Adding upstream version 5.10.209.upstream/5.10.209upstream

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

1 files changed, 1451 insertions, 0 deletions

diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
new file mode 100644
index 000000000..b8821665c
--- /dev/null
+++ b/kernel/rcu/srcutree.c
@@ -0,0 +1,1451 @@
+// SPDX-License-Identifier: GPL-2.0+
+/*
+ * Sleepable Read-Copy Update mechanism for mutual exclusion.
+ *
+ * Copyright (C) IBM Corporation, 2006
+ * Copyright (C) Fujitsu, 2012
+ *
+ * Authors: Paul McKenney <paulmck@linux.ibm.com>
+ *	   Lai Jiangshan <laijs@cn.fujitsu.com>
+ *
+ * For detailed explanation of Read-Copy Update mechanism see -
+ *		Documentation/RCU/ *.txt
+ *
+ */
+
+#define pr_fmt(fmt) "rcu: " fmt
+
+#include <linux/export.h>
+#include <linux/mutex.h>
+#include <linux/percpu.h>
+#include <linux/preempt.h>
+#include <linux/rcupdate_wait.h>
+#include <linux/sched.h>
+#include <linux/smp.h>
+#include <linux/delay.h>
+#include <linux/module.h>
+#include <linux/srcu.h>
+
+#include "rcu.h"
+#include "rcu_segcblist.h"
+
+/* Holdoff in nanoseconds for auto-expediting. */
+#define DEFAULT_SRCU_EXP_HOLDOFF (25 * 1000)
+static ulong exp_holdoff = DEFAULT_SRCU_EXP_HOLDOFF;
+module_param(exp_holdoff, ulong, 0444);
+
+/* Overflow-check frequency.  N bits roughly says every 2**N grace periods. */
+static ulong counter_wrap_check = (ULONG_MAX >> 2);
+module_param(counter_wrap_check, ulong, 0444);
+
+/* Early-boot callback-management, so early that no lock is required! */
+static LIST_HEAD(srcu_boot_list);
+static bool __read_mostly srcu_init_done;
+
+static void srcu_invoke_callbacks(struct work_struct *work);
+static void srcu_reschedule(struct srcu_struct *ssp, unsigned long delay);
+static void process_srcu(struct work_struct *work);
+static void srcu_delay_timer(struct timer_list *t);
+
+/* Wrappers for lock acquisition and release, see raw_spin_lock_rcu_node(). */
+#define spin_lock_rcu_node(p)					\
+do {									\
+	spin_lock(&ACCESS_PRIVATE(p, lock));			\
+	smp_mb__after_unlock_lock();					\
+} while (0)
+
+#define spin_unlock_rcu_node(p) spin_unlock(&ACCESS_PRIVATE(p, lock))
+
+#define spin_lock_irq_rcu_node(p)					\
+do {									\
+	spin_lock_irq(&ACCESS_PRIVATE(p, lock));			\
+	smp_mb__after_unlock_lock();					\
+} while (0)
+
+#define spin_unlock_irq_rcu_node(p)					\
+	spin_unlock_irq(&ACCESS_PRIVATE(p, lock))
+
+#define spin_lock_irqsave_rcu_node(p, flags)			\
+do {									\
+	spin_lock_irqsave(&ACCESS_PRIVATE(p, lock), flags);	\
+	smp_mb__after_unlock_lock();					\
+} while (0)
+
+#define spin_unlock_irqrestore_rcu_node(p, flags)			\
+	spin_unlock_irqrestore(&ACCESS_PRIVATE(p, lock), flags)	\
+
+/*
+ * Initialize SRCU combining tree.  Note that statically allocated
+ * srcu_struct structures might already have srcu_read_lock() and
+ * srcu_read_unlock() running against them.  So if the is_static parameter
+ * is set, don't initialize ->srcu_lock_count[] and ->srcu_unlock_count[].
+ */
+static void init_srcu_struct_nodes(struct srcu_struct *ssp, bool is_static)
+{
+	int cpu;
+	int i;
+	int level = 0;
+	int levelspread[RCU_NUM_LVLS];
+	struct srcu_data *sdp;
+	struct srcu_node *snp;
+	struct srcu_node *snp_first;
+
+	/* Initialize geometry if it has not already been initialized. */
+	rcu_init_geometry();
+
+	/* Work out the overall tree geometry. */
+	ssp->level[0] = &ssp->node[0];
+	for (i = 1; i < rcu_num_lvls; i++)
+		ssp->level[i] = ssp->level[i - 1] + num_rcu_lvl[i - 1];
+	rcu_init_levelspread(levelspread, num_rcu_lvl);
+
+	/* Each pass through this loop initializes one srcu_node structure. */
+	srcu_for_each_node_breadth_first(ssp, snp) {
+		spin_lock_init(&ACCESS_PRIVATE(snp, lock));
+		WARN_ON_ONCE(ARRAY_SIZE(snp->srcu_have_cbs) !=
+			     ARRAY_SIZE(snp->srcu_data_have_cbs));
+		for (i = 0; i < ARRAY_SIZE(snp->srcu_have_cbs); i++) {
+			snp->srcu_have_cbs[i] = 0;
+			snp->srcu_data_have_cbs[i] = 0;
+		}
+		snp->srcu_gp_seq_needed_exp = 0;
+		snp->grplo = -1;
+		snp->grphi = -1;
+		if (snp == &ssp->node[0]) {
+			/* Root node, special case. */
+			snp->srcu_parent = NULL;
+			continue;
+		}
+
+		/* Non-root node. */
+		if (snp == ssp->level[level + 1])
+			level++;
+		snp->srcu_parent = ssp->level[level - 1] +
+				   (snp - ssp->level[level]) /
+				   levelspread[level - 1];
+	}
+
+	/*
+	 * Initialize the per-CPU srcu_data array, which feeds into the
+	 * leaves of the srcu_node tree.
+	 */
+	WARN_ON_ONCE(ARRAY_SIZE(sdp->srcu_lock_count) !=
+		     ARRAY_SIZE(sdp->srcu_unlock_count));
+	level = rcu_num_lvls - 1;
+	snp_first = ssp->level[level];
+	for_each_possible_cpu(cpu) {
+		sdp = per_cpu_ptr(ssp->sda, cpu);
+		spin_lock_init(&ACCESS_PRIVATE(sdp, lock));
+		rcu_segcblist_init(&sdp->srcu_cblist);
+		sdp->srcu_cblist_invoking = false;
+		sdp->srcu_gp_seq_needed = ssp->srcu_gp_seq;
+		sdp->srcu_gp_seq_needed_exp = ssp->srcu_gp_seq;
+		sdp->mynode = &snp_first[cpu / levelspread[level]];
+		for (snp = sdp->mynode; snp != NULL; snp = snp->srcu_parent) {
+			if (snp->grplo < 0)
+				snp->grplo = cpu;
+			snp->grphi = cpu;
+		}
+		sdp->cpu = cpu;
+		INIT_WORK(&sdp->work, srcu_invoke_callbacks);
+		timer_setup(&sdp->delay_work, srcu_delay_timer, 0);
+		sdp->ssp = ssp;
+		sdp->grpmask = 1 << (cpu - sdp->mynode->grplo);
+		if (is_static)
+			continue;
+
+		/* Dynamically allocated, better be no srcu_read_locks()! */
+		for (i = 0; i < ARRAY_SIZE(sdp->srcu_lock_count); i++) {
+			sdp->srcu_lock_count[i] = 0;
+			sdp->srcu_unlock_count[i] = 0;
+		}
+	}
+}
+
+/*
+ * Initialize non-compile-time initialized fields, including the
+ * associated srcu_node and srcu_data structures.  The is_static
+ * parameter is passed through to init_srcu_struct_nodes(), and
+ * also tells us that ->sda has already been wired up to srcu_data.
+ */
+static int init_srcu_struct_fields(struct srcu_struct *ssp, bool is_static)
+{
+	mutex_init(&ssp->srcu_cb_mutex);
+	mutex_init(&ssp->srcu_gp_mutex);
+	ssp->srcu_idx = 0;
+	ssp->srcu_gp_seq = 0;
+	ssp->srcu_barrier_seq = 0;
+	mutex_init(&ssp->srcu_barrier_mutex);
+	atomic_set(&ssp->srcu_barrier_cpu_cnt, 0);
+	INIT_DELAYED_WORK(&ssp->work, process_srcu);
+	if (!is_static)
+		ssp->sda = alloc_percpu(struct srcu_data);
+	init_srcu_struct_nodes(ssp, is_static);
+	ssp->srcu_gp_seq_needed_exp = 0;
+	ssp->srcu_last_gp_end = ktime_get_mono_fast_ns();
+	smp_store_release(&ssp->srcu_gp_seq_needed, 0); /* Init done. */
+	return ssp->sda ? 0 : -ENOMEM;
+}
+
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
+
+int __init_srcu_struct(struct srcu_struct *ssp, const char *name,
+		       struct lock_class_key *key)
+{
+	/* Don't re-initialize a lock while it is held. */
+	debug_check_no_locks_freed((void *)ssp, sizeof(*ssp));
+	lockdep_init_map(&ssp->dep_map, name, key, 0);
+	spin_lock_init(&ACCESS_PRIVATE(ssp, lock));
+	return init_srcu_struct_fields(ssp, false);
+}
+EXPORT_SYMBOL_GPL(__init_srcu_struct);
+
+#else /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
+
+/**
+ * init_srcu_struct - initialize a sleep-RCU structure
+ * @ssp: structure to initialize.
+ *
+ * Must invoke this on a given srcu_struct before passing that srcu_struct
+ * to any other function.  Each srcu_struct represents a separate domain
+ * of SRCU protection.
+ */
+int init_srcu_struct(struct srcu_struct *ssp)
+{
+	spin_lock_init(&ACCESS_PRIVATE(ssp, lock));
+	return init_srcu_struct_fields(ssp, false);
+}
+EXPORT_SYMBOL_GPL(init_srcu_struct);
+
+#endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */
+
+/*
+ * First-use initialization of statically allocated srcu_struct
+ * structure.  Wiring up the combining tree is more than can be
+ * done with compile-time initialization, so this check is added
+ * to each update-side SRCU primitive.  Use ssp->lock, which -is-
+ * compile-time initialized, to resolve races involving multiple
+ * CPUs trying to garner first-use privileges.
+ */
+static void check_init_srcu_struct(struct srcu_struct *ssp)
+{
+	unsigned long flags;
+
+	/* The smp_load_acquire() pairs with the smp_store_release(). */
+	if (!rcu_seq_state(smp_load_acquire(&ssp->srcu_gp_seq_needed))) /*^^^*/
+		return; /* Already initialized. */
+	spin_lock_irqsave_rcu_node(ssp, flags);
+	if (!rcu_seq_state(ssp->srcu_gp_seq_needed)) {
+		spin_unlock_irqrestore_rcu_node(ssp, flags);
+		return;
+	}
+	init_srcu_struct_fields(ssp, true);
+	spin_unlock_irqrestore_rcu_node(ssp, flags);
+}
+
+/*
+ * Returns approximate total of the readers' ->srcu_lock_count[] values
+ * for the rank of per-CPU counters specified by idx.
+ */
+static unsigned long srcu_readers_lock_idx(struct srcu_struct *ssp, int idx)
+{
+	int cpu;
+	unsigned long sum = 0;
+
+	for_each_possible_cpu(cpu) {
+		struct srcu_data *cpuc = per_cpu_ptr(ssp->sda, cpu);
+
+		sum += READ_ONCE(cpuc->srcu_lock_count[idx]);
+	}
+	return sum;
+}
+
+/*
+ * Returns approximate total of the readers' ->srcu_unlock_count[] values
+ * for the rank of per-CPU counters specified by idx.
+ */
+static unsigned long srcu_readers_unlock_idx(struct srcu_struct *ssp, int idx)
+{
+	int cpu;
+	unsigned long sum = 0;
+
+	for_each_possible_cpu(cpu) {
+		struct srcu_data *cpuc = per_cpu_ptr(ssp->sda, cpu);
+
+		sum += READ_ONCE(cpuc->srcu_unlock_count[idx]);
+	}
+	return sum;
+}
+
+/*
+ * Return true if the number of pre-existing readers is determined to
+ * be zero.
+ */
+static bool srcu_readers_active_idx_check(struct srcu_struct *ssp, int idx)
+{
+	unsigned long unlocks;
+
+	unlocks = srcu_readers_unlock_idx(ssp, idx);
+
+	/*
+	 * Make sure that a lock is always counted if the corresponding
+	 * unlock is counted. Needs to be a smp_mb() as the read side may
+	 * contain a read from a variable that is written to before the
+	 * synchronize_srcu() in the write side. In this case smp_mb()s
+	 * A and B act like the store buffering pattern.
+	 *
+	 * This smp_mb() also pairs with smp_mb() C to prevent accesses
+	 * after the synchronize_srcu() from being executed before the
+	 * grace period ends.
+	 */
+	smp_mb(); /* A */
+
+	/*
+	 * If the locks are the same as the unlocks, then there must have
+	 * been no readers on this index at some time in between. This does
+	 * not mean that there are no more readers, as one could have read
+	 * the current index but not have incremented the lock counter yet.
+	 *
+	 * So suppose that the updater is preempted here for so long
+	 * that more than ULONG_MAX non-nested readers come and go in
+	 * the meantime.  It turns out that this cannot result in overflow
+	 * because if a reader modifies its unlock count after we read it
+	 * above, then that reader's next load of ->srcu_idx is guaranteed
+	 * to get the new value, which will cause it to operate on the
+	 * other bank of counters, where it cannot contribute to the
+	 * overflow of these counters.  This means that there is a maximum
+	 * of 2*NR_CPUS increments, which cannot overflow given current
+	 * systems, especially not on 64-bit systems.
+	 *
+	 * OK, how about nesting?  This does impose a limit on nesting
+	 * of floor(ULONG_MAX/NR_CPUS/2), which should be sufficient,
+	 * especially on 64-bit systems.
+	 */
+	return srcu_readers_lock_idx(ssp, idx) == unlocks;
+}
+
+/**
+ * srcu_readers_active - returns true if there are readers. and false
+ *                       otherwise
+ * @ssp: which srcu_struct to count active readers (holding srcu_read_lock).
+ *
+ * Note that this is not an atomic primitive, and can therefore suffer
+ * severe errors when invoked on an active srcu_struct.  That said, it
+ * can be useful as an error check at cleanup time.
+ */
+static bool srcu_readers_active(struct srcu_struct *ssp)
+{
+	int cpu;
+	unsigned long sum = 0;
+
+	for_each_possible_cpu(cpu) {
+		struct srcu_data *cpuc = per_cpu_ptr(ssp->sda, cpu);
+
+		sum += READ_ONCE(cpuc->srcu_lock_count[0]);
+		sum += READ_ONCE(cpuc->srcu_lock_count[1]);
+		sum -= READ_ONCE(cpuc->srcu_unlock_count[0]);
+		sum -= READ_ONCE(cpuc->srcu_unlock_count[1]);
+	}
+	return sum;
+}
+
+#define SRCU_INTERVAL		1
+
+/*
+ * Return grace-period delay, zero if there are expedited grace
+ * periods pending, SRCU_INTERVAL otherwise.
+ */
+static unsigned long srcu_get_delay(struct srcu_struct *ssp)
+{
+	if (ULONG_CMP_LT(READ_ONCE(ssp->srcu_gp_seq),
+			 READ_ONCE(ssp->srcu_gp_seq_needed_exp)))
+		return 0;
+	return SRCU_INTERVAL;
+}
+
+/**
+ * cleanup_srcu_struct - deconstruct a sleep-RCU structure
+ * @ssp: structure to clean up.
+ *
+ * Must invoke this after you are finished using a given srcu_struct that
+ * was initialized via init_srcu_struct(), else you leak memory.
+ */
+void cleanup_srcu_struct(struct srcu_struct *ssp)
+{
+	int cpu;
+
+	if (WARN_ON(!srcu_get_delay(ssp)))
+		return; /* Just leak it! */
+	if (WARN_ON(srcu_readers_active(ssp)))
+		return; /* Just leak it! */
+	flush_delayed_work(&ssp->work);
+	for_each_possible_cpu(cpu) {
+		struct srcu_data *sdp = per_cpu_ptr(ssp->sda, cpu);
+
+		del_timer_sync(&sdp->delay_work);
+		flush_work(&sdp->work);
+		if (WARN_ON(rcu_segcblist_n_cbs(&sdp->srcu_cblist)))
+			return; /* Forgot srcu_barrier(), so just leak it! */
+	}
+	if (WARN_ON(rcu_seq_state(READ_ONCE(ssp->srcu_gp_seq)) != SRCU_STATE_IDLE) ||
+	    WARN_ON(srcu_readers_active(ssp))) {
+		pr_info("%s: Active srcu_struct %p state: %d\n",
+			__func__, ssp, rcu_seq_state(READ_ONCE(ssp->srcu_gp_seq)));
+		return; /* Caller forgot to stop doing call_srcu()? */
+	}
+	free_percpu(ssp->sda);
+	ssp->sda = NULL;
+}
+EXPORT_SYMBOL_GPL(cleanup_srcu_struct);
+
+/*
+ * Counts the new reader in the appropriate per-CPU element of the
+ * srcu_struct.
+ * Returns an index that must be passed to the matching srcu_read_unlock().
+ */
+int __srcu_read_lock(struct srcu_struct *ssp)
+{
+	int idx;
+
+	idx = READ_ONCE(ssp->srcu_idx) & 0x1;
+	this_cpu_inc(ssp->sda->srcu_lock_count[idx]);
+	smp_mb(); /* B */  /* Avoid leaking the critical section. */
+	return idx;
+}
+EXPORT_SYMBOL_GPL(__srcu_read_lock);
+
+/*
+ * Removes the count for the old reader from the appropriate per-CPU
+ * element of the srcu_struct.  Note that this may well be a different
+ * CPU than that which was incremented by the corresponding srcu_read_lock().
+ */
+void __srcu_read_unlock(struct srcu_struct *ssp, int idx)
+{
+	smp_mb(); /* C */  /* Avoid leaking the critical section. */
+	this_cpu_inc(ssp->sda->srcu_unlock_count[idx]);
+}
+EXPORT_SYMBOL_GPL(__srcu_read_unlock);
+
+/*
+ * We use an adaptive strategy for synchronize_srcu() and especially for
+ * synchronize_srcu_expedited().  We spin for a fixed time period
+ * (defined below) to allow SRCU readers to exit their read-side critical
+ * sections.  If there are still some readers after a few microseconds,
+ * we repeatedly block for 1-millisecond time periods.
+ */
+#define SRCU_RETRY_CHECK_DELAY		5
+
+/*
+ * Start an SRCU grace period.
+ */
+static void srcu_gp_start(struct srcu_struct *ssp)
+{
+	struct srcu_data *sdp = this_cpu_ptr(ssp->sda);
+	int state;
+
+	lockdep_assert_held(&ACCESS_PRIVATE(ssp, lock));
+	WARN_ON_ONCE(ULONG_CMP_GE(ssp->srcu_gp_seq, ssp->srcu_gp_seq_needed));
+	spin_lock_rcu_node(sdp);  /* Interrupts already disabled. */
+	rcu_segcblist_advance(&sdp->srcu_cblist,
+			      rcu_seq_current(&ssp->srcu_gp_seq));
+	(void)rcu_segcblist_accelerate(&sdp->srcu_cblist,
+				       rcu_seq_snap(&ssp->srcu_gp_seq));
+	spin_unlock_rcu_node(sdp);  /* Interrupts remain disabled. */
+	smp_mb(); /* Order prior store to ->srcu_gp_seq_needed vs. GP start. */
+	rcu_seq_start(&ssp->srcu_gp_seq);
+	state = rcu_seq_state(ssp->srcu_gp_seq);
+	WARN_ON_ONCE(state != SRCU_STATE_SCAN1);
+}
+
+
+static void srcu_delay_timer(struct timer_list *t)
+{
+	struct srcu_data *sdp = container_of(t, struct srcu_data, delay_work);
+
+	queue_work_on(sdp->cpu, rcu_gp_wq, &sdp->work);
+}
+
+static void srcu_queue_delayed_work_on(struct srcu_data *sdp,
+				       unsigned long delay)
+{
+	if (!delay) {
+		queue_work_on(sdp->cpu, rcu_gp_wq, &sdp->work);
+		return;
+	}
+
+	timer_reduce(&sdp->delay_work, jiffies + delay);
+}
+
+/*
+ * Schedule callback invocation for the specified srcu_data structure,
+ * if possible, on the corresponding CPU.
+ */
+static void srcu_schedule_cbs_sdp(struct srcu_data *sdp, unsigned long delay)
+{
+	srcu_queue_delayed_work_on(sdp, delay);
+}
+
+/*
+ * Schedule callback invocation for all srcu_data structures associated
+ * with the specified srcu_node structure that have callbacks for the
+ * just-completed grace period, the one corresponding to idx.  If possible,
+ * schedule this invocation on the corresponding CPUs.
+ */
+static void srcu_schedule_cbs_snp(struct srcu_struct *ssp, struct srcu_node *snp,
+				  unsigned long mask, unsigned long delay)
+{
+	int cpu;
+
+	for (cpu = snp->grplo; cpu <= snp->grphi; cpu++) {
+		if (!(mask & (1 << (cpu - snp->grplo))))
+			continue;
+		srcu_schedule_cbs_sdp(per_cpu_ptr(ssp->sda, cpu), delay);
+	}
+}
+
+/*
+ * Note the end of an SRCU grace period.  Initiates callback invocation
+ * and starts a new grace period if needed.
+ *
+ * The ->srcu_cb_mutex acquisition does not protect any data, but
+ * instead prevents more than one grace period from starting while we
+ * are initiating callback invocation.  This allows the ->srcu_have_cbs[]
+ * array to have a finite number of elements.
+ */
+static void srcu_gp_end(struct srcu_struct *ssp)
+{
+	unsigned long cbdelay;
+	bool cbs;
+	bool last_lvl;
+	int cpu;
+	unsigned long flags;
+	unsigned long gpseq;
+	int idx;
+	unsigned long mask;
+	struct srcu_data *sdp;
+	struct srcu_node *snp;
+
+	/* Prevent more than one additional grace period. */
+	mutex_lock(&ssp->srcu_cb_mutex);
+
+	/* End the current grace period. */
+	spin_lock_irq_rcu_node(ssp);
+	idx = rcu_seq_state(ssp->srcu_gp_seq);
+	WARN_ON_ONCE(idx != SRCU_STATE_SCAN2);
+	cbdelay = srcu_get_delay(ssp);
+	WRITE_ONCE(ssp->srcu_last_gp_end, ktime_get_mono_fast_ns());
+	rcu_seq_end(&ssp->srcu_gp_seq);
+	gpseq = rcu_seq_current(&ssp->srcu_gp_seq);
+	if (ULONG_CMP_LT(ssp->srcu_gp_seq_needed_exp, gpseq))
+		WRITE_ONCE(ssp->srcu_gp_seq_needed_exp, gpseq);
+	spin_unlock_irq_rcu_node(ssp);
+	mutex_unlock(&ssp->srcu_gp_mutex);
+	/* A new grace period can start at this point.  But only one. */
+
+	/* Initiate callback invocation as needed. */
+	idx = rcu_seq_ctr(gpseq) % ARRAY_SIZE(snp->srcu_have_cbs);
+	srcu_for_each_node_breadth_first(ssp, snp) {
+		spin_lock_irq_rcu_node(snp);
+		cbs = false;
+		last_lvl = snp >= ssp->level[rcu_num_lvls - 1];
+		if (last_lvl)
+			cbs = snp->srcu_have_cbs[idx] == gpseq;
+		snp->srcu_have_cbs[idx] = gpseq;
+		rcu_seq_set_state(&snp->srcu_have_cbs[idx], 1);
+		if (ULONG_CMP_LT(snp->srcu_gp_seq_needed_exp, gpseq))
+			WRITE_ONCE(snp->srcu_gp_seq_needed_exp, gpseq);
+		mask = snp->srcu_data_have_cbs[idx];
+		snp->srcu_data_have_cbs[idx] = 0;
+		spin_unlock_irq_rcu_node(snp);
+		if (cbs)
+			srcu_schedule_cbs_snp(ssp, snp, mask, cbdelay);
+
+		/* Occasionally prevent srcu_data counter wrap. */
+		if (!(gpseq & counter_wrap_check) && last_lvl)
+			for (cpu = snp->grplo; cpu <= snp->grphi; cpu++) {
+				sdp = per_cpu_ptr(ssp->sda, cpu);
+				spin_lock_irqsave_rcu_node(sdp, flags);
+				if (ULONG_CMP_GE(gpseq,
+						 sdp->srcu_gp_seq_needed + 100))
+					sdp->srcu_gp_seq_needed = gpseq;
+				if (ULONG_CMP_GE(gpseq,
+						 sdp->srcu_gp_seq_needed_exp + 100))
+					sdp->srcu_gp_seq_needed_exp = gpseq;
+				spin_unlock_irqrestore_rcu_node(sdp, flags);
+			}
+	}
+
+	/* Callback initiation done, allow grace periods after next. */
+	mutex_unlock(&ssp->srcu_cb_mutex);
+
+	/* Start a new grace period if needed. */
+	spin_lock_irq_rcu_node(ssp);
+	gpseq = rcu_seq_current(&ssp->srcu_gp_seq);
+	if (!rcu_seq_state(gpseq) &&
+	    ULONG_CMP_LT(gpseq, ssp->srcu_gp_seq_needed)) {
+		srcu_gp_start(ssp);
+		spin_unlock_irq_rcu_node(ssp);
+		srcu_reschedule(ssp, 0);
+	} else {
+		spin_unlock_irq_rcu_node(ssp);
+	}
+}
+
+/*
+ * Funnel-locking scheme to scalably mediate many concurrent expedited
+ * grace-period requests.  This function is invoked for the first known
+ * expedited request for a grace period that has already been requested,
+ * but without expediting.  To start a completely new grace period,
+ * whether expedited or not, use srcu_funnel_gp_start() instead.
+ */
+static void srcu_funnel_exp_start(struct srcu_struct *ssp, struct srcu_node *snp,
+				  unsigned long s)
+{
+	unsigned long flags;
+
+	for (; snp != NULL; snp = snp->srcu_parent) {
+		if (rcu_seq_done(&ssp->srcu_gp_seq, s) ||
+		    ULONG_CMP_GE(READ_ONCE(snp->srcu_gp_seq_needed_exp), s))
+			return;
+		spin_lock_irqsave_rcu_node(snp, flags);
+		if (ULONG_CMP_GE(snp->srcu_gp_seq_needed_exp, s)) {
+			spin_unlock_irqrestore_rcu_node(snp, flags);
+			return;
+		}
+		WRITE_ONCE(snp->srcu_gp_seq_needed_exp, s);
+		spin_unlock_irqrestore_rcu_node(snp, flags);
+	}
+	spin_lock_irqsave_rcu_node(ssp, flags);
+	if (ULONG_CMP_LT(ssp->srcu_gp_seq_needed_exp, s))
+		WRITE_ONCE(ssp->srcu_gp_seq_needed_exp, s);
+	spin_unlock_irqrestore_rcu_node(ssp, flags);
+}
+
+/*
+ * Funnel-locking scheme to scalably mediate many concurrent grace-period
+ * requests.  The winner has to do the work of actually starting grace
+ * period s.  Losers must either ensure that their desired grace-period
+ * number is recorded on at least their leaf srcu_node structure, or they
+ * must take steps to invoke their own callbacks.
+ *
+ * Note that this function also does the work of srcu_funnel_exp_start(),
+ * in some cases by directly invoking it.
+ */
+static void srcu_funnel_gp_start(struct srcu_struct *ssp, struct srcu_data *sdp,
+				 unsigned long s, bool do_norm)
+{
+	unsigned long flags;
+	int idx = rcu_seq_ctr(s) % ARRAY_SIZE(sdp->mynode->srcu_have_cbs);
+	struct srcu_node *snp = sdp->mynode;
+	unsigned long snp_seq;
+
+	/* Each pass through the loop does one level of the srcu_node tree. */
+	for (; snp != NULL; snp = snp->srcu_parent) {
+		if (rcu_seq_done(&ssp->srcu_gp_seq, s) && snp != sdp->mynode)
+			return; /* GP already done and CBs recorded. */
+		spin_lock_irqsave_rcu_node(snp, flags);
+		if (ULONG_CMP_GE(snp->srcu_have_cbs[idx], s)) {
+			snp_seq = snp->srcu_have_cbs[idx];
+			if (snp == sdp->mynode && snp_seq == s)
+				snp->srcu_data_have_cbs[idx] |= sdp->grpmask;
+			spin_unlock_irqrestore_rcu_node(snp, flags);
+			if (snp == sdp->mynode && snp_seq != s) {
+				srcu_schedule_cbs_sdp(sdp, do_norm
+							   ? SRCU_INTERVAL
+							   : 0);
+				return;
+			}
+			if (!do_norm)
+				srcu_funnel_exp_start(ssp, snp, s);
+			return;
+		}
+		snp->srcu_have_cbs[idx] = s;
+		if (snp == sdp->mynode)
+			snp->srcu_data_have_cbs[idx] |= sdp->grpmask;
+		if (!do_norm && ULONG_CMP_LT(snp->srcu_gp_seq_needed_exp, s))
+			WRITE_ONCE(snp->srcu_gp_seq_needed_exp, s);
+		spin_unlock_irqrestore_rcu_node(snp, flags);
+	}
+
+	/* Top of tree, must ensure the grace period will be started. */
+	spin_lock_irqsave_rcu_node(ssp, flags);
+	if (ULONG_CMP_LT(ssp->srcu_gp_seq_needed, s)) {
+		/*
+		 * Record need for grace period s.  Pair with load
+		 * acquire setting up for initialization.
+		 */
+		smp_store_release(&ssp->srcu_gp_seq_needed, s); /*^^^*/
+	}
+	if (!do_norm && ULONG_CMP_LT(ssp->srcu_gp_seq_needed_exp, s))
+		WRITE_ONCE(ssp->srcu_gp_seq_needed_exp, s);
+
+	/* If grace period not already done and none in progress, start it. */
+	if (!rcu_seq_done(&ssp->srcu_gp_seq, s) &&
+	    rcu_seq_state(ssp->srcu_gp_seq) == SRCU_STATE_IDLE) {
+		WARN_ON_ONCE(ULONG_CMP_GE(ssp->srcu_gp_seq, ssp->srcu_gp_seq_needed));
+		srcu_gp_start(ssp);
+		if (likely(srcu_init_done))
+			queue_delayed_work(rcu_gp_wq, &ssp->work,
+					   srcu_get_delay(ssp));
+		else if (list_empty(&ssp->work.work.entry))
+			list_add(&ssp->work.work.entry, &srcu_boot_list);
+	}
+	spin_unlock_irqrestore_rcu_node(ssp, flags);
+}
+
+/*
+ * Wait until all readers counted by array index idx complete, but
+ * loop an additional time if there is an expedited grace period pending.
+ * The caller must ensure that ->srcu_idx is not changed while checking.
+ */
+static bool try_check_zero(struct srcu_struct *ssp, int idx, int trycount)
+{
+	for (;;) {
+		if (srcu_readers_active_idx_check(ssp, idx))
+			return true;
+		if (--trycount + !srcu_get_delay(ssp) <= 0)
+			return false;
+		udelay(SRCU_RETRY_CHECK_DELAY);
+	}
+}
+
+/*
+ * Increment the ->srcu_idx counter so that future SRCU readers will
+ * use the other rank of the ->srcu_(un)lock_count[] arrays.  This allows
+ * us to wait for pre-existing readers in a starvation-free manner.
+ */
+static void srcu_flip(struct srcu_struct *ssp)
+{
+	/*
+	 * Ensure that if this updater saw a given reader's increment
+	 * from __srcu_read_lock(), that reader was using an old value
+	 * of ->srcu_idx.  Also ensure that if a given reader sees the
+	 * new value of ->srcu_idx, this updater's earlier scans cannot
+	 * have seen that reader's increments (which is OK, because this
+	 * grace period need not wait on that reader).
+	 */
+	smp_mb(); /* E */  /* Pairs with B and C. */
+
+	WRITE_ONCE(ssp->srcu_idx, ssp->srcu_idx + 1);
+
+	/*
+	 * Ensure that if the updater misses an __srcu_read_unlock()
+	 * increment, that task's next __srcu_read_lock() will see the
+	 * above counter update.  Note that both this memory barrier
+	 * and the one in srcu_readers_active_idx_check() provide the
+	 * guarantee for __srcu_read_lock().
+	 */
+	smp_mb(); /* D */  /* Pairs with C. */
+}
+
+/*
+ * If SRCU is likely idle, return true, otherwise return false.
+ *
+ * Note that it is OK for several current from-idle requests for a new
+ * grace period from idle to specify expediting because they will all end
+ * up requesting the same grace period anyhow.  So no loss.
+ *
+ * Note also that if any CPU (including the current one) is still invoking
+ * callbacks, this function will nevertheless say "idle".  This is not
+ * ideal, but the overhead of checking all CPUs' callback lists is even
+ * less ideal, especially on large systems.  Furthermore, the wakeup
+ * can happen before the callback is fully removed, so we have no choice
+ * but to accept this type of error.
+ *
+ * This function is also subject to counter-wrap errors, but let's face
+ * it, if this function was preempted for enough time for the counters
+ * to wrap, it really doesn't matter whether or not we expedite the grace
+ * period.  The extra overhead of a needlessly expedited grace period is
+ * negligible when amortized over that time period, and the extra latency
+ * of a needlessly non-expedited grace period is similarly negligible.
+ */
+static bool srcu_might_be_idle(struct srcu_struct *ssp)
+{
+	unsigned long curseq;
+	unsigned long flags;
+	struct srcu_data *sdp;
+	unsigned long t;
+	unsigned long tlast;
+
+	check_init_srcu_struct(ssp);
+	/* If the local srcu_data structure has callbacks, not idle.  */
+	sdp = raw_cpu_ptr(ssp->sda);
+	spin_lock_irqsave_rcu_node(sdp, flags);
+	if (rcu_segcblist_pend_cbs(&sdp->srcu_cblist)) {
+		spin_unlock_irqrestore_rcu_node(sdp, flags);
+		return false; /* Callbacks already present, so not idle. */
+	}
+	spin_unlock_irqrestore_rcu_node(sdp, flags);
+
+	/*
+	 * No local callbacks, so probabalistically probe global state.
+	 * Exact information would require acquiring locks, which would
+	 * kill scalability, hence the probabalistic nature of the probe.
+	 */
+
+	/* First, see if enough time has passed since the last GP. */
+	t = ktime_get_mono_fast_ns();
+	tlast = READ_ONCE(ssp->srcu_last_gp_end);
+	if (exp_holdoff == 0 ||
+	    time_in_range_open(t, tlast, tlast + exp_holdoff))
+		return false; /* Too soon after last GP. */
+
+	/* Next, check for probable idleness. */
+	curseq = rcu_seq_current(&ssp->srcu_gp_seq);
+	smp_mb(); /* Order ->srcu_gp_seq with ->srcu_gp_seq_needed. */
+	if (ULONG_CMP_LT(curseq, READ_ONCE(ssp->srcu_gp_seq_needed)))
+		return false; /* Grace period in progress, so not idle. */
+	smp_mb(); /* Order ->srcu_gp_seq with prior access. */
+	if (curseq != rcu_seq_current(&ssp->srcu_gp_seq))
+		return false; /* GP # changed, so not idle. */
+	return true; /* With reasonable probability, idle! */
+}
+
+/*
+ * SRCU callback function to leak a callback.
+ */
+static void srcu_leak_callback(struct rcu_head *rhp)
+{
+}
+
+/*
+ * Start an SRCU grace period, and also queue the callback if non-NULL.
+ */
+static unsigned long srcu_gp_start_if_needed(struct srcu_struct *ssp,
+					     struct rcu_head *rhp, bool do_norm)
+{
+	unsigned long flags;
+	int idx;
+	bool needexp = false;
+	bool needgp = false;
+	unsigned long s;
+	struct srcu_data *sdp;
+
+	check_init_srcu_struct(ssp);
+	idx = srcu_read_lock(ssp);
+	sdp = raw_cpu_ptr(ssp->sda);
+	spin_lock_irqsave_rcu_node(sdp, flags);
+	if (rhp)
+		rcu_segcblist_enqueue(&sdp->srcu_cblist, rhp);
+	rcu_segcblist_advance(&sdp->srcu_cblist,
+			      rcu_seq_current(&ssp->srcu_gp_seq));
+	s = rcu_seq_snap(&ssp->srcu_gp_seq);
+	(void)rcu_segcblist_accelerate(&sdp->srcu_cblist, s);
+	if (ULONG_CMP_LT(sdp->srcu_gp_seq_needed, s)) {
+		sdp->srcu_gp_seq_needed = s;
+		needgp = true;
+	}
+	if (!do_norm && ULONG_CMP_LT(sdp->srcu_gp_seq_needed_exp, s)) {
+		sdp->srcu_gp_seq_needed_exp = s;
+		needexp = true;
+	}
+	spin_unlock_irqrestore_rcu_node(sdp, flags);
+	if (needgp)
+		srcu_funnel_gp_start(ssp, sdp, s, do_norm);
+	else if (needexp)
+		srcu_funnel_exp_start(ssp, sdp->mynode, s);
+	srcu_read_unlock(ssp, idx);
+	return s;
+}
+
+/*
+ * Enqueue an SRCU callback on the srcu_data structure associated with
+ * the current CPU and the specified srcu_struct structure, initiating
+ * grace-period processing if it is not already running.
+ *
+ * Note that all CPUs must agree that the grace period extended beyond
+ * all pre-existing SRCU read-side critical section.  On systems with
+ * more than one CPU, this means that when "func()" is invoked, each CPU
+ * is guaranteed to have executed a full memory barrier since the end of
+ * its last corresponding SRCU read-side critical section whose beginning
+ * preceded the call to call_srcu().  It also means that each CPU executing
+ * an SRCU read-side critical section that continues beyond the start of
+ * "func()" must have executed a memory barrier after the call_srcu()
+ * but before the beginning of that SRCU read-side critical section.
+ * Note that these guarantees include CPUs that are offline, idle, or
+ * executing in user mode, as well as CPUs that are executing in the kernel.
+ *
+ * Furthermore, if CPU A invoked call_srcu() and CPU B invoked the
+ * resulting SRCU callback function "func()", then both CPU A and CPU
+ * B are guaranteed to execute a full memory barrier during the time
+ * interval between the call to call_srcu() and the invocation of "func()".
+ * This guarantee applies even if CPU A and CPU B are the same CPU (but
+ * again only if the system has more than one CPU).
+ *
+ * Of course, these guarantees apply only for invocations of call_srcu(),
+ * srcu_read_lock(), and srcu_read_unlock() that are all passed the same
+ * srcu_struct structure.
+ */
+static void __call_srcu(struct srcu_struct *ssp, struct rcu_head *rhp,
+			rcu_callback_t func, bool do_norm)
+{
+	if (debug_rcu_head_queue(rhp)) {
+		/* Probable double call_srcu(), so leak the callback. */
+		WRITE_ONCE(rhp->func, srcu_leak_callback);
+		WARN_ONCE(1, "call_srcu(): Leaked duplicate callback\n");
+		return;
+	}
+	rhp->func = func;
+	(void)srcu_gp_start_if_needed(ssp, rhp, do_norm);
+}
+
+/**
+ * call_srcu() - Queue a callback for invocation after an SRCU grace period
+ * @ssp: srcu_struct in queue the callback
+ * @rhp: structure to be used for queueing the SRCU callback.
+ * @func: function to be invoked after the SRCU grace period
+ *
+ * The callback function will be invoked some time after a full SRCU
+ * grace period elapses, in other words after all pre-existing SRCU
+ * read-side critical sections have completed.  However, the callback
+ * function might well execute concurrently with other SRCU read-side
+ * critical sections that started after call_srcu() was invoked.  SRCU
+ * read-side critical sections are delimited by srcu_read_lock() and
+ * srcu_read_unlock(), and may be nested.
+ *
+ * The callback will be invoked from process context, but must nevertheless
+ * be fast and must not block.
+ */
+void call_srcu(struct srcu_struct *ssp, struct rcu_head *rhp,
+	       rcu_callback_t func)
+{
+	__call_srcu(ssp, rhp, func, true);
+}
+EXPORT_SYMBOL_GPL(call_srcu);
+
+/*
+ * Helper function for synchronize_srcu() and synchronize_srcu_expedited().
+ */
+static void __synchronize_srcu(struct srcu_struct *ssp, bool do_norm)
+{
+	struct rcu_synchronize rcu;
+
+	RCU_LOCKDEP_WARN(lock_is_held(&ssp->dep_map) ||
+			 lock_is_held(&rcu_bh_lock_map) ||
+			 lock_is_held(&rcu_lock_map) ||
+			 lock_is_held(&rcu_sched_lock_map),
+			 "Illegal synchronize_srcu() in same-type SRCU (or in RCU) read-side critical section");
+
+	if (rcu_scheduler_active == RCU_SCHEDULER_INACTIVE)
+		return;
+	might_sleep();
+	check_init_srcu_struct(ssp);
+	init_completion(&rcu.completion);
+	init_rcu_head_on_stack(&rcu.head);
+	__call_srcu(ssp, &rcu.head, wakeme_after_rcu, do_norm);
+	wait_for_completion(&rcu.completion);
+	destroy_rcu_head_on_stack(&rcu.head);
+
+	/*
+	 * Make sure that later code is ordered after the SRCU grace
+	 * period.  This pairs with the spin_lock_irq_rcu_node()
+	 * in srcu_invoke_callbacks().  Unlike Tree RCU, this is needed
+	 * because the current CPU might have been totally uninvolved with
+	 * (and thus unordered against) that grace period.
+	 */
+	smp_mb();
+}
+
+/**
+ * synchronize_srcu_expedited - Brute-force SRCU grace period
+ * @ssp: srcu_struct with which to synchronize.
+ *
+ * Wait for an SRCU grace period to elapse, but be more aggressive about
+ * spinning rather than blocking when waiting.
+ *
+ * Note that synchronize_srcu_expedited() has the same deadlock and
+ * memory-ordering properties as does synchronize_srcu().
+ */
+void synchronize_srcu_expedited(struct srcu_struct *ssp)
+{
+	__synchronize_srcu(ssp, rcu_gp_is_normal());
+}
+EXPORT_SYMBOL_GPL(synchronize_srcu_expedited);
+
+/**
+ * synchronize_srcu - wait for prior SRCU read-side critical-section completion
+ * @ssp: srcu_struct with which to synchronize.
+ *
+ * Wait for the count to drain to zero of both indexes. To avoid the
+ * possible starvation of synchronize_srcu(), it waits for the count of
+ * the index=((->srcu_idx & 1) ^ 1) to drain to zero at first,
+ * and then flip the srcu_idx and wait for the count of the other index.
+ *
+ * Can block; must be called from process context.
+ *
+ * Note that it is illegal to call synchronize_srcu() from the corresponding
+ * SRCU read-side critical section; doing so will result in deadlock.
+ * However, it is perfectly legal to call synchronize_srcu() on one
+ * srcu_struct from some other srcu_struct's read-side critical section,
+ * as long as the resulting graph of srcu_structs is acyclic.
+ *
+ * There are memory-ordering constraints implied by synchronize_srcu().
+ * On systems with more than one CPU, when synchronize_srcu() returns,
+ * each CPU is guaranteed to have executed a full memory barrier since
+ * the end of its last corresponding SRCU read-side critical section
+ * whose beginning preceded the call to synchronize_srcu().  In addition,
+ * each CPU having an SRCU read-side critical section that extends beyond
+ * the return from synchronize_srcu() is guaranteed to have executed a
+ * full memory barrier after the beginning of synchronize_srcu() and before
+ * the beginning of that SRCU read-side critical section.  Note that these
+ * guarantees include CPUs that are offline, idle, or executing in user mode,
+ * as well as CPUs that are executing in the kernel.
+ *
+ * Furthermore, if CPU A invoked synchronize_srcu(), which returned
+ * to its caller on CPU B, then both CPU A and CPU B are guaranteed
+ * to have executed a full memory barrier during the execution of
+ * synchronize_srcu().  This guarantee applies even if CPU A and CPU B
+ * are the same CPU, but again only if the system has more than one CPU.
+ *
+ * Of course, these memory-ordering guarantees apply only when
+ * synchronize_srcu(), srcu_read_lock(), and srcu_read_unlock() are
+ * passed the same srcu_struct structure.
+ *
+ * If SRCU is likely idle, expedite the first request.  This semantic
+ * was provided by Classic SRCU, and is relied upon by its users, so TREE
+ * SRCU must also provide it.  Note that detecting idleness is heuristic
+ * and subject to both false positives and negatives.
+ */
+void synchronize_srcu(struct srcu_struct *ssp)
+{
+	if (srcu_might_be_idle(ssp) || rcu_gp_is_expedited())
+		synchronize_srcu_expedited(ssp);
+	else
+		__synchronize_srcu(ssp, true);
+}
+EXPORT_SYMBOL_GPL(synchronize_srcu);
+
+/**
+ * get_state_synchronize_srcu - Provide an end-of-grace-period cookie
+ * @ssp: srcu_struct to provide cookie for.
+ *
+ * This function returns a cookie that can be passed to
+ * poll_state_synchronize_srcu(), which will return true if a full grace
+ * period has elapsed in the meantime.  It is the caller's responsibility
+ * to make sure that grace period happens, for example, by invoking
+ * call_srcu() after return from get_state_synchronize_srcu().
+ */
+unsigned long get_state_synchronize_srcu(struct srcu_struct *ssp)
+{
+	// Any prior manipulation of SRCU-protected data must happen
+	// before the load from ->srcu_gp_seq.
+	smp_mb();
+	return rcu_seq_snap(&ssp->srcu_gp_seq);
+}
+EXPORT_SYMBOL_GPL(get_state_synchronize_srcu);
+
+/**
+ * start_poll_synchronize_srcu - Provide cookie and start grace period
+ * @ssp: srcu_struct to provide cookie for.
+ *
+ * This function returns a cookie that can be passed to
+ * poll_state_synchronize_srcu(), which will return true if a full grace
+ * period has elapsed in the meantime.  Unlike get_state_synchronize_srcu(),
+ * this function also ensures that any needed SRCU grace period will be
+ * started.  This convenience does come at a cost in terms of CPU overhead.
+ */
+unsigned long start_poll_synchronize_srcu(struct srcu_struct *ssp)
+{
+	return srcu_gp_start_if_needed(ssp, NULL, true);
+}
+EXPORT_SYMBOL_GPL(start_poll_synchronize_srcu);
+
+/**
+ * poll_state_synchronize_srcu - Has cookie's grace period ended?
+ * @ssp: srcu_struct to provide cookie for.
+ * @cookie: Return value from get_state_synchronize_srcu() or start_poll_synchronize_srcu().
+ *
+ * This function takes the cookie that was returned from either
+ * get_state_synchronize_srcu() or start_poll_synchronize_srcu(), and
+ * returns @true if an SRCU grace period elapsed since the time that the
+ * cookie was created.
+ */
+bool poll_state_synchronize_srcu(struct srcu_struct *ssp, unsigned long cookie)
+{
+	if (!rcu_seq_done(&ssp->srcu_gp_seq, cookie))
+		return false;
+	// Ensure that the end of the SRCU grace period happens before
+	// any subsequent code that the caller might execute.
+	smp_mb(); // ^^^
+	return true;
+}
+EXPORT_SYMBOL_GPL(poll_state_synchronize_srcu);
+
+/*
+ * Callback function for srcu_barrier() use.
+ */
+static void srcu_barrier_cb(struct rcu_head *rhp)
+{
+	struct srcu_data *sdp;
+	struct srcu_struct *ssp;
+
+	sdp = container_of(rhp, struct srcu_data, srcu_barrier_head);
+	ssp = sdp->ssp;
+	if (atomic_dec_and_test(&ssp->srcu_barrier_cpu_cnt))
+		complete(&ssp->srcu_barrier_completion);
+}
+
+/**
+ * srcu_barrier - Wait until all in-flight call_srcu() callbacks complete.
+ * @ssp: srcu_struct on which to wait for in-flight callbacks.
+ */
+void srcu_barrier(struct srcu_struct *ssp)
+{
+	int cpu;
+	struct srcu_data *sdp;
+	unsigned long s = rcu_seq_snap(&ssp->srcu_barrier_seq);
+
+	check_init_srcu_struct(ssp);
+	mutex_lock(&ssp->srcu_barrier_mutex);
+	if (rcu_seq_done(&ssp->srcu_barrier_seq, s)) {
+		smp_mb(); /* Force ordering following return. */
+		mutex_unlock(&ssp->srcu_barrier_mutex);
+		return; /* Someone else did our work for us. */
+	}
+	rcu_seq_start(&ssp->srcu_barrier_seq);
+	init_completion(&ssp->srcu_barrier_completion);
+
+	/* Initial count prevents reaching zero until all CBs are posted. */
+	atomic_set(&ssp->srcu_barrier_cpu_cnt, 1);
+
+	/*
+	 * Each pass through this loop enqueues a callback, but only
+	 * on CPUs already having callbacks enqueued.  Note that if
+	 * a CPU already has callbacks enqueue, it must have already
+	 * registered the need for a future grace period, so all we
+	 * need do is enqueue a callback that will use the same
+	 * grace period as the last callback already in the queue.
+	 */
+	for_each_possible_cpu(cpu) {
+		sdp = per_cpu_ptr(ssp->sda, cpu);
+		spin_lock_irq_rcu_node(sdp);
+		atomic_inc(&ssp->srcu_barrier_cpu_cnt);
+		sdp->srcu_barrier_head.func = srcu_barrier_cb;
+		debug_rcu_head_queue(&sdp->srcu_barrier_head);
+		if (!rcu_segcblist_entrain(&sdp->srcu_cblist,
+					   &sdp->srcu_barrier_head)) {
+			debug_rcu_head_unqueue(&sdp->srcu_barrier_head);
+			atomic_dec(&ssp->srcu_barrier_cpu_cnt);
+		}
+		spin_unlock_irq_rcu_node(sdp);
+	}
+
+	/* Remove the initial count, at which point reaching zero can happen. */
+	if (atomic_dec_and_test(&ssp->srcu_barrier_cpu_cnt))
+		complete(&ssp->srcu_barrier_completion);
+	wait_for_completion(&ssp->srcu_barrier_completion);
+
+	rcu_seq_end(&ssp->srcu_barrier_seq);
+	mutex_unlock(&ssp->srcu_barrier_mutex);
+}
+EXPORT_SYMBOL_GPL(srcu_barrier);
+
+/**
+ * srcu_batches_completed - return batches completed.
+ * @ssp: srcu_struct on which to report batch completion.
+ *
+ * Report the number of batches, correlated with, but not necessarily
+ * precisely the same as, the number of grace periods that have elapsed.
+ */
+unsigned long srcu_batches_completed(struct srcu_struct *ssp)
+{
+	return READ_ONCE(ssp->srcu_idx);
+}
+EXPORT_SYMBOL_GPL(srcu_batches_completed);
+
+/*
+ * Core SRCU state machine.  Push state bits of ->srcu_gp_seq
+ * to SRCU_STATE_SCAN2, and invoke srcu_gp_end() when scan has
+ * completed in that state.
+ */
+static void srcu_advance_state(struct srcu_struct *ssp)
+{
+	int idx;
+
+	mutex_lock(&ssp->srcu_gp_mutex);
+
+	/*
+	 * Because readers might be delayed for an extended period after
+	 * fetching ->srcu_idx for their index, at any point in time there
+	 * might well be readers using both idx=0 and idx=1.  We therefore
+	 * need to wait for readers to clear from both index values before
+	 * invoking a callback.
+	 *
+	 * The load-acquire ensures that we see the accesses performed
+	 * by the prior grace period.
+	 */
+	idx = rcu_seq_state(smp_load_acquire(&ssp->srcu_gp_seq)); /* ^^^ */
+	if (idx == SRCU_STATE_IDLE) {
+		spin_lock_irq_rcu_node(ssp);
+		if (ULONG_CMP_GE(ssp->srcu_gp_seq, ssp->srcu_gp_seq_needed)) {
+			WARN_ON_ONCE(rcu_seq_state(ssp->srcu_gp_seq));
+			spin_unlock_irq_rcu_node(ssp);
+			mutex_unlock(&ssp->srcu_gp_mutex);
+			return;
+		}
+		idx = rcu_seq_state(READ_ONCE(ssp->srcu_gp_seq));
+		if (idx == SRCU_STATE_IDLE)
+			srcu_gp_start(ssp);
+		spin_unlock_irq_rcu_node(ssp);
+		if (idx != SRCU_STATE_IDLE) {
+			mutex_unlock(&ssp->srcu_gp_mutex);
+			return; /* Someone else started the grace period. */
+		}
+	}
+
+	if (rcu_seq_state(READ_ONCE(ssp->srcu_gp_seq)) == SRCU_STATE_SCAN1) {
+		idx = 1 ^ (ssp->srcu_idx & 1);
+		if (!try_check_zero(ssp, idx, 1)) {
+			mutex_unlock(&ssp->srcu_gp_mutex);
+			return; /* readers present, retry later. */
+		}
+		srcu_flip(ssp);
+		spin_lock_irq_rcu_node(ssp);
+		rcu_seq_set_state(&ssp->srcu_gp_seq, SRCU_STATE_SCAN2);
+		spin_unlock_irq_rcu_node(ssp);
+	}
+
+	if (rcu_seq_state(READ_ONCE(ssp->srcu_gp_seq)) == SRCU_STATE_SCAN2) {
+
+		/*
+		 * SRCU read-side critical sections are normally short,
+		 * so check at least twice in quick succession after a flip.
+		 */
+		idx = 1 ^ (ssp->srcu_idx & 1);
+		if (!try_check_zero(ssp, idx, 2)) {
+			mutex_unlock(&ssp->srcu_gp_mutex);
+			return; /* readers present, retry later. */
+		}
+		srcu_gp_end(ssp);  /* Releases ->srcu_gp_mutex. */
+	}
+}
+
+/*
+ * Invoke a limited number of SRCU callbacks that have passed through
+ * their grace period.  If there are more to do, SRCU will reschedule
+ * the workqueue.  Note that needed memory barriers have been executed
+ * in this task's context by srcu_readers_active_idx_check().
+ */
+static void srcu_invoke_callbacks(struct work_struct *work)
+{
+	bool more;
+	struct rcu_cblist ready_cbs;
+	struct rcu_head *rhp;
+	struct srcu_data *sdp;
+	struct srcu_struct *ssp;
+
+	sdp = container_of(work, struct srcu_data, work);
+
+	ssp = sdp->ssp;
+	rcu_cblist_init(&ready_cbs);
+	spin_lock_irq_rcu_node(sdp);
+	rcu_segcblist_advance(&sdp->srcu_cblist,
+			      rcu_seq_current(&ssp->srcu_gp_seq));
+	if (sdp->srcu_cblist_invoking ||
+	    !rcu_segcblist_ready_cbs(&sdp->srcu_cblist)) {
+		spin_unlock_irq_rcu_node(sdp);
+		return;  /* Someone else on the job or nothing to do. */
+	}
+
+	/* We are on the job!  Extract and invoke ready callbacks. */
+	sdp->srcu_cblist_invoking = true;
+	rcu_segcblist_extract_done_cbs(&sdp->srcu_cblist, &ready_cbs);
+	spin_unlock_irq_rcu_node(sdp);
+	rhp = rcu_cblist_dequeue(&ready_cbs);
+	for (; rhp != NULL; rhp = rcu_cblist_dequeue(&ready_cbs)) {
+		debug_rcu_head_unqueue(rhp);
+		local_bh_disable();
+		rhp->func(rhp);
+		local_bh_enable();
+	}
+
+	/*
+	 * Update counts, accelerate new callbacks, and if needed,
+	 * schedule another round of callback invocation.
+	 */
+	spin_lock_irq_rcu_node(sdp);
+	rcu_segcblist_insert_count(&sdp->srcu_cblist, &ready_cbs);
+	(void)rcu_segcblist_accelerate(&sdp->srcu_cblist,
+				       rcu_seq_snap(&ssp->srcu_gp_seq));
+	sdp->srcu_cblist_invoking = false;
+	more = rcu_segcblist_ready_cbs(&sdp->srcu_cblist);
+	spin_unlock_irq_rcu_node(sdp);
+	if (more)
+		srcu_schedule_cbs_sdp(sdp, 0);
+}
+
+/*
+ * Finished one round of SRCU grace period.  Start another if there are
+ * more SRCU callbacks queued, otherwise put SRCU into not-running state.
+ */
+static void srcu_reschedule(struct srcu_struct *ssp, unsigned long delay)
+{
+	bool pushgp = true;
+
+	spin_lock_irq_rcu_node(ssp);
+	if (ULONG_CMP_GE(ssp->srcu_gp_seq, ssp->srcu_gp_seq_needed)) {
+		if (!WARN_ON_ONCE(rcu_seq_state(ssp->srcu_gp_seq))) {
+			/* All requests fulfilled, time to go idle. */
+			pushgp = false;
+		}
+	} else if (!rcu_seq_state(ssp->srcu_gp_seq)) {
+		/* Outstanding request and no GP.  Start one. */
+		srcu_gp_start(ssp);
+	}
+	spin_unlock_irq_rcu_node(ssp);
+
+	if (pushgp)
+		queue_delayed_work(rcu_gp_wq, &ssp->work, delay);
+}
+
+/*
+ * This is the work-queue function that handles SRCU grace periods.
+ */
+static void process_srcu(struct work_struct *work)
+{
+	struct srcu_struct *ssp;
+
+	ssp = container_of(work, struct srcu_struct, work.work);
+
+	srcu_advance_state(ssp);
+	srcu_reschedule(ssp, srcu_get_delay(ssp));
+}
+
+void srcutorture_get_gp_data(enum rcutorture_type test_type,
+			     struct srcu_struct *ssp, int *flags,
+			     unsigned long *gp_seq)
+{
+	if (test_type != SRCU_FLAVOR)
+		return;
+	*flags = 0;
+	*gp_seq = rcu_seq_current(&ssp->srcu_gp_seq);
+}
+EXPORT_SYMBOL_GPL(srcutorture_get_gp_data);
+
+void srcu_torture_stats_print(struct srcu_struct *ssp, char *tt, char *tf)
+{
+	int cpu;
+	int idx;
+	unsigned long s0 = 0, s1 = 0;
+
+	idx = ssp->srcu_idx & 0x1;
+	pr_alert("%s%s Tree SRCU g%ld per-CPU(idx=%d):",
+		 tt, tf, rcu_seq_current(&ssp->srcu_gp_seq), idx);
+	for_each_possible_cpu(cpu) {
+		unsigned long l0, l1;
+		unsigned long u0, u1;
+		long c0, c1;
+		struct srcu_data *sdp;
+
+		sdp = per_cpu_ptr(ssp->sda, cpu);
+		u0 = data_race(sdp->srcu_unlock_count[!idx]);
+		u1 = data_race(sdp->srcu_unlock_count[idx]);
+
+		/*
+		 * Make sure that a lock is always counted if the corresponding
+		 * unlock is counted.
+		 */
+		smp_rmb();
+
+		l0 = data_race(sdp->srcu_lock_count[!idx]);
+		l1 = data_race(sdp->srcu_lock_count[idx]);
+
+		c0 = l0 - u0;
+		c1 = l1 - u1;
+		pr_cont(" %d(%ld,%ld %c)",
+			cpu, c0, c1,
+			"C."[rcu_segcblist_empty(&sdp->srcu_cblist)]);
+		s0 += c0;
+		s1 += c1;
+	}
+	pr_cont(" T(%ld,%ld)\n", s0, s1);
+}
+EXPORT_SYMBOL_GPL(srcu_torture_stats_print);
+
+static int __init srcu_bootup_announce(void)
+{
+	pr_info("Hierarchical SRCU implementation.\n");
+	if (exp_holdoff != DEFAULT_SRCU_EXP_HOLDOFF)
+		pr_info("\tNon-default auto-expedite holdoff of %lu ns.\n", exp_holdoff);
+	return 0;
+}
+early_initcall(srcu_bootup_announce);
+
+void __init srcu_init(void)
+{
+	struct srcu_struct *ssp;
+
+	srcu_init_done = true;
+	while (!list_empty(&srcu_boot_list)) {
+		ssp = list_first_entry(&srcu_boot_list, struct srcu_struct,
+				      work.work.entry);
+		check_init_srcu_struct(ssp);
+		list_del_init(&ssp->work.work.entry);
+		queue_work(rcu_gp_wq, &ssp->work.work);
+	}
+}
+
+#ifdef CONFIG_MODULES
+
+/* Initialize any global-scope srcu_struct structures used by this module. */
+static int srcu_module_coming(struct module *mod)
+{
+	int i;
+	struct srcu_struct **sspp = mod->srcu_struct_ptrs;
+	int ret;
+
+	for (i = 0; i < mod->num_srcu_structs; i++) {
+		ret = init_srcu_struct(*(sspp++));
+		if (WARN_ON_ONCE(ret))
+			return ret;
+	}
+	return 0;
+}
+
+/* Clean up any global-scope srcu_struct structures used by this module. */
+static void srcu_module_going(struct module *mod)
+{
+	int i;
+	struct srcu_struct **sspp = mod->srcu_struct_ptrs;
+
+	for (i = 0; i < mod->num_srcu_structs; i++)
+		cleanup_srcu_struct(*(sspp++));
+}
+
+/* Handle one module, either coming or going. */
+static int srcu_module_notify(struct notifier_block *self,
+			      unsigned long val, void *data)
+{
+	struct module *mod = data;
+	int ret = 0;
+
+	switch (val) {
+	case MODULE_STATE_COMING:
+		ret = srcu_module_coming(mod);
+		break;
+	case MODULE_STATE_GOING:
+		srcu_module_going(mod);
+		break;
+	default:
+		break;
+	}
+	return ret;
+}
+
+static struct notifier_block srcu_module_nb = {
+	.notifier_call = srcu_module_notify,
+	.priority = 0,
+};
+
+static __init int init_srcu_module_notifier(void)
+{
+	int ret;
+
+	ret = register_module_notifier(&srcu_module_nb);
+	if (ret)
+		pr_warn("Failed to register srcu module notifier\n");
+	return ret;
+}
+late_initcall(init_srcu_module_notifier);
+
+#endif /* #ifdef CONFIG_MODULES */