread_ptr;
+	unsigned long lazy_jiffies;
+	rcu_tasks_gp_func_t gp_func;
+	pregp_func_t pregp_func;
+	pertask_func_t pertask_func;
+	postscan_func_t postscan_func;
+	holdouts_func_t holdouts_func;
+	postgp_func_t postgp_func;
+	call_rcu_func_t call_func;
+	struct rcu_tasks_percpu __percpu *rtpcpu;
+	int percpu_enqueue_shift;
+	int percpu_enqueue_lim;
+	int percpu_dequeue_lim;
+	unsigned long percpu_dequeue_gpseq;
+	struct mutex barrier_q_mutex;
+	atomic_t barrier_q_count;
+	struct completion barrier_q_completion;
+	unsigned long barrier_q_seq;
+	char *name;
+	char *kname;
+};
+
+static void call_rcu_tasks_iw_wakeup(struct irq_work *iwp);
+
+#define DEFINE_RCU_TASKS(rt_name, gp, call, n)						\
+static DEFINE_PER_CPU(struct rcu_tasks_percpu, rt_name ## __percpu) = {			\
+	.lock = __RAW_SPIN_LOCK_UNLOCKED(rt_name ## __percpu.cbs_pcpu_lock),		\
+	.rtp_irq_work = IRQ_WORK_INIT_HARD(call_rcu_tasks_iw_wakeup),			\
+};											\
+static struct rcu_tasks rt_name =							\
+{											\
+	.cbs_wait = __RCUWAIT_INITIALIZER(rt_name.wait),				\
+	.cbs_gbl_lock = __RAW_SPIN_LOCK_UNLOCKED(rt_name.cbs_gbl_lock),			\
+	.tasks_gp_mutex = __MUTEX_INITIALIZER(rt_name.tasks_gp_mutex),			\
+	.gp_func = gp,									\
+	.call_func = call,								\
+	.rtpcpu = &rt_name ## __percpu,							\
+	.lazy_jiffies = DIV_ROUND_UP(HZ, 4),						\
+	.name = n,									\
+	.percpu_enqueue_shift = order_base_2(CONFIG_NR_CPUS),				\
+	.percpu_enqueue_lim = 1,							\
+	.percpu_dequeue_lim = 1,							\
+	.barrier_q_mutex = __MUTEX_INITIALIZER(rt_name.barrier_q_mutex),		\
+	.barrier_q_seq = (0UL - 50UL) << RCU_SEQ_CTR_SHIFT,				\
+	.kname = #rt_name,								\
+}
+
+#ifdef CONFIG_TASKS_RCU
+/* Track exiting tasks in order to allow them to be waited for. */
+DEFINE_STATIC_SRCU(tasks_rcu_exit_srcu);
+
+/* Report delay in synchronize_srcu() completion in rcu_tasks_postscan(). */
+static void tasks_rcu_exit_srcu_stall(struct timer_list *unused);
+static DEFINE_TIMER(tasks_rcu_exit_srcu_stall_timer, tasks_rcu_exit_srcu_stall);
+#endif
+
+/* Avoid IPIing CPUs early in the grace period. */
+#define RCU_TASK_IPI_DELAY (IS_ENABLED(CONFIG_TASKS_TRACE_RCU_READ_MB) ? HZ / 2 : 0)
+static int rcu_task_ipi_delay __read_mostly = RCU_TASK_IPI_DELAY;
+module_param(rcu_task_ipi_delay, int, 0644);
+
+/* Control stall timeouts.  Disable with <= 0, otherwise jiffies till stall. */
+#define RCU_TASK_BOOT_STALL_TIMEOUT (HZ * 30)
+#define RCU_TASK_STALL_TIMEOUT (HZ * 60 * 10)
+static int rcu_task_stall_timeout __read_mostly = RCU_TASK_STALL_TIMEOUT;
+module_param(rcu_task_stall_timeout, int, 0644);
+#define RCU_TASK_STALL_INFO (HZ * 10)
+static int rcu_task_stall_info __read_mostly = RCU_TASK_STALL_INFO;
+module_param(rcu_task_stall_info, int, 0644);
+static int rcu_task_stall_info_mult __read_mostly = 3;
+module_param(rcu_task_stall_info_mult, int, 0444);
+
+static int rcu_task_enqueue_lim __read_mostly = -1;
+module_param(rcu_task_enqueue_lim, int, 0444);
+
+static bool rcu_task_cb_adjust;
+static int rcu_task_contend_lim __read_mostly = 100;
+module_param(rcu_task_contend_lim, int, 0444);
+static int rcu_task_collapse_lim __read_mostly = 10;
+module_param(rcu_task_collapse_lim, int, 0444);
+static int rcu_task_lazy_lim __read_mostly = 32;
+module_param(rcu_task_lazy_lim, int, 0444);
+
+/* RCU tasks grace-period state for debugging. */
+#define RTGS_INIT		 0
+#define RTGS_WAIT_WAIT_CBS	 1
+#define RTGS_WAIT_GP		 2
+#define RTGS_PRE_WAIT_GP	 3
+#define RTGS_SCAN_TASKLIST	 4
+#define RTGS_POST_SCAN_TASKLIST	 5
+#define RTGS_WAIT_SCAN_HOLDOUTS	 6
+#define RTGS_SCAN_HOLDOUTS	 7
+#define RTGS_POST_GP		 8
+#define RTGS_WAIT_READERS	 9
+#define RTGS_INVOKE_CBS		10
+#define RTGS_WAIT_CBS		11
+#ifndef CONFIG_TINY_RCU
+static const char * const rcu_tasks_gp_state_names[] = {
+	"RTGS_INIT",
+	"RTGS_WAIT_WAIT_CBS",
+	"RTGS_WAIT_GP",
+	"RTGS_PRE_WAIT_GP",
+	"RTGS_SCAN_TASKLIST",
+	"RTGS_POST_SCAN_TASKLIST",
+	"RTGS_WAIT_SCAN_HOLDOUTS",
+	"RTGS_SCAN_HOLDOUTS",
+	"RTGS_POST_GP",
+	"RTGS_WAIT_READERS",
+	"RTGS_INVOKE_CBS",
+	"RTGS_WAIT_CBS",
+};
+#endif /* #ifndef CONFIG_TINY_RCU */
+
+////////////////////////////////////////////////////////////////////////
+//
+// Generic code.
+
+static void rcu_tasks_invoke_cbs_wq(struct work_struct *wp);
+
+/* Record grace-period phase and time. */
+static void set_tasks_gp_state(struct rcu_tasks *rtp, int newstate)
+{
+	rtp->gp_state = newstate;
+	rtp->gp_jiffies = jiffies;
+}
+
+#ifndef CONFIG_TINY_RCU
+/* Return state name. */
+static const char *tasks_gp_state_getname(struct rcu_tasks *rtp)
+{
+	int i = data_race(rtp->gp_state); // Let KCSAN detect update races
+	int j = READ_ONCE(i); // Prevent the compiler from reading twice
+
+	if (j >= ARRAY_SIZE(rcu_tasks_gp_state_names))
+		return "???";
+	return rcu_tasks_gp_state_names[j];
+}
+#endif /* #ifndef CONFIG_TINY_RCU */
+
+// Initialize per-CPU callback lists for the specified flavor of
+// Tasks RCU.  Do not enqueue callbacks before this function is invoked.
+static void cblist_init_generic(struct rcu_tasks *rtp)
+{
+	int cpu;
+	unsigned long flags;
+	int lim;
+	int shift;
+
+	if (rcu_task_enqueue_lim < 0) {
+		rcu_task_enqueue_lim = 1;
+		rcu_task_cb_adjust = true;
+	} else if (rcu_task_enqueue_lim == 0) {
+		rcu_task_enqueue_lim = 1;
+	}
+	lim = rcu_task_enqueue_lim;
+
+	if (lim > nr_cpu_ids)
+		lim = nr_cpu_ids;
+	shift = ilog2(nr_cpu_ids / lim);
+	if (((nr_cpu_ids - 1) >> shift) >= lim)
+		shift++;
+	WRITE_ONCE(rtp->percpu_enqueue_shift, shift);
+	WRITE_ONCE(rtp->percpu_dequeue_lim, lim);
+	smp_store_release(&rtp->percpu_enqueue_lim, lim);
+	for_each_possible_cpu(cpu) {
+		struct rcu_tasks_percpu *rtpcp = per_cpu_ptr(rtp->rtpcpu, cpu);
+
+		WARN_ON_ONCE(!rtpcp);
+		if (cpu)
+			raw_spin_lock_init(&ACCESS_PRIVATE(rtpcp, lock));
+		local_irq_save(flags);  // serialize initialization
+		if (rcu_segcblist_empty(&rtpcp->cblist))
+			rcu_segcblist_init(&rtpcp->cblist);
+		local_irq_restore(flags);
+		INIT_WORK(&rtpcp->rtp_work, rcu_tasks_invoke_cbs_wq);
+		rtpcp->cpu = cpu;
+		rtpcp->rtpp = rtp;
+		if (!rtpcp->rtp_blkd_tasks.next)
+			INIT_LIST_HEAD(&rtpcp->rtp_blkd_tasks);
+	}
+
+	pr_info("%s: Setting shift to %d and lim to %d rcu_task_cb_adjust=%d.\n", rtp->name,
+			data_race(rtp->percpu_enqueue_shift), data_race(rtp->percpu_enqueue_lim), rcu_task_cb_adjust);
+}
+
+// Compute wakeup time for lazy callback timer.
+static unsigned long rcu_tasks_lazy_time(struct rcu_tasks *rtp)
+{
+	return jiffies + rtp->lazy_jiffies;
+}
+
+// Timer handler that unlazifies lazy callbacks.
+static void call_rcu_tasks_generic_timer(struct timer_list *tlp)
+{
+	unsigned long flags;
+	bool needwake = false;
+	struct rcu_tasks *rtp;
+	struct rcu_tasks_percpu *rtpcp = from_timer(rtpcp, tlp, lazy_timer);
+
+	rtp = rtpcp->rtpp;
+	raw_spin_lock_irqsave_rcu_node(rtpcp, flags);
+	if (!rcu_segcblist_empty(&rtpcp->cblist) && rtp->lazy_jiffies) {
+		if (!rtpcp->urgent_gp)
+			rtpcp->urgent_gp = 1;
+		needwake = true;
+		mod_timer(&rtpcp->lazy_timer, rcu_tasks_lazy_time(rtp));
+	}
+	raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
+	if (needwake)
+		rcuwait_wake_up(&rtp->cbs_wait);
+}
+
+// IRQ-work handler that does deferred wakeup for call_rcu_tasks_generic().
+static void call_rcu_tasks_iw_wakeup(struct irq_work *iwp)
+{
+	struct rcu_tasks *rtp;
+	struct rcu_tasks_percpu *rtpcp = container_of(iwp, struct rcu_tasks_percpu, rtp_irq_work);
+
+	rtp = rtpcp->rtpp;
+	rcuwait_wake_up(&rtp->cbs_wait);
+}
+
+// Enqueue a callback for the specified flavor of Tasks RCU.
+static void call_rcu_tasks_generic(struct rcu_head *rhp, rcu_callback_t func,
+				   struct rcu_tasks *rtp)
+{
+	int chosen_cpu;
+	unsigned long flags;
+	bool havekthread = smp_load_acquire(&rtp->kthread_ptr);
+	int ideal_cpu;
+	unsigned long j;
+	bool needadjust = false;
+	bool needwake;
+	struct rcu_tasks_percpu *rtpcp;
+
+	rhp->next = NULL;
+	rhp->func = func;
+	local_irq_save(flags);
+	rcu_read_lock();
+	ideal_cpu = smp_processor_id() >> READ_ONCE(rtp->percpu_enqueue_shift);
+	chosen_cpu = cpumask_next(ideal_cpu - 1, cpu_possible_mask);
+	rtpcp = per_cpu_ptr(rtp->rtpcpu, chosen_cpu);
+	if (!raw_spin_trylock_rcu_node(rtpcp)) { // irqs already disabled.
+		raw_spin_lock_rcu_node(rtpcp); // irqs already disabled.
+		j = jiffies;
+		if (rtpcp->rtp_jiffies != j) {
+			rtpcp->rtp_jiffies = j;
+			rtpcp->rtp_n_lock_retries = 0;
+		}
+		if (rcu_task_cb_adjust && ++rtpcp->rtp_n_lock_retries > rcu_task_contend_lim &&
+		    READ_ONCE(rtp->percpu_enqueue_lim) != nr_cpu_ids)
+			needadjust = true;  // Defer adjustment to avoid deadlock.
+	}
+	// Queuing callbacks before initialization not yet supported.
+	if (WARN_ON_ONCE(!rcu_segcblist_is_enabled(&rtpcp->cblist)))
+		rcu_segcblist_init(&rtpcp->cblist);
+	needwake = (func == wakeme_after_rcu) ||
+		   (rcu_segcblist_n_cbs(&rtpcp->cblist) == rcu_task_lazy_lim);
+	if (havekthread && !needwake && !timer_pending(&rtpcp->lazy_timer)) {
+		if (rtp->lazy_jiffies)
+			mod_timer(&rtpcp->lazy_timer, rcu_tasks_lazy_time(rtp));
+		else
+			needwake = rcu_segcblist_empty(&rtpcp->cblist);
+	}
+	if (needwake)
+		rtpcp->urgent_gp = 3;
+	rcu_segcblist_enqueue(&rtpcp->cblist, rhp);
+	raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
+	if (unlikely(needadjust)) {
+		raw_spin_lock_irqsave(&rtp->cbs_gbl_lock, flags);
+		if (rtp->percpu_enqueue_lim != nr_cpu_ids) {
+			WRITE_ONCE(rtp->percpu_enqueue_shift, 0);
+			WRITE_ONCE(rtp->percpu_dequeue_lim, nr_cpu_ids);
+			smp_store_release(&rtp->percpu_enqueue_lim, nr_cpu_ids);
+			pr_info("Switching %s to per-CPU callback queuing.\n", rtp->name);
+		}
+		raw_spin_unlock_irqrestore(&rtp->cbs_gbl_lock, flags);
+	}
+	rcu_read_unlock();
+	/* We can't create the thread unless interrupts are enabled. */
+	if (needwake && READ_ONCE(rtp->kthread_ptr))
+		irq_work_queue(&rtpcp->rtp_irq_work);
+}
+
+// RCU callback function for rcu_barrier_tasks_generic().
+static void rcu_barrier_tasks_generic_cb(struct rcu_head *rhp)
+{
+	struct rcu_tasks *rtp;
+	struct rcu_tasks_percpu *rtpcp;
+
+	rtpcp = container_of(rhp, struct rcu_tasks_percpu, barrier_q_head);
+	rtp = rtpcp->rtpp;
+	if (atomic_dec_and_test(&rtp->barrier_q_count))
+		complete(&rtp->barrier_q_completion);
+}
+
+// Wait for all in-flight callbacks for the specified RCU Tasks flavor.
+// Operates in a manner similar to rcu_barrier().
+static void rcu_barrier_tasks_generic(struct rcu_tasks *rtp)
+{
+	int cpu;
+	unsigned long flags;
+	struct rcu_tasks_percpu *rtpcp;
+	unsigned long s = rcu_seq_snap(&rtp->barrier_q_seq);
+
+	mutex_lock(&rtp->barrier_q_mutex);
+	if (rcu_seq_done(&rtp->barrier_q_seq, s)) {
+		smp_mb();
+		mutex_unlock(&rtp->barrier_q_mutex);
+		return;
+	}
+	rcu_seq_start(&rtp->barrier_q_seq);
+	init_completion(&rtp->barrier_q_completion);
+	atomic_set(&rtp->barrier_q_count, 2);
+	for_each_possible_cpu(cpu) {
+		if (cpu >= smp_load_acquire(&rtp->percpu_dequeue_lim))
+			break;
+		rtpcp = per_cpu_ptr(rtp->rtpcpu, cpu);
+		rtpcp->barrier_q_head.func = rcu_barrier_tasks_generic_cb;
+		raw_spin_lock_irqsave_rcu_node(rtpcp, flags);
+		if (rcu_segcblist_entrain(&rtpcp->cblist, &rtpcp->barrier_q_head))
+			atomic_inc(&rtp->barrier_q_count);
+		raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
+	}
+	if (atomic_sub_and_test(2, &rtp->barrier_q_count))
+		complete(&rtp->barrier_q_completion);
+	wait_for_completion(&rtp->barrier_q_completion);
+	rcu_seq_end(&rtp->barrier_q_seq);
+	mutex_unlock(&rtp->barrier_q_mutex);
+}
+
+// Advance callbacks and indicate whether either a grace period or
+// callback invocation is needed.
+static int rcu_tasks_need_gpcb(struct rcu_tasks *rtp)
+{
+	int cpu;
+	unsigned long flags;
+	bool gpdone = poll_state_synchronize_rcu(rtp->percpu_dequeue_gpseq);
+	long n;
+	long ncbs = 0;
+	long ncbsnz = 0;
+	int needgpcb = 0;
+
+	for (cpu = 0; cpu < smp_load_acquire(&rtp->percpu_dequeue_lim); cpu++) {
+		struct rcu_tasks_percpu *rtpcp = per_cpu_ptr(rtp->rtpcpu, cpu);
+
+		/* Advance and accelerate any new callbacks. */
+		if (!rcu_segcblist_n_cbs(&rtpcp->cblist))
+			continue;
+		raw_spin_lock_irqsave_rcu_node(rtpcp, flags);
+		// Should we shrink down to a single callback queue?
+		n = rcu_segcblist_n_cbs(&rtpcp->cblist);
+		if (n) {
+			ncbs += n;
+			if (cpu > 0)
+				ncbsnz += n;
+		}
+		rcu_segcblist_advance(&rtpcp->cblist, rcu_seq_current(&rtp->tasks_gp_seq));
+		(void)rcu_segcblist_accelerate(&rtpcp->cblist, rcu_seq_snap(&rtp->tasks_gp_seq));
+		if (rtpcp->urgent_gp > 0 && rcu_segcblist_pend_cbs(&rtpcp->cblist)) {
+			if (rtp->lazy_jiffies)
+				rtpcp->urgent_gp--;
+			needgpcb |= 0x3;
+		} else if (rcu_segcblist_empty(&rtpcp->cblist)) {
+			rtpcp->urgent_gp = 0;
+		}
+		if (rcu_segcblist_ready_cbs(&rtpcp->cblist))
+			needgpcb |= 0x1;
+		raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
+	}
+
+	// Shrink down to a single callback queue if appropriate.
+	// This is done in two stages: (1) If there are no more than
+	// rcu_task_collapse_lim callbacks on CPU 0 and none on any other
+	// CPU, limit enqueueing to CPU 0.  (2) After an RCU grace period,
+	// if there has not been an increase in callbacks, limit dequeuing
+	// to CPU 0.  Note the matching RCU read-side critical section in
+	// call_rcu_tasks_generic().
+	if (rcu_task_cb_adjust && ncbs <= rcu_task_collapse_lim) {
+		raw_spin_lock_irqsave(&rtp->cbs_gbl_lock, flags);
+		if (rtp->percpu_enqueue_lim > 1) {
+			WRITE_ONCE(rtp->percpu_enqueue_shift, order_base_2(nr_cpu_ids));
+			smp_store_release(&rtp->percpu_enqueue_lim, 1);
+			rtp->percpu_dequeue_gpseq = get_state_synchronize_rcu();
+			gpdone = false;
+			pr_info("Starting switch %s to CPU-0 callback queuing.\n", rtp->name);
+		}
+		raw_spin_unlock_irqrestore(&rtp->cbs_gbl_lock, flags);
+	}
+	if (rcu_task_cb_adjust && !ncbsnz && gpdone) {
+		raw_spin_lock_irqsave(&rtp->cbs_gbl_lock, flags);
+		if (rtp->percpu_enqueue_lim < rtp->percpu_dequeue_lim) {
+			WRITE_ONCE(rtp->percpu_dequeue_lim, 1);
+			pr_info("Completing switch %s to CPU-0 callback queuing.\n", rtp->name);
+		}
+		if (rtp->percpu_dequeue_lim == 1) {
+			for (cpu = rtp->percpu_dequeue_lim; cpu < nr_cpu_ids; cpu++) {
+				struct rcu_tasks_percpu *rtpcp = per_cpu_ptr(rtp->rtpcpu, cpu);
+
+				WARN_ON_ONCE(rcu_segcblist_n_cbs(&rtpcp->cblist));
+			}
+		}
+		raw_spin_unlock_irqrestore(&rtp->cbs_gbl_lock, flags);
+	}
+
+	return needgpcb;
+}
+
+// Advance callbacks and invoke any that are ready.
+static void rcu_tasks_invoke_cbs(struct rcu_tasks *rtp, struct rcu_tasks_percpu *rtpcp)
+{
+	int cpu;
+	int cpunext;
+	int cpuwq;
+	unsigned long flags;
+	int len;
+	struct rcu_head *rhp;
+	struct rcu_cblist rcl = RCU_CBLIST_INITIALIZER(rcl);
+	struct rcu_tasks_percpu *rtpcp_next;
+
+	cpu = rtpcp->cpu;
+	cpunext = cpu * 2 + 1;
+	if (cpunext < smp_load_acquire(&rtp->percpu_dequeue_lim)) {
+		rtpcp_next = per_cpu_ptr(rtp->rtpcpu, cpunext);
+		cpuwq = rcu_cpu_beenfullyonline(cpunext) ? cpunext : WORK_CPU_UNBOUND;
+		queue_work_on(cpuwq, system_wq, &rtpcp_next->rtp_work);
+		cpunext++;
+		if (cpunext < smp_load_acquire(&rtp->percpu_dequeue_lim)) {
+			rtpcp_next = per_cpu_ptr(rtp->rtpcpu, cpunext);
+			cpuwq = rcu_cpu_beenfullyonline(cpunext) ? cpunext : WORK_CPU_UNBOUND;
+			queue_work_on(cpuwq, system_wq, &rtpcp_next->rtp_work);
+		}
+	}
+
+	if (rcu_segcblist_empty(&rtpcp->cblist) || !cpu_possible(cpu))
+		return;
+	raw_spin_lock_irqsave_rcu_node(rtpcp, flags);
+	rcu_segcblist_advance(&rtpcp->cblist, rcu_seq_current(&rtp->tasks_gp_seq));
+	rcu_segcblist_extract_done_cbs(&rtpcp->cblist, &rcl);
+	raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
+	len = rcl.len;
+	for (rhp = rcu_cblist_dequeue(&rcl); rhp; rhp = rcu_cblist_dequeue(&rcl)) {
+		local_bh_disable();
+		rhp->func(rhp);
+		local_bh_enable();
+		cond_resched();
+	}
+	raw_spin_lock_irqsave_rcu_node(rtpcp, flags);
+	rcu_segcblist_add_len(&rtpcp->cblist, -len);
+	(void)rcu_segcblist_accelerate(&rtpcp->cblist, rcu_seq_snap(&rtp->tasks_gp_seq));
+	raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
+}
+
+// Workqueue flood to advance callbacks and invoke any that are ready.
+static void rcu_tasks_invoke_cbs_wq(struct work_struct *wp)
+{
+	struct rcu_tasks *rtp;
+	struct rcu_tasks_percpu *rtpcp = container_of(wp, struct rcu_tasks_percpu, rtp_work);
+
+	rtp = rtpcp->rtpp;
+	rcu_tasks_invoke_cbs(rtp, rtpcp);
+}
+
+// Wait for one grace period.
+static void rcu_tasks_one_gp(struct rcu_tasks *rtp, bool midboot)
+{
+	int needgpcb;
+
+	mutex_lock(&rtp->tasks_gp_mutex);
+
+	// If there were none, wait a bit and start over.
+	if (unlikely(midboot)) {
+		needgpcb = 0x2;
+	} else {
+		mutex_unlock(&rtp->tasks_gp_mutex);
+		set_tasks_gp_state(rtp, RTGS_WAIT_CBS);
+		rcuwait_wait_event(&rtp->cbs_wait,
+				   (needgpcb = rcu_tasks_need_gpcb(rtp)),
+				   TASK_IDLE);
+		mutex_lock(&rtp->tasks_gp_mutex);
+	}
+
+	if (needgpcb & 0x2) {
+		// Wait for one grace period.
+		set_tasks_gp_state(rtp, RTGS_WAIT_GP);
+		rtp->gp_start = jiffies;
+		rcu_seq_start(&rtp->tasks_gp_seq);
+		rtp->gp_func(rtp);
+		rcu_seq_end(&rtp->tasks_gp_seq);
+	}
+
+	// Invoke callbacks.
+	set_tasks_gp_state(rtp, RTGS_INVOKE_CBS);
+	rcu_tasks_invoke_cbs(rtp, per_cpu_ptr(rtp->rtpcpu, 0));
+	mutex_unlock(&rtp->tasks_gp_mutex);
+}
+
+// RCU-tasks kthread that detects grace periods and invokes callbacks.
+static int __noreturn rcu_tasks_kthread(void *arg)
+{
+	int cpu;
+	struct rcu_tasks *rtp = arg;
+
+	for_each_possible_cpu(cpu) {
+		struct rcu_tasks_percpu *rtpcp = per_cpu_ptr(rtp->rtpcpu, cpu);
+
+		timer_setup(&rtpcp->lazy_timer, call_rcu_tasks_generic_timer, 0);
+		rtpcp->urgent_gp = 1;
+	}
+
+	/* Run on housekeeping CPUs by default.  Sysadm can move if desired. */
+	housekeeping_affine(current, HK_TYPE_RCU);
+	smp_store_release(&rtp->kthread_ptr, current); // Let GPs start!
+
+	/*
+	 * Each pass through the following loop makes one check for
+	 * newly arrived callbacks, and, if there are some, waits for
+	 * one RCU-tasks grace period and then invokes the callbacks.
+	 * This loop is terminated by the system going down.  ;-)
+	 */
+	for (;;) {
+		// Wait for one grace period and invoke any callbacks
+		// that are ready.
+		rcu_tasks_one_gp(rtp, false);
+
+		// Paranoid sleep to keep this from entering a tight loop.
+		schedule_timeout_idle(rtp->gp_sleep);
+	}
+}
+
+// Wait for a grace period for the specified flavor of Tasks RCU.
+static void synchronize_rcu_tasks_generic(struct rcu_tasks *rtp)
+{
+	/* Complain if the scheduler has not started.  */
+	if (WARN_ONCE(rcu_scheduler_active == RCU_SCHEDULER_INACTIVE,
+			 "synchronize_%s() called too soon", rtp->name))
+		return;
+
+	// If the grace-period kthread is running, use it.
+	if (READ_ONCE(rtp->kthread_ptr)) {
+		wait_rcu_gp(rtp->call_func);
+		return;
+	}
+	rcu_tasks_one_gp(rtp, true);
+}
+
+/* Spawn RCU-tasks grace-period kthread. */
+static void __init rcu_spawn_tasks_kthread_generic(struct rcu_tasks *rtp)
+{
+	struct task_struct *t;
+
+	t = kthread_run(rcu_tasks_kthread, rtp, "%s_kthread", rtp->kname);
+	if (WARN_ONCE(IS_ERR(t), "%s: Could not start %s grace-period kthread, OOM is now expected behavior\n", __func__, rtp->name))
+		return;
+	smp_mb(); /* Ensure others see full kthread. */
+}
+
+#ifndef CONFIG_TINY_RCU
+
+/*
+ * Print any non-default Tasks RCU settings.
+ */
+static void __init rcu_tasks_bootup_oddness(void)
+{
+#if defined(CONFIG_TASKS_RCU) || defined(CONFIG_TASKS_TRACE_RCU)
+	int rtsimc;
+
+	if (rcu_task_stall_timeout != RCU_TASK_STALL_TIMEOUT)
+		pr_info("\tTasks-RCU CPU stall warnings timeout set to %d (rcu_task_stall_timeout).\n", rcu_task_stall_timeout);
+	rtsimc = clamp(rcu_task_stall_info_mult, 1, 10);
+	if (rtsimc != rcu_task_stall_info_mult) {
+		pr_info("\tTasks-RCU CPU stall info multiplier clamped to %d (rcu_task_stall_info_mult).\n", rtsimc);
+		rcu_task_stall_info_mult = rtsimc;
+	}
+#endif /* #ifdef CONFIG_TASKS_RCU */
+#ifdef CONFIG_TASKS_RCU
+	pr_info("\tTrampoline variant of Tasks RCU enabled.\n");
+#endif /* #ifdef CONFIG_TASKS_RCU */
+#ifdef CONFIG_TASKS_RUDE_RCU
+	pr_info("\tRude variant of Tasks RCU enabled.\n");
+#endif /* #ifdef CONFIG_TASKS_RUDE_RCU */
+#ifdef CONFIG_TASKS_TRACE_RCU
+	pr_info("\tTracing variant of Tasks RCU enabled.\n");
+#endif /* #ifdef CONFIG_TASKS_TRACE_RCU */
+}
+
+#endif /* #ifndef CONFIG_TINY_RCU */
+
+#ifndef CONFIG_TINY_RCU
+/* Dump out rcutorture-relevant state common to all RCU-tasks flavors. */
+static void show_rcu_tasks_generic_gp_kthread(struct rcu_tasks *rtp, char *s)
+{
+	int cpu;
+	bool havecbs = false;
+	bool haveurgent = false;
+	bool haveurgentcbs = false;
+
+	for_each_possible_cpu(cpu) {
+		struct rcu_tasks_percpu *rtpcp = per_cpu_ptr(rtp->rtpcpu, cpu);
+
+		if (!data_race(rcu_segcblist_empty(&rtpcp->cblist)))
+			havecbs = true;
+		if (data_race(rtpcp->urgent_gp))
+			haveurgent = true;
+		if (!data_race(rcu_segcblist_empty(&rtpcp->cblist)) && data_race(rtpcp->urgent_gp))
+			haveurgentcbs = true;
+		if (havecbs && haveurgent && haveurgentcbs)
+			break;
+	}
+	pr_info("%s: %s(%d) since %lu g:%lu i:%lu/%lu %c%c%c%c l:%lu %s\n",
+		rtp->kname,
+		tasks_gp_state_getname(rtp), data_race(rtp->gp_state),
+		jiffies - data_race(rtp->gp_jiffies),
+		data_race(rcu_seq_current(&rtp->tasks_gp_seq)),
+		data_race(rtp->n_ipis_fails), data_race(rtp->n_ipis),
+		".k"[!!data_race(rtp->kthread_ptr)],
+		".C"[havecbs],
+		".u"[haveurgent],
+		".U"[haveurgentcbs],
+		rtp->lazy_jiffies,
+		s);
+}
+#endif // #ifndef CONFIG_TINY_RCU
+
+static void exit_tasks_rcu_finish_trace(struct task_struct *t);
+
+#if defined(CONFIG_TASKS_RCU) || defined(CONFIG_TASKS_TRACE_RCU)
+
+////////////////////////////////////////////////////////////////////////
+//
+// Shared code between task-list-scanning variants of Tasks RCU.
+
+/* Wait for one RCU-tasks grace period. */
+static void rcu_tasks_wait_gp(struct rcu_tasks *rtp)
+{
+	struct task_struct *g;
+	int fract;
+	LIST_HEAD(holdouts);
+	unsigned long j;
+	unsigned long lastinfo;
+	unsigned long lastreport;
+	bool reported = false;
+	int rtsi;
+	struct task_struct *t;
+
+	set_tasks_gp_state(rtp, RTGS_PRE_WAIT_GP);
+	rtp->pregp_func(&holdouts);
+
+	/*
+	 * There were callbacks, so we need to wait for an RCU-tasks
+	 * grace period.  Start off by scanning the task list for tasks
+	 * that are not already voluntarily blocked.  Mark these tasks
+	 * and make a list of them in holdouts.
+	 */
+	set_tasks_gp_state(rtp, RTGS_SCAN_TASKLIST);
+	if (rtp->pertask_func) {
+		rcu_read_lock();
+		for_each_process_thread(g, t)
+			rtp->pertask_func(t, &holdouts);
+		rcu_read_unlock();
+	}
+
+	set_tasks_gp_state(rtp, RTGS_POST_SCAN_TASKLIST);
+	rtp->postscan_func(&holdouts);
+
+	/*
+	 * Each pass through the following loop scans the list of holdout
+	 * tasks, removing any that are no longer holdouts.  When the list
+	 * is empty, we are done.
+	 */
+	lastreport = jiffies;
+	lastinfo = lastreport;
+	rtsi = READ_ONCE(rcu_task_stall_info);
+
+	// Start off with initial wait and slowly back off to 1 HZ wait.
+	fract = rtp->init_fract;
+
+	while (!list_empty(&holdouts)) {
+		ktime_t exp;
+		bool firstreport;
+		bool needreport;
+		int rtst;
+
+		// Slowly back off waiting for holdouts
+		set_tasks_gp_state(rtp, RTGS_WAIT_SCAN_HOLDOUTS);
+		if (!IS_ENABLED(CONFIG_PREEMPT_RT)) {
+			schedule_timeout_idle(fract);
+		} else {
+			exp = jiffies_to_nsecs(fract);
+			__set_current_state(TASK_IDLE);
+			schedule_hrtimeout_range(&exp, jiffies_to_nsecs(HZ / 2), HRTIMER_MODE_REL_HARD);
+		}
+
+		if (fract < HZ)
+			fract++;
+
+		rtst = READ_ONCE(rcu_task_stall_timeout);
+		needreport = rtst > 0 && time_after(jiffies, lastreport + rtst);
+		if (needreport) {
+			lastreport = jiffies;
+			reported = true;
+		}
+		firstreport = true;
+		WARN_ON(signal_pending(current));
+		set_tasks_gp_state(rtp, RTGS_SCAN_HOLDOUTS);
+		rtp->holdouts_func(&holdouts, needreport, &firstreport);
+
+		// Print pre-stall informational messages if needed.
+		j = jiffies;
+		if (rtsi > 0 && !reported && time_after(j, lastinfo + rtsi)) {
+			lastinfo = j;
+			rtsi = rtsi * rcu_task_stall_info_mult;
+			pr_info("%s: %s grace period number %lu (since boot) is %lu jiffies old.\n",
+				__func__, rtp->kname, rtp->tasks_gp_seq, j - rtp->gp_start);
+		}
+	}
+
+	set_tasks_gp_state(rtp, RTGS_POST_GP);
+	rtp->postgp_func(rtp);
+}
+
+#endif /* #if defined(CONFIG_TASKS_RCU) || defined(CONFIG_TASKS_TRACE_RCU) */
+
+#ifdef CONFIG_TASKS_RCU
+
+////////////////////////////////////////////////////////////////////////
+//
+// Simple variant of RCU whose quiescent states are voluntary context
+// switch, cond_resched_tasks_rcu_qs(), user-space execution, and idle.
+// As such, grace periods can take one good long time.  There are no
+// read-side primitives similar to rcu_read_lock() and rcu_read_unlock()
+// because this implementation is intended to get the system into a safe
+// state for some of the manipulations involved in tracing and the like.
+// Finally, this implementation does not support high call_rcu_tasks()
+// rates from multiple CPUs.  If this is required, per-CPU callback lists
+// will be needed.
+//
+// The implementation uses rcu_tasks_wait_gp(), which relies on function
+// pointers in the rcu_tasks structure.  The rcu_spawn_tasks_kthread()
+// function sets these function pointers up so that rcu_tasks_wait_gp()
+// invokes these functions in this order:
+//
+// rcu_tasks_pregp_step():
+//	Invokes synchronize_rcu() in order to wait for all in-flight
+//	t->on_rq and t->nvcsw transitions to complete.	This works because
+//	all such transitions are carried out with interrupts disabled.
+// rcu_tasks_pertask(), invoked on every non-idle task:
+//	For every runnable non-idle task other than the current one, use
+//	get_task_struct() to pin down that task, snapshot that task's
+//	number of voluntary context switches, and add that task to the
+//	holdout list.
+// rcu_tasks_postscan():
+//	Invoke synchronize_srcu() to ensure that all tasks that were
+//	in the process of exiting (and which thus might not know to
+//	synchronize with this RCU Tasks grace period) have completed
+//	exiting.
+// check_all_holdout_tasks(), repeatedly until holdout list is empty:
+//	Scans the holdout list, attempting to identify a quiescent state
+//	for each task on the list.  If there is a quiescent state, the
+//	corresponding task is removed from the holdout list.
+// rcu_tasks_postgp():
+//	Invokes synchronize_rcu() in order to ensure that all prior
+//	t->on_rq and t->nvcsw transitions are seen by all CPUs and tasks
+//	to have happened before the end of this RCU Tasks grace period.
+//	Again, this works because all such transitions are carried out
+//	with interrupts disabled.
+//
+// For each exiting task, the exit_tasks_rcu_start() and
+// exit_tasks_rcu_finish() functions begin and end, respectively, the SRCU
+// read-side critical sections waited for by rcu_tasks_postscan().
+//
+// Pre-grace-period update-side code is ordered before the grace
+// via the raw_spin_lock.*rcu_node().  Pre-grace-period read-side code
+// is ordered before the grace period via synchronize_rcu() call in
+// rcu_tasks_pregp_step() and by the scheduler's locks and interrupt
+// disabling.
+
+/* Pre-grace-period preparation. */
+static void rcu_tasks_pregp_step(struct list_head *hop)
+{
+	/*
+	 * Wait for all pre-existing t->on_rq and t->nvcsw transitions
+	 * to complete.  Invoking synchronize_rcu() suffices because all
+	 * these transitions occur with interrupts disabled.  Without this
+	 * synchronize_rcu(), a read-side critical section that started
+	 * before the grace period might be incorrectly seen as having
+	 * started after the grace period.
+	 *
+	 * This synchronize_rcu() also dispenses with the need for a
+	 * memory barrier on the first store to t->rcu_tasks_holdout,
+	 * as it forces the store to happen after the beginning of the
+	 * grace period.
+	 */
+	synchronize_rcu();
+}
+
+/* Check for quiescent states since the pregp's synchronize_rcu() */
+static bool rcu_tasks_is_holdout(struct task_struct *t)
+{
+	int cpu;
+
+	/* Has the task been seen voluntarily sleeping? */
+	if (!READ_ONCE(t->on_rq))
+		return false;
+
+	/*
+	 * Idle tasks (or idle injection) within the idle loop are RCU-tasks
+	 * quiescent states. But CPU boot code performed by the idle task
+	 * isn't a quiescent state.
+	 */
+	if (is_idle_task(t))
+		return false;
+
+	cpu = task_cpu(t);
+
+	/* Idle tasks on offline CPUs are RCU-tasks quiescent states. */
+	if (t == idle_task(cpu) && !rcu_cpu_online(cpu))
+		return false;
+
+	return true;
+}
+
+/* Per-task initial processing. */
+static void rcu_tasks_pertask(struct task_struct *t, struct list_head *hop)
+{
+	if (t != current && rcu_tasks_is_holdout(t)) {
+		get_task_struct(t);
+		t->rcu_tasks_nvcsw = READ_ONCE(t->nvcsw);
+		WRITE_ONCE(t->rcu_tasks_holdout, true);
+		list_add(&t->rcu_tasks_holdout_list, hop);
+	}
+}
+
+/* Processing between scanning taskslist and draining the holdout list. */
+static void rcu_tasks_postscan(struct list_head *hop)
+{
+	int rtsi = READ_ONCE(rcu_task_stall_info);
+
+	if (!IS_ENABLED(CONFIG_TINY_RCU)) {
+		tasks_rcu_exit_srcu_stall_timer.expires = jiffies + rtsi;
+		add_timer(&tasks_rcu_exit_srcu_stall_timer);
+	}
+
+	/*
+	 * Exiting tasks may escape the tasklist scan. Those are vulnerable
+	 * until their final schedule() with TASK_DEAD state. To cope with
+	 * this, divide the fragile exit path part in two intersecting
+	 * read side critical sections:
+	 *
+	 * 1) An _SRCU_ read side starting before calling exit_notify(),
+	 *    which may remove the task from the tasklist, and ending after
+	 *    the final preempt_disable() call in do_exit().
+	 *
+	 * 2) An _RCU_ read side starting with the final preempt_disable()
+	 *    call in do_exit() and ending with the final call to schedule()
+	 *    with TASK_DEAD state.
+	 *
+	 * This handles the part 1). And postgp will handle part 2) with a
+	 * call to synchronize_rcu().
+	 */
+	synchronize_srcu(&tasks_rcu_exit_srcu);
+
+	if (!IS_ENABLED(CONFIG_TINY_RCU))
+		del_timer_sync(&tasks_rcu_exit_srcu_stall_timer);
+}
+
+/* See if tasks are still holding out, complain if so. */
+static void check_holdout_task(struct task_struct *t,
+			       bool needreport, bool *firstreport)
+{
+	int cpu;
+
+	if (!READ_ONCE(t->rcu_tasks_holdout) ||
+	    t->rcu_tasks_nvcsw != READ_ONCE(t->nvcsw) ||
+	    !rcu_tasks_is_holdout(t) ||
+	    (IS_ENABLED(CONFIG_NO_HZ_FULL) &&
+	     !is_idle_task(t) && t->rcu_tasks_idle_cpu >= 0)) {
+		WRITE_ONCE(t->rcu_tasks_holdout, false);
+		list_del_init(&t->rcu_tasks_holdout_list);
+		put_task_struct(t);
+		return;
+	}
+	rcu_request_urgent_qs_task(t);
+	if (!needreport)
+		return;
+	if (*firstreport) {
+		pr_err("INFO: rcu_tasks detected stalls on tasks:\n");
+		*firstreport = false;
+	}
+	cpu = task_cpu(t);
+	pr_alert("%p: %c%c nvcsw: %lu/%lu holdout: %d idle_cpu: %d/%d\n",
+		 t, ".I"[is_idle_task(t)],
+		 "N."[cpu < 0 || !tick_nohz_full_cpu(cpu)],
+		 t->rcu_tasks_nvcsw, t->nvcsw, t->rcu_tasks_holdout,
+		 t->rcu_tasks_idle_cpu, cpu);
+	sched_show_task(t);
+}
+
+/* Scan the holdout lists for tasks no longer holding out. */
+static void check_all_holdout_tasks(struct list_head *hop,
+				    bool needreport, bool *firstreport)
+{
+	struct task_struct *t, *t1;
+
+	list_for_each_entry_safe(t, t1, hop, rcu_tasks_holdout_list) {
+		check_holdout_task(t, needreport, firstreport);
+		cond_resched();
+	}
+}
+
+/* Finish off the Tasks-RCU grace period. */
+static void rcu_tasks_postgp(struct rcu_tasks *rtp)
+{
+	/*
+	 * Because ->on_rq and ->nvcsw are not guaranteed to have a full
+	 * memory barriers prior to them in the schedule() path, memory
+	 * reordering on other CPUs could cause their RCU-tasks read-side
+	 * critical sections to extend past the end of the grace period.
+	 * However, because these ->nvcsw updates are carried out with
+	 * interrupts disabled, we can use synchronize_rcu() to force the
+	 * needed ordering on all such CPUs.
+	 *
+	 * This synchronize_rcu() also confines all ->rcu_tasks_holdout
+	 * accesses to be within the grace period, avoiding the need for
+	 * memory barriers for ->rcu_tasks_holdout accesses.
+	 *
+	 * In addition, this synchronize_rcu() waits for exiting tasks
+	 * to complete their final preempt_disable() region of execution,
+	 * cleaning up after synchronize_srcu(&tasks_rcu_exit_srcu),
+	 * enforcing the whole region before tasklist removal until
+	 * the final schedule() with TASK_DEAD state to be an RCU TASKS
+	 * read side critical section.
+	 */
+	synchronize_rcu();
+}
+
+void call_rcu_tasks(struct rcu_head *rhp, rcu_callback_t func);
+DEFINE_RCU_TASKS(rcu_tasks, rcu_tasks_wait_gp, call_rcu_tasks, "RCU Tasks");
+
+static void tasks_rcu_exit_srcu_stall(struct timer_list *unused)
+{
+#ifndef CONFIG_TINY_RCU
+	int rtsi;
+
+	rtsi = READ_ONCE(rcu_task_stall_info);
+	pr_info("%s: %s grace period number %lu (since boot) gp_state: %s is %lu jiffies old.\n",
+		__func__, rcu_tasks.kname, rcu_tasks.tasks_gp_seq,
+		tasks_gp_state_getname(&rcu_tasks), jiffies - rcu_tasks.gp_jiffies);
+	pr_info("Please check any exiting tasks stuck between calls to exit_tasks_rcu_start() and exit_tasks_rcu_finish()\n");
+	tasks_rcu_exit_srcu_stall_timer.expires = jiffies + rtsi;
+	add_timer(&tasks_rcu_exit_srcu_stall_timer);
+#endif // #ifndef CONFIG_TINY_RCU
+}
+
+/**
+ * call_rcu_tasks() - Queue an RCU for invocation task-based grace period
+ * @rhp: structure to be used for queueing the RCU updates.
+ * @func: actual callback function to be invoked after the grace period
+ *
+ * The callback function will be invoked some time after a full grace
+ * period elapses, in other words after all currently executing RCU
+ * read-side critical sections have completed. call_rcu_tasks() assumes
+ * that the read-side critical sections end at a voluntary context
+ * switch (not a preemption!), cond_resched_tasks_rcu_qs(), entry into idle,
+ * or transition to usermode execution.  As such, there are no read-side
+ * primitives analogous to rcu_read_lock() and rcu_read_unlock() because
+ * this primitive is intended to determine that all tasks have passed
+ * through a safe state, not so much for data-structure synchronization.
+ *
+ * See the description of call_rcu() for more detailed information on
+ * memory ordering guarantees.
+ */
+void call_rcu_tasks(struct rcu_head *rhp, rcu_callback_t func)
+{
+	call_rcu_tasks_generic(rhp, func, &rcu_tasks);
+}
+EXPORT_SYMBOL_GPL(call_rcu_tasks);
+
+/**
+ * synchronize_rcu_tasks - wait until an rcu-tasks grace period has elapsed.
+ *
+ * Control will return to the caller some time after a full rcu-tasks
+ * grace period has elapsed, in other words after all currently
+ * executing rcu-tasks read-side critical sections have elapsed.  These
+ * read-side critical sections are delimited by calls to schedule(),
+ * cond_resched_tasks_rcu_qs(), idle execution, userspace execution, calls
+ * to synchronize_rcu_tasks(), and (in theory, anyway) cond_resched().
+ *
+ * This is a very specialized primitive, intended only for a few uses in
+ * tracing and other situations requiring manipulation of function
+ * preambles and profiling hooks.  The synchronize_rcu_tasks() function
+ * is not (yet) intended for heavy use from multiple CPUs.
+ *
+ * See the description of synchronize_rcu() for more detailed information
+ * on memory ordering guarantees.
+ */
+void synchronize_rcu_tasks(void)
+{
+	synchronize_rcu_tasks_generic(&rcu_tasks);
+}
+EXPORT_SYMBOL_GPL(synchronize_rcu_tasks);
+
+/**
+ * rcu_barrier_tasks - Wait for in-flight call_rcu_tasks() callbacks.
+ *
+ * Although the current implementation is guaranteed to wait, it is not
+ * obligated to, for example, if there are no pending callbacks.
+ */
+void rcu_barrier_tasks(void)
+{
+	rcu_barrier_tasks_generic(&rcu_tasks);
+}
+EXPORT_SYMBOL_GPL(rcu_barrier_tasks);
+
+int rcu_tasks_lazy_ms = -1;
+module_param(rcu_tasks_lazy_ms, int, 0444);
+
+static int __init rcu_spawn_tasks_kthread(void)
+{
+	cblist_init_generic(&rcu_tasks);
+	rcu_tasks.gp_sleep = HZ / 10;
+	rcu_tasks.init_fract = HZ / 10;
+	if (rcu_tasks_lazy_ms >= 0)
+		rcu_tasks.lazy_jiffies = msecs_to_jiffies(rcu_tasks_lazy_ms);
+	rcu_tasks.pregp_func = rcu_tasks_pregp_step;
+	rcu_tasks.pertask_func = rcu_tasks_pertask;
+	rcu_tasks.postscan_func = rcu_tasks_postscan;
+	rcu_tasks.holdouts_func = check_all_holdout_tasks;
+	rcu_tasks.postgp_func = rcu_tasks_postgp;
+	rcu_spawn_tasks_kthread_generic(&rcu_tasks);
+	return 0;
+}
+
+#if !defined(CONFIG_TINY_RCU)
+void show_rcu_tasks_classic_gp_kthread(void)
+{
+	show_rcu_tasks_generic_gp_kthread(&rcu_tasks, "");
+}
+EXPORT_SYMBOL_GPL(show_rcu_tasks_classic_gp_kthread);
+#endif // !defined(CONFIG_TINY_RCU)
+
+struct task_struct *get_rcu_tasks_gp_kthread(void)
+{
+	return rcu_tasks.kthread_ptr;
+}
+EXPORT_SYMBOL_GPL(get_rcu_tasks_gp_kthread);
+
+/*
+ * Contribute to protect against tasklist scan blind spot while the
+ * task is exiting and may be removed from the tasklist. See
+ * corresponding synchronize_srcu() for further details.
+ */
+void exit_tasks_rcu_start(void) __acquires(&tasks_rcu_exit_srcu)
+{
+	current->rcu_tasks_idx = __srcu_read_lock(&tasks_rcu_exit_srcu);
+}
+
+/*
+ * Contribute to protect against tasklist scan blind spot while the
+ * task is exiting and may be removed from the tasklist. See
+ * corresponding synchronize_srcu() for further details.
+ */
+void exit_tasks_rcu_stop(void) __releases(&tasks_rcu_exit_srcu)
+{
+	struct task_struct *t = current;
+
+	__srcu_read_unlock(&tasks_rcu_exit_srcu, t->rcu_tasks_idx);
+}
+
+/*
+ * Contribute to protect against tasklist scan blind spot while the
+ * task is exiting and may be removed from the tasklist. See
+ * corresponding synchronize_srcu() for further details.
+ */
+void exit_tasks_rcu_finish(void)
+{
+	exit_tasks_rcu_stop();
+	exit_tasks_rcu_finish_trace(current);
+}
+
+#else /* #ifdef CONFIG_TASKS_RCU */
+void exit_tasks_rcu_start(void) { }
+void exit_tasks_rcu_stop(void) { }
+void exit_tasks_rcu_finish(void) { exit_tasks_rcu_finish_trace(current); }
+#endif /* #else #ifdef CONFIG_TASKS_RCU */
+
+#ifdef CONFIG_TASKS_RUDE_RCU
+
+////////////////////////////////////////////////////////////////////////
+//
+// "Rude" variant of Tasks RCU, inspired by Steve Rostedt's trick of
+// passing an empty function to schedule_on_each_cpu().  This approach
+// provides an asynchronous call_rcu_tasks_rude() API and batching of
+// concurrent calls to the synchronous synchronize_rcu_tasks_rude() API.
+// This invokes schedule_on_each_cpu() in order to send IPIs far and wide
+// and induces otherwise unnecessary context switches on all online CPUs,
+// whether idle or not.
+//
+// Callback handling is provided by the rcu_tasks_kthread() function.
+//
+// Ordering is provided by the scheduler's context-switch code.
+
+// Empty function to allow workqueues to force a context switch.
+static void rcu_tasks_be_rude(struct work_struct *work)
+{
+}
+
+// Wait for one rude RCU-tasks grace period.
+static void rcu_tasks_rude_wait_gp(struct rcu_tasks *rtp)
+{
+	rtp->n_ipis += cpumask_weight(cpu_online_mask);
+	schedule_on_each_cpu(rcu_tasks_be_rude);
+}
+
+void call_rcu_tasks_rude(struct rcu_head *rhp, rcu_callback_t func);
+DEFINE_RCU_TASKS(rcu_tasks_rude, rcu_tasks_rude_wait_gp, call_rcu_tasks_rude,
+		 "RCU Tasks Rude");
+
+/**
+ * call_rcu_tasks_rude() - Queue a callback rude task-based grace period
+ * @rhp: structure to be used for queueing the RCU updates.
+ * @func: actual callback function to be invoked after the grace period
+ *
+ * The callback function will be invoked some time after a full grace
+ * period elapses, in other words after all currently executing RCU
+ * read-side critical sections have completed. call_rcu_tasks_rude()
+ * assumes that the read-side critical sections end at context switch,
+ * cond_resched_tasks_rcu_qs(), or transition to usermode execution (as
+ * usermode execution is schedulable). As such, there are no read-side
+ * primitives analogous to rcu_read_lock() and rcu_read_unlock() because
+ * this primitive is intended to determine that all tasks have passed
+ * through a safe state, not so much for data-structure synchronization.
+ *
+ * See the description of call_rcu() for more detailed information on
+ * memory ordering guarantees.
+ */
+void call_rcu_tasks_rude(struct rcu_head *rhp, rcu_callback_t func)
+{
+	call_rcu_tasks_generic(rhp, func, &rcu_tasks_rude);
+}
+EXPORT_SYMBOL_GPL(call_rcu_tasks_rude);
+
+/**
+ * synchronize_rcu_tasks_rude - wait for a rude rcu-tasks grace period
+ *
+ * Control will return to the caller some time after a rude rcu-tasks
+ * grace period has elapsed, in other words after all currently
+ * executing rcu-tasks read-side critical sections have elapsed.  These
+ * read-side critical sections are delimited by calls to schedule(),
+ * cond_resched_tasks_rcu_qs(), userspace execution (which is a schedulable
+ * context), and (in theory, anyway) cond_resched().
+ *
+ * This is a very specialized primitive, intended only for a few uses in
+ * tracing and other situations requiring manipulation of function preambles
+ * and profiling hooks.  The synchronize_rcu_tasks_rude() function is not
+ * (yet) intended for heavy use from multiple CPUs.
+ *
+ * See the description of synchronize_rcu() for more detailed information
+ * on memory ordering guarantees.
+ */
+void synchronize_rcu_tasks_rude(void)
+{
+	synchronize_rcu_tasks_generic(&rcu_tasks_rude);
+}
+EXPORT_SYMBOL_GPL(synchronize_rcu_tasks_rude);
+
+/**
+ * rcu_barrier_tasks_rude - Wait for in-flight call_rcu_tasks_rude() callbacks.
+ *
+ * Although the current implementation is guaranteed to wait, it is not
+ * obligated to, for example, if there are no pending callbacks.
+ */
+void rcu_barrier_tasks_rude(void)
+{
+	rcu_barrier_tasks_generic(&rcu_tasks_rude);
+}
+EXPORT_SYMBOL_GPL(rcu_barrier_tasks_rude);
+
+int rcu_tasks_rude_lazy_ms = -1;
+module_param(rcu_tasks_rude_lazy_ms, int, 0444);
+
+static int __init rcu_spawn_tasks_rude_kthread(void)
+{
+	cblist_init_generic(&rcu_tasks_rude);
+	rcu_tasks_rude.gp_sleep = HZ / 10;
+	if (rcu_tasks_rude_lazy_ms >= 0)
+		rcu_tasks_rude.lazy_jiffies = msecs_to_jiffies(rcu_tasks_rude_lazy_ms);
+	rcu_spawn_tasks_kthread_generic(&rcu_tasks_rude);
+	return 0;
+}
+
+#if !defined(CONFIG_TINY_RCU)
+void show_rcu_tasks_rude_gp_kthread(void)
+{
+	show_rcu_tasks_generic_gp_kthread(&rcu_tasks_rude, "");
+}
+EXPORT_SYMBOL_GPL(show_rcu_tasks_rude_gp_kthread);
+#endif // !defined(CONFIG_TINY_RCU)
+
+struct task_struct *get_rcu_tasks_rude_gp_kthread(void)
+{
+	return rcu_tasks_rude.kthread_ptr;
+}
+EXPORT_SYMBOL_GPL(get_rcu_tasks_rude_gp_kthread);
+
+#endif /* #ifdef CONFIG_TASKS_RUDE_RCU */
+
+////////////////////////////////////////////////////////////////////////
+//
+// Tracing variant of Tasks RCU.  This variant is designed to be used
+// to protect tracing hooks, including those of BPF.  This variant
+// therefore:
+//
+// 1.	Has explicit read-side markers to allow finite grace periods
+//	in the face of in-kernel loops for PREEMPT=n builds.
+//
+// 2.	Protects code in the idle loop, exception entry/exit, and
+//	CPU-hotplug code paths, similar to the capabilities of SRCU.
+//
+// 3.	Avoids expensive read-side instructions, having overhead similar
+//	to that of Preemptible RCU.
+//
+// There are of course downsides.  For example, the grace-period code
+// can send IPIs to CPUs, even when those CPUs are in the idle loop or
+// in nohz_full userspace.  If needed, these downsides can be at least
+// partially remedied.
+//
+// Perhaps most important, this variant of RCU does not affect the vanilla
+// flavors, rcu_preempt and rcu_sched.  The fact that RCU Tasks Trace
+// readers can operate from idle, offline, and exception entry/exit in no
+// way allows rcu_preempt and rcu_sched readers to also do so.
+//
+// The implementation uses rcu_tasks_wait_gp(), which relies on function
+// pointers in the rcu_tasks structure.  The rcu_spawn_tasks_trace_kthread()
+// function sets these function pointers up so that rcu_tasks_wait_gp()
+// invokes these functions in this order:
+//
+// rcu_tasks_trace_pregp_step():
+//	Disables CPU hotplug, adds all currently executing tasks to the
+//	holdout list, then checks the state of all tasks that blocked
+//	or were preempted within their current RCU Tasks Trace read-side
+//	critical section, adding them to the holdout list if appropriate.
+//	Finally, this function re-enables CPU hotplug.
+// The ->pertask_func() pointer is NULL, so there is no per-task processing.
+// rcu_tasks_trace_postscan():
+//	Invokes synchronize_rcu() to wait for late-stage exiting tasks
+//	to finish exiting.
+// check_all_holdout_tasks_trace(), repeatedly until holdout list is empty:
+//	Scans the holdout list, attempting to identify a quiescent state
+//	for each task on the list.  If there is a quiescent state, the
+//	corresponding task is removed from the holdout list.  Once this
+//	list is empty, the grace period has completed.
+// rcu_tasks_trace_postgp():
+//	Provides the needed full memory barrier and does debug checks.
+//
+// The exit_tasks_rcu_finish_trace() synchronizes with exiting tasks.
+//
+// Pre-grace-period update-side code is ordered before the grace period
+// via the ->cbs_lock and barriers in rcu_tasks_kthread().  Pre-grace-period
+// read-side code is ordered before the grace period by atomic operations
+// on .b.need_qs flag of each task involved in this process, or by scheduler
+// context-switch ordering (for locked-down non-running readers).
+
+// The lockdep state must be outside of #ifdef to be useful.
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
+static struct lock_class_key rcu_lock_trace_key;
+struct lockdep_map rcu_trace_lock_map =
+	STATIC_LOCKDEP_MAP_INIT("rcu_read_lock_trace", &rcu_lock_trace_key);
+EXPORT_SYMBOL_GPL(rcu_trace_lock_map);
+#endif /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
+
+#ifdef CONFIG_TASKS_TRACE_RCU
+
+// Record outstanding IPIs to each CPU.  No point in sending two...
+static DEFINE_PER_CPU(bool, trc_ipi_to_cpu);
+
+// The number of detections of task quiescent state relying on
+// heavyweight readers executing explicit memory barriers.
+static unsigned long n_heavy_reader_attempts;
+static unsigned long n_heavy_reader_updates;
+static unsigned long n_heavy_reader_ofl_updates;
+static unsigned long n_trc_holdouts;
+
+void call_rcu_tasks_trace(struct rcu_head *rhp, rcu_callback_t func);
+DEFINE_RCU_TASKS(rcu_tasks_trace, rcu_tasks_wait_gp, call_rcu_tasks_trace,
+		 "RCU Tasks Trace");
+
+/* Load from ->trc_reader_special.b.need_qs with proper ordering. */
+static u8 rcu_ld_need_qs(struct task_struct *t)
+{
+	smp_mb(); // Enforce full grace-period ordering.
+	return smp_load_acquire(&t->trc_reader_special.b.need_qs);
+}
+
+/* Store to ->trc_reader_special.b.need_qs with proper ordering. */
+static void rcu_st_need_qs(struct task_struct *t, u8 v)
+{
+	smp_store_release(&t->trc_reader_special.b.need_qs, v);
+	smp_mb(); // Enforce full grace-period ordering.
+}
+
+/*
+ * Do a cmpxchg() on ->trc_reader_special.b.need_qs, allowing for
+ * the four-byte operand-size restriction of some platforms.
+ * Returns the old value, which is often ignored.
+ */
+u8 rcu_trc_cmpxchg_need_qs(struct task_struct *t, u8 old, u8 new)
+{
+	union rcu_special ret;
+	union rcu_special trs_old = READ_ONCE(t->trc_reader_special);
+	union rcu_special trs_new = trs_old;
+
+	if (trs_old.b.need_qs != old)
+		return trs_old.b.need_qs;
+	trs_new.b.need_qs = new;
+	ret.s = cmpxchg(&t->trc_reader_special.s, trs_old.s, trs_new.s);
+	return ret.b.need_qs;
+}
+EXPORT_SYMBOL_GPL(rcu_trc_cmpxchg_need_qs);
+
+/*
+ * If we are the last reader, signal the grace-period kthread.
+ * Also remove from the per-CPU list of blocked tasks.
+ */
+void rcu_read_unlock_trace_special(struct task_struct *t)
+{
+	unsigned long flags;
+	struct rcu_tasks_percpu *rtpcp;
+	union rcu_special trs;
+
+	// Open-coded full-word version of rcu_ld_need_qs().
+	smp_mb(); // Enforce full grace-period ordering.
+	trs = smp_load_acquire(&t->trc_reader_special);
+
+	if (IS_ENABLED(CONFIG_TASKS_TRACE_RCU_READ_MB) && t->trc_reader_special.b.need_mb)
+		smp_mb(); // Pairs with update-side barriers.
+	// Update .need_qs before ->trc_reader_nesting for irq/NMI handlers.
+	if (trs.b.need_qs == (TRC_NEED_QS_CHECKED | TRC_NEED_QS)) {
+		u8 result = rcu_trc_cmpxchg_need_qs(t, TRC_NEED_QS_CHECKED | TRC_NEED_QS,
+						       TRC_NEED_QS_CHECKED);
+
+		WARN_ONCE(result != trs.b.need_qs, "%s: result = %d", __func__, result);
+	}
+	if (trs.b.blocked) {
+		rtpcp = per_cpu_ptr(rcu_tasks_trace.rtpcpu, t->trc_blkd_cpu);
+		raw_spin_lock_irqsave_rcu_node(rtpcp, flags);
+		list_del_init(&t->trc_blkd_node);
+		WRITE_ONCE(t->trc_reader_special.b.blocked, false);
+		raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
+	}
+	WRITE_ONCE(t->trc_reader_nesting, 0);
+}
+EXPORT_SYMBOL_GPL(rcu_read_unlock_trace_special);
+
+/* Add a newly blocked reader task to its CPU's list. */
+void rcu_tasks_trace_qs_blkd(struct task_struct *t)
+{
+	unsigned long flags;
+	struct rcu_tasks_percpu *rtpcp;
+
+	local_irq_save(flags);
+	rtpcp = this_cpu_ptr(rcu_tasks_trace.rtpcpu);
+	raw_spin_lock_rcu_node(rtpcp); // irqs already disabled
+	t->trc_blkd_cpu = smp_processor_id();
+	if (!rtpcp->rtp_blkd_tasks.next)
+		INIT_LIST_HEAD(&rtpcp->rtp_blkd_tasks);
+	list_add(&t->trc_blkd_node, &rtpcp->rtp_blkd_tasks);
+	WRITE_ONCE(t->trc_reader_special.b.blocked, true);
+	raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
+}
+EXPORT_SYMBOL_GPL(rcu_tasks_trace_qs_blkd);
+
+/* Add a task to the holdout list, if it is not already on the list. */
+static void trc_add_holdout(struct task_struct *t, struct list_head *bhp)
+{
+	if (list_empty(&t->trc_holdout_list)) {
+		get_task_struct(t);
+		list_add(&t->trc_holdout_list, bhp);
+		n_trc_holdouts++;
+	}
+}
+
+/* Remove a task from the holdout list, if it is in fact present. */
+static void trc_del_holdout(struct task_struct *t)
+{
+	if (!list_empty(&t->trc_holdout_list)) {
+		list_del_init(&t->trc_holdout_list);
+		put_task_struct(t);
+		n_trc_holdouts--;
+	}
+}
+
+/* IPI handler to check task state. */
+static void trc_read_check_handler(void *t_in)
+{
+	int nesting;
+	struct task_struct *t = current;
+	struct task_struct *texp = t_in;
+
+	// If the task is no longer running on this CPU, leave.
+	if (unlikely(texp != t))
+		goto reset_ipi; // Already on holdout list, so will check later.
+
+	// If the task is not in a read-side critical section, and
+	// if this is the last reader, awaken the grace-period kthread.
+	nesting = READ_ONCE(t->trc_reader_nesting);
+	if (likely(!nesting)) {
+		rcu_trc_cmpxchg_need_qs(t, 0, TRC_NEED_QS_CHECKED);
+		goto reset_ipi;
+	}
+	// If we are racing with an rcu_read_unlock_trace(), try again later.
+	if (unlikely(nesting < 0))
+		goto reset_ipi;
+
+	// Get here if the task is in a read-side critical section.
+	// Set its state so that it will update state for the grace-period
+	// kthread upon exit from that critical section.
+	rcu_trc_cmpxchg_need_qs(t, 0, TRC_NEED_QS | TRC_NEED_QS_CHECKED);
+
+reset_ipi:
+	// Allow future IPIs to be sent on CPU and for task.
+	// Also order this IPI handler against any later manipulations of
+	// the intended task.
+	smp_store_release(per_cpu_ptr(&trc_ipi_to_cpu, smp_processor_id()), false); // ^^^
+	smp_store_release(&texp->trc_ipi_to_cpu, -1); // ^^^
+}
+
+/* Callback function for scheduler to check locked-down task.  */
+static int trc_inspect_reader(struct task_struct *t, void *bhp_in)
+{
+	struct list_head *bhp = bhp_in;
+	int cpu = task_cpu(t);
+	int nesting;
+	bool ofl = cpu_is_offline(cpu);
+
+	if (task_curr(t) && !ofl) {
+		// If no chance of heavyweight readers, do it the hard way.
+		if (!IS_ENABLED(CONFIG_TASKS_TRACE_RCU_READ_MB))
+			return -EINVAL;
+
+		// If heavyweight readers are enabled on the remote task,
+		// we can inspect its state despite its currently running.
+		// However, we cannot safely change its state.
+		n_heavy_reader_attempts++;
+		// Check for "running" idle tasks on offline CPUs.
+		if (!rcu_dynticks_zero_in_eqs(cpu, &t->trc_reader_nesting))
+			return -EINVAL; // No quiescent state, do it the hard way.
+		n_heavy_reader_updates++;
+		nesting = 0;
+	} else {
+		// The task is not running, so C-language access is safe.
+		nesting = t->trc_reader_nesting;
+		WARN_ON_ONCE(ofl && task_curr(t) && (t != idle_task(task_cpu(t))));
+		if (IS_ENABLED(CONFIG_TASKS_TRACE_RCU_READ_MB) && ofl)
+			n_heavy_reader_ofl_updates++;
+	}
+
+	// If not exiting a read-side critical section, mark as checked
+	// so that the grace-period kthread will remove it from the
+	// holdout list.
+	if (!nesting) {
+		rcu_trc_cmpxchg_need_qs(t, 0, TRC_NEED_QS_CHECKED);
+		return 0;  // In QS, so done.
+	}
+	if (nesting < 0)
+		return -EINVAL; // Reader transitioning, try again later.
+
+	// The task is in a read-side critical section, so set up its
+	// state so that it will update state upon exit from that critical
+	// section.
+	if (!rcu_trc_cmpxchg_need_qs(t, 0, TRC_NEED_QS | TRC_NEED_QS_CHECKED))
+		trc_add_holdout(t, bhp);
+	return 0;
+}
+
+/* Attempt to extract the state for the specified task. */
+static void trc_wait_for_one_reader(struct task_struct *t,
+				    struct list_head *bhp)
+{
+	int cpu;
+
+	// If a previous IPI is still in flight, let it complete.
+	if (smp_load_acquire(&t->trc_ipi_to_cpu) != -1) // Order IPI
+		return;
+
+	// The current task had better be in a quiescent state.
+	if (t == current) {
+		rcu_trc_cmpxchg_need_qs(t, 0, TRC_NEED_QS_CHECKED);
+		WARN_ON_ONCE(READ_ONCE(t->trc_reader_nesting));
+		return;
+	}
+
+	// Attempt to nail down the task for inspection.
+	get_task_struct(t);
+	if (!task_call_func(t, trc_inspect_reader, bhp)) {
+		put_task_struct(t);
+		return;
+	}
+	put_task_struct(t);
+
+	// If this task is not yet on the holdout list, then we are in
+	// an RCU read-side critical section.  Otherwise, the invocation of
+	// trc_add_holdout() that added it to the list did the necessary
+	// get_task_struct().  Either way, the task cannot be freed out
+	// from under this code.
+
+	// If currently running, send an IPI, either way, add to list.
+	trc_add_holdout(t, bhp);
+	if (task_curr(t) &&
+	    time_after(jiffies + 1, rcu_tasks_trace.gp_start + rcu_task_ipi_delay)) {
+		// The task is currently running, so try IPIing it.
+		cpu = task_cpu(t);
+
+		// If there is already an IPI outstanding, let it happen.
+		if (per_cpu(trc_ipi_to_cpu, cpu) || t->trc_ipi_to_cpu >= 0)
+			return;
+
+		per_cpu(trc_ipi_to_cpu, cpu) = true;
+		t->trc_ipi_to_cpu = cpu;
+		rcu_tasks_trace.n_ipis++;
+		if (smp_call_function_single(cpu, trc_read_check_handler, t, 0)) {
+			// Just in case there is some other reason for
+			// failure than the target CPU being offline.
+			WARN_ONCE(1, "%s():  smp_call_function_single() failed for CPU: %d\n",
+				  __func__, cpu);
+			rcu_tasks_trace.n_ipis_fails++;
+			per_cpu(trc_ipi_to_cpu, cpu) = false;
+			t->trc_ipi_to_cpu = -1;
+		}
+	}
+}
+
+/*
+ * Initialize for first-round processing for the specified task.
+ * Return false if task is NULL or already taken care of, true otherwise.
+ */
+static bool rcu_tasks_trace_pertask_prep(struct task_struct *t, bool notself)
+{
+	// During early boot when there is only the one boot CPU, there
+	// is no idle task for the other CPUs.	Also, the grace-period
+	// kthread is always in a quiescent state.  In addition, just return
+	// if this task is already on the list.
+	if (unlikely(t == NULL) || (t == current && notself) || !list_empty(&t->trc_holdout_list))
+		return false;
+
+	rcu_st_need_qs(t, 0);
+	t->trc_ipi_to_cpu = -1;
+	return true;
+}
+
+/* Do first-round processing for the specified task. */
+static void rcu_tasks_trace_pertask(struct task_struct *t, struct list_head *hop)
+{
+	if (rcu_tasks_trace_pertask_prep(t, true))
+		trc_wait_for_one_reader(t, hop);
+}
+
+/* Initialize for a new RCU-tasks-trace grace period. */
+static void rcu_tasks_trace_pregp_step(struct list_head *hop)
+{
+	LIST_HEAD(blkd_tasks);
+	int cpu;
+	unsigned long flags;
+	struct rcu_tasks_percpu *rtpcp;
+	struct task_struct *t;
+
+	// There shouldn't be any old IPIs, but...
+	for_each_possible_cpu(cpu)
+		WARN_ON_ONCE(per_cpu(trc_ipi_to_cpu, cpu));
+
+	// Disable CPU hotplug across the CPU scan for the benefit of
+	// any IPIs that might be needed.  This also waits for all readers
+	// in CPU-hotplug code paths.
+	cpus_read_lock();
+
+	// These rcu_tasks_trace_pertask_prep() calls are serialized to
+	// allow safe access to the hop list.
+	for_each_online_cpu(cpu) {
+		rcu_read_lock();
+		t = cpu_curr_snapshot(cpu);
+		if (rcu_tasks_trace_pertask_prep(t, true))
+			trc_add_holdout(t, hop);
+		rcu_read_unlock();
+		cond_resched_tasks_rcu_qs();
+	}
+
+	// Only after all running tasks have been accounted for is it
+	// safe to take care of the tasks that have blocked within their
+	// current RCU tasks trace read-side critical section.
+	for_each_possible_cpu(cpu) {
+		rtpcp = per_cpu_ptr(rcu_tasks_trace.rtpcpu, cpu);
+		raw_spin_lock_irqsave_rcu_node(rtpcp, flags);
+		list_splice_init(&rtpcp->rtp_blkd_tasks, &blkd_tasks);
+		while (!list_empty(&blkd_tasks)) {
+			rcu_read_lock();
+			t = list_first_entry(&blkd_tasks, struct task_struct, trc_blkd_node);
+			list_del_init(&t->trc_blkd_node);
+			list_add(&t->trc_blkd_node, &rtpcp->rtp_blkd_tasks);
+			raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
+			rcu_tasks_trace_pertask(t, hop);
+			rcu_read_unlock();
+			raw_spin_lock_irqsave_rcu_node(rtpcp, flags);
+		}
+		raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
+		cond_resched_tasks_rcu_qs();
+	}
+
+	// Re-enable CPU hotplug now that the holdout list is populated.
+	cpus_read_unlock();
+}
+
+/*
+ * Do intermediate processing between task and holdout scans.
+ */
+static void rcu_tasks_trace_postscan(struct list_head *hop)
+{
+	// Wait for late-stage exiting tasks to finish exiting.
+	// These might have passed the call to exit_tasks_rcu_finish().
+
+	// If you remove the following line, update rcu_trace_implies_rcu_gp()!!!
+	synchronize_rcu();
+	// Any tasks that exit after this point will set
+	// TRC_NEED_QS_CHECKED in ->trc_reader_special.b.need_qs.
+}
+
+/* Communicate task state back to the RCU tasks trace stall warning request. */
+struct trc_stall_chk_rdr {
+	int nesting;
+	int ipi_to_cpu;
+	u8 needqs;
+};
+
+static int trc_check_slow_task(struct task_struct *t, void *arg)
+{
+	struct trc_stall_chk_rdr *trc_rdrp = arg;
+
+	if (task_curr(t) && cpu_online(task_cpu(t)))
+		return false; // It is running, so decline to inspect it.
+	trc_rdrp->nesting = READ_ONCE(t->trc_reader_nesting);
+	trc_rdrp->ipi_to_cpu = READ_ONCE(t->trc_ipi_to_cpu);
+	trc_rdrp->needqs = rcu_ld_need_qs(t);
+	return true;
+}
+
+/* Show the state of a task stalling the current RCU tasks trace GP. */
+static void show_stalled_task_trace(struct task_struct *t, bool *firstreport)
+{
+	int cpu;
+	struct trc_stall_chk_rdr trc_rdr;
+	bool is_idle_tsk = is_idle_task(t);
+
+	if (*firstreport) {
+		pr_err("INFO: rcu_tasks_trace detected stalls on tasks:\n");
+		*firstreport = false;
+	}
+	cpu = task_cpu(t);
+	if (!task_call_func(t, trc_check_slow_task, &trc_rdr))
+		pr_alert("P%d: %c%c\n",
+			 t->pid,
+			 ".I"[t->trc_ipi_to_cpu >= 0],
+			 ".i"[is_idle_tsk]);
+	else
+		pr_alert("P%d: %c%c%c%c nesting: %d%c%c cpu: %d%s\n",
+			 t->pid,
+			 ".I"[trc_rdr.ipi_to_cpu >= 0],
+			 ".i"[is_idle_tsk],
+			 ".N"[cpu >= 0 && tick_nohz_full_cpu(cpu)],
+			 ".B"[!!data_race(t->trc_reader_special.b.blocked)],
+			 trc_rdr.nesting,
+			 " !CN"[trc_rdr.needqs & 0x3],
+			 " ?"[trc_rdr.needqs > 0x3],
+			 cpu, cpu_online(cpu) ? "" : "(offline)");
+	sched_show_task(t);
+}
+
+/* List stalled IPIs for RCU tasks trace. */
+static void show_stalled_ipi_trace(void)
+{
+	int cpu;
+
+	for_each_possible_cpu(cpu)
+		if (per_cpu(trc_ipi_to_cpu, cpu))
+			pr_alert("\tIPI outstanding to CPU %d\n", cpu);
+}
+
+/* Do one scan of the holdout list. */
+static void check_all_holdout_tasks_trace(struct list_head *hop,
+					  bool needreport, bool *firstreport)
+{
+	struct task_struct *g, *t;
+
+	// Disable CPU hotplug across the holdout list scan for IPIs.
+	cpus_read_lock();
+
+	list_for_each_entry_safe(t, g, hop, trc_holdout_list) {
+		// If safe and needed, try to check the current task.
+		if (READ_ONCE(t->trc_ipi_to_cpu) == -1 &&
+		    !(rcu_ld_need_qs(t) & TRC_NEED_QS_CHECKED))
+			trc_wait_for_one_reader(t, hop);
+
+		// If check succeeded, remove this task from the list.
+		if (smp_load_acquire(&t->trc_ipi_to_cpu) == -1 &&
+		    rcu_ld_need_qs(t) == TRC_NEED_QS_CHECKED)
+			trc_del_holdout(t);
+		else if (needreport)
+			show_stalled_task_trace(t, firstreport);
+		cond_resched_tasks_rcu_qs();
+	}
+
+	// Re-enable CPU hotplug now that the holdout list scan has completed.
+	cpus_read_unlock();
+
+	if (needreport) {
+		if (*firstreport)
+			pr_err("INFO: rcu_tasks_trace detected stalls? (Late IPI?)\n");
+		show_stalled_ipi_trace();
+	}
+}
+
+static void rcu_tasks_trace_empty_fn(void *unused)
+{
+}
+
+/* Wait for grace period to complete and provide ordering. */
+static void rcu_tasks_trace_postgp(struct rcu_tasks *rtp)
+{
+	int cpu;
+
+	// Wait for any lingering IPI handlers to complete.  Note that
+	// if a CPU has gone offline or transitioned to userspace in the
+	// meantime, all IPI handlers should have been drained beforehand.
+	// Yes, this assumes that CPUs process IPIs in order.  If that ever
+	// changes, there will need to be a recheck and/or timed wait.
+	for_each_online_cpu(cpu)
+		if (WARN_ON_ONCE(smp_load_acquire(per_cpu_ptr(&trc_ipi_to_cpu, cpu))))
+			smp_call_function_single(cpu, rcu_tasks_trace_empty_fn, NULL, 1);
+
+	smp_mb(); // Caller's code must be ordered after wakeup.
+		  // Pairs with pretty much every ordering primitive.
+}
+
+/* Report any needed quiescent state for this exiting task. */
+static void exit_tasks_rcu_finish_trace(struct task_struct *t)
+{
+	union rcu_special trs = READ_ONCE(t->trc_reader_special);
+
+	rcu_trc_cmpxchg_need_qs(t, 0, TRC_NEED_QS_CHECKED);
+	WARN_ON_ONCE(READ_ONCE(t->trc_reader_nesting));
+	if (WARN_ON_ONCE(rcu_ld_need_qs(t) & TRC_NEED_QS || trs.b.blocked))
+		rcu_read_unlock_trace_special(t);
+	else
+		WRITE_ONCE(t->trc_reader_nesting, 0);
+}
+
+/**
+ * call_rcu_tasks_trace() - Queue a callback trace task-based grace period
+ * @rhp: structure to be used for queueing the RCU updates.
+ * @func: actual callback function to be invoked after the grace period
+ *
+ * The callback function will be invoked some time after a trace rcu-tasks
+ * grace period elapses, in other words after all currently executing
+ * trace rcu-tasks read-side critical sections have completed. These
+ * read-side critical sections are delimited by calls to rcu_read_lock_trace()
+ * and rcu_read_unlock_trace().
+ *
+ * See the description of call_rcu() for more detailed information on
+ * memory ordering guarantees.
+ */
+void call_rcu_tasks_trace(struct rcu_head *rhp, rcu_callback_t func)
+{
+	call_rcu_tasks_generic(rhp, func, &rcu_tasks_trace);
+}
+EXPORT_SYMBOL_GPL(call_rcu_tasks_trace);
+
+/**
+ * synchronize_rcu_tasks_trace - wait for a trace rcu-tasks grace period
+ *
+ * Control will return to the caller some time after a trace rcu-tasks
+ * grace period has elapsed, in other words after all currently executing
+ * trace rcu-tasks read-side critical sections have elapsed. These read-side
+ * critical sections are delimited by calls to rcu_read_lock_trace()
+ * and rcu_read_unlock_trace().
+ *
+ * This is a very specialized primitive, intended only for a few uses in
+ * tracing and other situations requiring manipulation of function preambles
+ * and profiling hooks.  The synchronize_rcu_tasks_trace() function is not
+ * (yet) intended for heavy use from multiple CPUs.
+ *
+ * See the description of synchronize_rcu() for more detailed information
+ * on memory ordering guarantees.
+ */
+void synchronize_rcu_tasks_trace(void)
+{
+	RCU_LOCKDEP_WARN(lock_is_held(&rcu_trace_lock_map), "Illegal synchronize_rcu_tasks_trace() in RCU Tasks Trace read-side critical section");
+	synchronize_rcu_tasks_generic(&rcu_tasks_trace);
+}
+EXPORT_SYMBOL_GPL(synchronize_rcu_tasks_trace);
+
+/**
+ * rcu_barrier_tasks_trace - Wait for in-flight call_rcu_tasks_trace() callbacks.
+ *
+ * Although the current implementation is guaranteed to wait, it is not
+ * obligated to, for example, if there are no pending callbacks.
+ */
+void rcu_barrier_tasks_trace(void)
+{
+	rcu_barrier_tasks_generic(&rcu_tasks_trace);
+}
+EXPORT_SYMBOL_GPL(rcu_barrier_tasks_trace);
+
+int rcu_tasks_trace_lazy_ms = -1;
+module_param(rcu_tasks_trace_lazy_ms, int, 0444);
+
+static int __init rcu_spawn_tasks_trace_kthread(void)
+{
+	cblist_init_generic(&rcu_tasks_trace);
+	if (IS_ENABLED(CONFIG_TASKS_TRACE_RCU_READ_MB)) {
+		rcu_tasks_trace.gp_sleep = HZ / 10;
+		rcu_tasks_trace.init_fract = HZ / 10;
+	} else {
+		rcu_tasks_trace.gp_sleep = HZ / 200;
+		if (rcu_tasks_trace.gp_sleep <= 0)
+			rcu_tasks_trace.gp_sleep = 1;
+		rcu_tasks_trace.init_fract = HZ / 200;
+		if (rcu_tasks_trace.init_fract <= 0)
+			rcu_tasks_trace.init_fract = 1;
+	}
+	if (rcu_tasks_trace_lazy_ms >= 0)
+		rcu_tasks_trace.lazy_jiffies = msecs_to_jiffies(rcu_tasks_trace_lazy_ms);
+	rcu_tasks_trace.pregp_func = rcu_tasks_trace_pregp_step;
+	rcu_tasks_trace.postscan_func = rcu_tasks_trace_postscan;
+	rcu_tasks_trace.holdouts_func = check_all_holdout_tasks_trace;
+	rcu_tasks_trace.postgp_func = rcu_tasks_trace_postgp;
+	rcu_spawn_tasks_kthread_generic(&rcu_tasks_trace);
+	return 0;
+}
+
+#if !defined(CONFIG_TINY_RCU)
+void show_rcu_tasks_trace_gp_kthread(void)
+{
+	char buf[64];
+
+	sprintf(buf, "N%lu h:%lu/%lu/%lu",
+		data_race(n_trc_holdouts),
+		data_race(n_heavy_reader_ofl_updates),
+		data_race(n_heavy_reader_updates),
+		data_race(n_heavy_reader_attempts));
+	show_rcu_tasks_generic_gp_kthread(&rcu_tasks_trace, buf);
+}
+EXPORT_SYMBOL_GPL(show_rcu_tasks_trace_gp_kthread);
+#endif // !defined(CONFIG_TINY_RCU)
+
+struct task_struct *get_rcu_tasks_trace_gp_kthread(void)
+{
+	return rcu_tasks_trace.kthread_ptr;
+}
+EXPORT_SYMBOL_GPL(get_rcu_tasks_trace_gp_kthread);
+
+#else /* #ifdef CONFIG_TASKS_TRACE_RCU */
+static void exit_tasks_rcu_finish_trace(struct task_struct *t) { }
+#endif /* #else #ifdef CONFIG_TASKS_TRACE_RCU */
+
+#ifndef CONFIG_TINY_RCU
+void show_rcu_tasks_gp_kthreads(void)
+{
+	show_rcu_tasks_classic_gp_kthread();
+	show_rcu_tasks_rude_gp_kthread();
+	show_rcu_tasks_trace_gp_kthread();
+}
+#endif /* #ifndef CONFIG_TINY_RCU */
+
+#ifdef CONFIG_PROVE_RCU
+struct rcu_tasks_test_desc {
+	struct rcu_head rh;
+	const char *name;
+	bool notrun;
+	unsigned long runstart;
+};
+
+static struct rcu_tasks_test_desc tests[] = {
+	{
+		.name = "call_rcu_tasks()",
+		/* If not defined, the test is skipped. */
+		.notrun = IS_ENABLED(CONFIG_TASKS_RCU),
+	},
+	{
+		.name = "call_rcu_tasks_rude()",
+		/* If not defined, the test is skipped. */
+		.notrun = IS_ENABLED(CONFIG_TASKS_RUDE_RCU),
+	},
+	{
+		.name = "call_rcu_tasks_trace()",
+		/* If not defined, the test is skipped. */
+		.notrun = IS_ENABLED(CONFIG_TASKS_TRACE_RCU)
+	}
+};
+
+static void test_rcu_tasks_callback(struct rcu_head *rhp)
+{
+	struct rcu_tasks_test_desc *rttd =
+		container_of(rhp, struct rcu_tasks_test_desc, rh);
+
+	pr_info("Callback from %s invoked.\n", rttd->name);
+
+	rttd->notrun = false;
+}
+
+static void rcu_tasks_initiate_self_tests(void)
+{
+	pr_info("Running RCU-tasks wait API self tests\n");
+#ifdef CONFIG_TASKS_RCU
+	tests[0].runstart = jiffies;
+	synchronize_rcu_tasks();
+	call_rcu_tasks(&tests[0].rh, test_rcu_tasks_callback);
+#endif
+
+#ifdef CONFIG_TASKS_RUDE_RCU
+	tests[1].runstart = jiffies;
+	synchronize_rcu_tasks_rude();
+	call_rcu_tasks_rude(&tests[1].rh, test_rcu_tasks_callback);
+#endif
+
+#ifdef CONFIG_TASKS_TRACE_RCU
+	tests[2].runstart = jiffies;
+	synchronize_rcu_tasks_trace();
+	call_rcu_tasks_trace(&tests[2].rh, test_rcu_tasks_callback);
+#endif
+}
+
+/*
+ * Return:  0 - test passed
+ *	    1 - test failed, but have not timed out yet
+ *	   -1 - test failed and timed out
+ */
+static int rcu_tasks_verify_self_tests(void)
+{
+	int ret = 0;
+	int i;
+	unsigned long bst = rcu_task_stall_timeout;
+
+	if (bst <= 0 || bst > RCU_TASK_BOOT_STALL_TIMEOUT)
+		bst = RCU_TASK_BOOT_STALL_TIMEOUT;
+	for (i = 0; i < ARRAY_SIZE(tests); i++) {
+		while (tests[i].notrun) {		// still hanging.
+			if (time_after(jiffies, tests[i].runstart + bst)) {
+				pr_err("%s has failed boot-time tests.\n", tests[i].name);
+				ret = -1;
+				break;
+			}
+			ret = 1;
+			break;
+		}
+	}
+	WARN_ON(ret < 0);
+
+	return ret;
+}
+
+/*
+ * Repeat the rcu_tasks_verify_self_tests() call once every second until the
+ * test passes or has timed out.
+ */
+static struct delayed_work rcu_tasks_verify_work;
+static void rcu_tasks_verify_work_fn(struct work_struct *work __maybe_unused)
+{
+	int ret = rcu_tasks_verify_self_tests();
+
+	if (ret <= 0)
+		return;
+
+	/* Test fails but not timed out yet, reschedule another check */
+	schedule_delayed_work(&rcu_tasks_verify_work, HZ);
+}
+
+static int rcu_tasks_verify_schedule_work(void)
+{
+	INIT_DELAYED_WORK(&rcu_tasks_verify_work, rcu_tasks_verify_work_fn);
+	rcu_tasks_verify_work_fn(NULL);
+	return 0;
+}
+late_initcall(rcu_tasks_verify_schedule_work);
+#else /* #ifdef CONFIG_PROVE_RCU */
+static void rcu_tasks_initiate_self_tests(void) { }
+#endif /* #else #ifdef CONFIG_PROVE_RCU */
+
+void __init rcu_init_tasks_generic(void)
+{
+#ifdef CONFIG_TASKS_RCU
+	rcu_spawn_tasks_kthread();
+#endif
+
+#ifdef CONFIG_TASKS_RUDE_RCU
+	rcu_spawn_tasks_rude_kthread();
+#endif
+
+#ifdef CONFIG_TASKS_TRACE_RCU
+	rcu_spawn_tasks_trace_kthread();
+#endif
+
+	// Run the self-tests.
+	rcu_tasks_initiate_self_tests();
+}
+
+#else /* #ifdef CONFIG_TASKS_RCU_GENERIC */
+static inline void rcu_tasks_bootup_oddness(void) {}
+#endif /* #else #ifdef CONFIG_TASKS_RCU_GENERIC */