/* SPDX-License-Identifier: GPL-2.0+ */ /* * Task-based RCU implementations. * * Copyright (C) 2020 Paul E. McKenney */ #ifdef CONFIG_TASKS_RCU_GENERIC //////////////////////////////////////////////////////////////////////// // // Generic data structures. struct rcu_tasks; typedef void (*rcu_tasks_gp_func_t)(struct rcu_tasks *rtp); typedef void (*pregp_func_t)(void); 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); /** * Definition for a Tasks-RCU-like mechanism. * @cbs_head: Head of callback list. * @cbs_tail: Tail pointer for callback list. * @cbs_wq: Wait queue allowning new callback to get kthread's attention. * @cbs_lock: Lock protecting callback list. * @kthread_ptr: This flavor's grace-period/callback-invocation kthread. * @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. * @n_gps: 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. * @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). * @holdout_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. * @name: This flavor's textual name. * @kname: This flavor's kthread name. */ struct rcu_tasks { struct rcu_head *cbs_head; struct rcu_head **cbs_tail; struct wait_queue_head cbs_wq; raw_spinlock_t cbs_lock; int gp_state; int gp_sleep; int init_fract; unsigned long gp_jiffies; unsigned long gp_start; unsigned long n_gps; unsigned long n_ipis; unsigned long n_ipis_fails; struct task_struct *kthread_ptr; 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; char *name; char *kname; }; #define DEFINE_RCU_TASKS(rt_name, gp, call, n) \ static struct rcu_tasks rt_name = \ { \ .cbs_tail = &rt_name.cbs_head, \ .cbs_wq = __WAIT_QUEUE_HEAD_INITIALIZER(rt_name.cbs_wq), \ .cbs_lock = __RAW_SPIN_LOCK_UNLOCKED(rt_name.cbs_lock), \ .gp_func = gp, \ .call_func = call, \ .name = n, \ .kname = #rt_name, \ } /* Track exiting tasks in order to allow them to be waited for. */ DEFINE_STATIC_SRCU(tasks_rcu_exit_srcu); /* 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_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); /* 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. /* 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 */ // 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) { unsigned long flags; bool needwake; rhp->next = NULL; rhp->func = func; raw_spin_lock_irqsave(&rtp->cbs_lock, flags); needwake = !rtp->cbs_head; WRITE_ONCE(*rtp->cbs_tail, rhp); rtp->cbs_tail = &rhp->next; raw_spin_unlock_irqrestore(&rtp->cbs_lock, flags); /* We can't create the thread unless interrupts are enabled. */ if (needwake && READ_ONCE(rtp->kthread_ptr)) wake_up(&rtp->cbs_wq); } // 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; /* Wait for the grace period. */ wait_rcu_gp(rtp->call_func); } /* RCU-tasks kthread that detects grace periods and invokes callbacks. */ static int __noreturn rcu_tasks_kthread(void *arg) { unsigned long flags; struct rcu_head *list; struct rcu_head *next; struct rcu_tasks *rtp = arg; /* Run on housekeeping CPUs by default. Sysadm can move if desired. */ housekeeping_affine(current, HK_FLAG_RCU); WRITE_ONCE(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 (;;) { set_tasks_gp_state(rtp, RTGS_WAIT_CBS); /* Pick up any new callbacks. */ raw_spin_lock_irqsave(&rtp->cbs_lock, flags); smp_mb__after_spinlock(); // Order updates vs. GP. list = rtp->cbs_head; rtp->cbs_head = NULL; rtp->cbs_tail = &rtp->cbs_head; raw_spin_unlock_irqrestore(&rtp->cbs_lock, flags); /* If there were none, wait a bit and start over. */ if (!list) { wait_event_interruptible(rtp->cbs_wq, READ_ONCE(rtp->cbs_head)); if (!rtp->cbs_head) { WARN_ON(signal_pending(current)); set_tasks_gp_state(rtp, RTGS_WAIT_WAIT_CBS); schedule_timeout_idle(HZ/10); } continue; } // Wait for one grace period. set_tasks_gp_state(rtp, RTGS_WAIT_GP); rtp->gp_start = jiffies; rtp->gp_func(rtp); rtp->n_gps++; /* Invoke the callbacks. */ set_tasks_gp_state(rtp, RTGS_INVOKE_CBS); while (list) { next = list->next; local_bh_disable(); list->func(list); local_bh_enable(); list = next; cond_resched(); } /* Paranoid sleep to keep this from entering a tight loop */ schedule_timeout_idle(rtp->gp_sleep); } } /* 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) 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); #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) { pr_info("%s: %s(%d) since %lu g:%lu i:%lu/%lu %c%c %s\n", rtp->kname, tasks_gp_state_getname(rtp), data_race(rtp->gp_state), jiffies - data_race(rtp->gp_jiffies), data_race(rtp->n_gps), data_race(rtp->n_ipis_fails), data_race(rtp->n_ipis), ".k"[!!data_race(rtp->kthread_ptr)], ".C"[!!data_race(rtp->cbs_head)], 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, *t; unsigned long lastreport; LIST_HEAD(holdouts); int fract; set_tasks_gp_state(rtp, RTGS_PRE_WAIT_GP); rtp->pregp_func(); /* * 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); 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; // Start off with initial wait and slowly back off to 1 HZ wait. fract = rtp->init_fract; if (fract > HZ) fract = HZ; for (;;) { bool firstreport; bool needreport; int rtst; if (list_empty(&holdouts)) break; /* Slowly back off waiting for holdouts */ set_tasks_gp_state(rtp, RTGS_WAIT_SCAN_HOLDOUTS); schedule_timeout_idle(HZ/fract); if (fract > 1) fract--; rtst = READ_ONCE(rcu_task_stall_timeout); needreport = rtst > 0 && time_after(jiffies, lastreport + rtst); if (needreport) lastreport = jiffies; firstreport = true; WARN_ON(signal_pending(current)); set_tasks_gp_state(rtp, RTGS_SCAN_HOLDOUTS); rtp->holdouts_func(&holdouts, needreport, &firstreport); } 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_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. /* Pre-grace-period preparation. */ static void rcu_tasks_pregp_step(void) { /* * 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(); } /* Per-task initial processing. */ static void rcu_tasks_pertask(struct task_struct *t, struct list_head *hop) { if (t != current && READ_ONCE(t->on_rq) && !is_idle_task(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) { /* * 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); } /* 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) || !READ_ONCE(t->on_rq) || (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"); /** * 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_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-strcuture 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) { /* There is only one callback queue, so this is easy. ;-) */ synchronize_rcu_tasks(); } EXPORT_SYMBOL_GPL(rcu_barrier_tasks); static int __init rcu_spawn_tasks_kthread(void) { rcu_tasks.gp_sleep = HZ / 10; rcu_tasks.init_fract = 10; 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; } #ifndef CONFIG_TINY_RCU static void show_rcu_tasks_classic_gp_kthread(void) { show_rcu_tasks_generic_gp_kthread(&rcu_tasks, ""); } #endif /* #ifndef CONFIG_TINY_RCU */ /* * 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 */ static inline void show_rcu_tasks_classic_gp_kthread(void) { } 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_rude() API. // This sends IPIs far and wide and induces otherwise unnecessary context // switches on all online CPUs, whether idle or not. // 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_rcu_qs(), 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-strcuture 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, 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) { /* There is only one callback queue, so this is easy. ;-) */ synchronize_rcu_tasks_rude(); } EXPORT_SYMBOL_GPL(rcu_barrier_tasks_rude); static int __init rcu_spawn_tasks_rude_kthread(void) { rcu_tasks_rude.gp_sleep = HZ / 10; rcu_spawn_tasks_kthread_generic(&rcu_tasks_rude); return 0; } #ifndef CONFIG_TINY_RCU static void show_rcu_tasks_rude_gp_kthread(void) { show_rcu_tasks_generic_gp_kthread(&rcu_tasks_rude, ""); } #endif /* #ifndef CONFIG_TINY_RCU */ #else /* #ifdef CONFIG_TASKS_RUDE_RCU */ static void show_rcu_tasks_rude_gp_kthread(void) {} #endif /* #else #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 instruction, having overhead similar // to that of Preemptible RCU. // // There are of course downsides. The grace-period code can send IPIs to // CPUs, even when those CPUs are in the idle loop or in nohz_full userspace. // It is necessary to scan the full tasklist, much as for Tasks RCU. There // is a single callback queue guarded by a single lock, again, much as for // Tasks RCU. 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 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 static atomic_t trc_n_readers_need_end; // Number of waited-for readers. static DECLARE_WAIT_QUEUE_HEAD(trc_wait); // List of holdout tasks. // 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; 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"); /* * This irq_work handler allows rcu_read_unlock_trace() to be invoked * while the scheduler locks are held. */ static void rcu_read_unlock_iw(struct irq_work *iwp) { wake_up(&trc_wait); } static DEFINE_IRQ_WORK(rcu_tasks_trace_iw, rcu_read_unlock_iw); /* If we are the last reader, wake up the grace-period kthread. */ void rcu_read_unlock_trace_special(struct task_struct *t, int nesting) { int nq = READ_ONCE(t->trc_reader_special.b.need_qs); 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 (nq) WRITE_ONCE(t->trc_reader_special.b.need_qs, false); WRITE_ONCE(t->trc_reader_nesting, nesting); if (nq && atomic_dec_and_test(&trc_n_readers_need_end)) irq_work_queue(&rcu_tasks_trace_iw); } EXPORT_SYMBOL_GPL(rcu_read_unlock_trace_special); /* 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); } } /* 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); } } /* IPI handler to check task state. */ static void trc_read_check_handler(void *t_in) { 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. if (likely(!READ_ONCE(t->trc_reader_nesting))) { WRITE_ONCE(t->trc_reader_checked, true); goto reset_ipi; } // If we are racing with an rcu_read_unlock_trace(), try again later. if (unlikely(READ_ONCE(t->trc_reader_nesting) < 0)) goto reset_ipi; WRITE_ONCE(t->trc_reader_checked, true); // Get here if the task is in a read-side critical section. Set // its state so that it will awaken the grace-period kthread upon // exit from that critical section. atomic_inc(&trc_n_readers_need_end); // One more to wait on. WARN_ON_ONCE(READ_ONCE(t->trc_reader_special.b.need_qs)); WRITE_ONCE(t->trc_reader_special.b.need_qs, true); 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(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 bool trc_inspect_reader(struct task_struct *t, void *arg) { int cpu = task_cpu(t); int nesting; bool ofl = cpu_is_offline(cpu); if (task_curr(t)) { WARN_ON_ONCE(ofl && !is_idle_task(t)); // If no chance of heavyweight readers, do it the hard way. if (!ofl && !IS_ENABLED(CONFIG_TASKS_TRACE_RCU_READ_MB)) return false; // 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++; if (!ofl && // Check for "running" idle tasks on offline CPUs. !rcu_dynticks_zero_in_eqs(cpu, &t->trc_reader_nesting)) return false; // No quiescent state, do it the hard way. n_heavy_reader_updates++; if (ofl) n_heavy_reader_ofl_updates++; nesting = 0; } else { // The task is not running, so C-language access is safe. nesting = t->trc_reader_nesting; } // If not exiting a read-side critical section, mark as checked // so that the grace-period kthread will remove it from the // holdout list. t->trc_reader_checked = nesting >= 0; if (nesting <= 0) return !nesting; // If in QS, done, otherwise try again later. // The task is in a read-side critical section, so set up its // state so that it will awaken the grace-period kthread upon exit // from that critical section. atomic_inc(&trc_n_readers_need_end); // One more to wait on. WARN_ON_ONCE(READ_ONCE(t->trc_reader_special.b.need_qs)); WRITE_ONCE(t->trc_reader_special.b.need_qs, true); return true; } /* 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) { t->trc_reader_checked = true; WARN_ON_ONCE(READ_ONCE(t->trc_reader_nesting)); return; } // Attempt to nail down the task for inspection. get_task_struct(t); if (try_invoke_on_locked_down_task(t, trc_inspect_reader, NULL)) { put_task_struct(t); return; } put_task_struct(t); // 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 a new RCU-tasks-trace grace period. */ static void rcu_tasks_trace_pregp_step(void) { int cpu; // Allow for fast-acting IPIs. atomic_set(&trc_n_readers_need_end, 1); // 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 tasklist scan. // This also waits for all readers in CPU-hotplug code paths. cpus_read_lock(); } /* Do first-round processing for the specified task. */ static void rcu_tasks_trace_pertask(struct task_struct *t, struct list_head *hop) { // During early boot when there is only the one boot CPU, there // is no idle task for the other CPUs. Just return. if (unlikely(t == NULL)) return; WRITE_ONCE(t->trc_reader_special.b.need_qs, false); WRITE_ONCE(t->trc_reader_checked, false); t->trc_ipi_to_cpu = -1; trc_wait_for_one_reader(t, hop); } /* * Do intermediate processing between task and holdout scans and * pick up the idle tasks. */ static void rcu_tasks_trace_postscan(struct list_head *hop) { int cpu; for_each_possible_cpu(cpu) rcu_tasks_trace_pertask(idle_task(cpu), hop); // Re-enable CPU hotplug now that the tasklist scan has completed. cpus_read_unlock(); // 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_reader_checked. } /* 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; if (*firstreport) { pr_err("INFO: rcu_tasks_trace detected stalls on tasks:\n"); *firstreport = false; } // FIXME: This should attempt to use try_invoke_on_nonrunning_task(). cpu = task_cpu(t); pr_alert("P%d: %c%c%c nesting: %d%c cpu: %d\n", t->pid, ".I"[READ_ONCE(t->trc_ipi_to_cpu) > 0], ".i"[is_idle_task(t)], ".N"[cpu > 0 && tick_nohz_full_cpu(cpu)], READ_ONCE(t->trc_reader_nesting), " N"[!!READ_ONCE(t->trc_reader_special.b.need_qs)], cpu); 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. 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 && !READ_ONCE(t->trc_reader_checked)) trc_wait_for_one_reader(t, hop); // If check succeeded, remove this task from the list. if (READ_ONCE(t->trc_reader_checked)) trc_del_holdout(t); else if (needreport) show_stalled_task_trace(t, firstreport); } // 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; bool firstreport; struct task_struct *g, *t; LIST_HEAD(holdouts); long ret; // 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 (smp_load_acquire(per_cpu_ptr(&trc_ipi_to_cpu, cpu))) smp_call_function_single(cpu, rcu_tasks_trace_empty_fn, NULL, 1); // Remove the safety count. smp_mb__before_atomic(); // Order vs. earlier atomics atomic_dec(&trc_n_readers_need_end); smp_mb__after_atomic(); // Order vs. later atomics // Wait for readers. set_tasks_gp_state(rtp, RTGS_WAIT_READERS); for (;;) { ret = wait_event_idle_exclusive_timeout( trc_wait, atomic_read(&trc_n_readers_need_end) == 0, READ_ONCE(rcu_task_stall_timeout)); if (ret) break; // Count reached zero. // Stall warning time, so make a list of the offenders. rcu_read_lock(); for_each_process_thread(g, t) if (READ_ONCE(t->trc_reader_special.b.need_qs)) trc_add_holdout(t, &holdouts); rcu_read_unlock(); firstreport = true; list_for_each_entry_safe(t, g, &holdouts, trc_holdout_list) { if (READ_ONCE(t->trc_reader_special.b.need_qs)) show_stalled_task_trace(t, &firstreport); trc_del_holdout(t); // Release task_struct reference. } if (firstreport) pr_err("INFO: rcu_tasks_trace detected stalls? (Counter/taskslist mismatch?)\n"); show_stalled_ipi_trace(); pr_err("\t%d holdouts\n", atomic_read(&trc_n_readers_need_end)); } 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) { WRITE_ONCE(t->trc_reader_checked, true); WARN_ON_ONCE(READ_ONCE(t->trc_reader_nesting)); WRITE_ONCE(t->trc_reader_nesting, 0); if (WARN_ON_ONCE(READ_ONCE(t->trc_reader_special.b.need_qs))) rcu_read_unlock_trace_special(t, 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 full grace * period elapses, in other words after all currently executing RCU * read-side critical sections have completed. call_rcu_tasks_trace() * assumes that the read-side critical sections end at context switch, * cond_resched_rcu_qs(), 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-strcuture synchronization. * * 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 * 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) { /* There is only one callback queue, so this is easy. ;-) */ synchronize_rcu_tasks_trace(); } EXPORT_SYMBOL_GPL(rcu_barrier_tasks_trace); static int __init rcu_spawn_tasks_trace_kthread(void) { if (IS_ENABLED(CONFIG_TASKS_TRACE_RCU_READ_MB)) { rcu_tasks_trace.gp_sleep = HZ / 10; rcu_tasks_trace.init_fract = 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 / 5; if (rcu_tasks_trace.init_fract <= 0) rcu_tasks_trace.init_fract = 1; } rcu_tasks_trace.pregp_func = rcu_tasks_trace_pregp_step; rcu_tasks_trace.pertask_func = rcu_tasks_trace_pertask; 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; } #ifndef CONFIG_TINY_RCU static void show_rcu_tasks_trace_gp_kthread(void) { char buf[64]; sprintf(buf, "N%d h:%lu/%lu/%lu", atomic_read(&trc_n_readers_need_end), 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); } #endif /* #ifndef CONFIG_TINY_RCU */ #else /* #ifdef CONFIG_TASKS_TRACE_RCU */ static void exit_tasks_rcu_finish_trace(struct task_struct *t) { } static inline void show_rcu_tasks_trace_gp_kthread(void) {} #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 */ 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 } #else /* #ifdef CONFIG_TASKS_RCU_GENERIC */ static inline void rcu_tasks_bootup_oddness(void) {} void show_rcu_tasks_gp_kthreads(void) {} #endif /* #else #ifdef CONFIG_TASKS_RCU_GENERIC */