/* * Read-Copy Update mechanism for mutual exclusion * * This program is free software; you can redistribute it and/or modify * it under the terms of the GNU General Public License as published by * the Free Software Foundation; either version 2 of the License, or * (at your option) any later version. * * This program is distributed in the hope that it will be useful, * but WITHOUT ANY WARRANTY; without even the implied warranty of * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the * GNU General Public License for more details. * * You should have received a copy of the GNU General Public License * along with this program; if not, you can access it online at * http://www.gnu.org/licenses/gpl-2.0.html. * * Copyright IBM Corporation, 2001 * * Authors: Dipankar Sarma * Manfred Spraul * * Based on the original work by Paul McKenney * and inputs from Rusty Russell, Andrea Arcangeli and Andi Kleen. * Papers: * http://www.rdrop.com/users/paulmck/paper/rclockpdcsproof.pdf * http://lse.sourceforge.net/locking/rclock_OLS.2001.05.01c.sc.pdf (OLS2001) * * For detailed explanation of Read-Copy Update mechanism see - * http://lse.sourceforge.net/locking/rcupdate.html * */ #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #define CREATE_TRACE_POINTS #include "rcu.h" #ifdef MODULE_PARAM_PREFIX #undef MODULE_PARAM_PREFIX #endif #define MODULE_PARAM_PREFIX "rcupdate." #ifndef CONFIG_TINY_RCU extern int rcu_expedited; /* from sysctl */ module_param(rcu_expedited, int, 0); extern int rcu_normal; /* from sysctl */ module_param(rcu_normal, int, 0); static int rcu_normal_after_boot; module_param(rcu_normal_after_boot, int, 0); #endif /* #ifndef CONFIG_TINY_RCU */ #ifdef CONFIG_DEBUG_LOCK_ALLOC /** * rcu_read_lock_sched_held() - might we be in RCU-sched read-side critical section? * * If CONFIG_DEBUG_LOCK_ALLOC is selected, returns nonzero iff in an * RCU-sched read-side critical section. In absence of * CONFIG_DEBUG_LOCK_ALLOC, this assumes we are in an RCU-sched read-side * critical section unless it can prove otherwise. Note that disabling * of preemption (including disabling irqs) counts as an RCU-sched * read-side critical section. This is useful for debug checks in functions * that required that they be called within an RCU-sched read-side * critical section. * * Check debug_lockdep_rcu_enabled() to prevent false positives during boot * and while lockdep is disabled. * * Note that if the CPU is in the idle loop from an RCU point of * view (ie: that we are in the section between rcu_idle_enter() and * rcu_idle_exit()) then rcu_read_lock_held() returns false even if the CPU * did an rcu_read_lock(). The reason for this is that RCU ignores CPUs * that are in such a section, considering these as in extended quiescent * state, so such a CPU is effectively never in an RCU read-side critical * section regardless of what RCU primitives it invokes. This state of * affairs is required --- we need to keep an RCU-free window in idle * where the CPU may possibly enter into low power mode. This way we can * notice an extended quiescent state to other CPUs that started a grace * period. Otherwise we would delay any grace period as long as we run in * the idle task. * * Similarly, we avoid claiming an SRCU read lock held if the current * CPU is offline. */ int rcu_read_lock_sched_held(void) { int lockdep_opinion = 0; if (!debug_lockdep_rcu_enabled()) return 1; if (!rcu_is_watching()) return 0; if (!rcu_lockdep_current_cpu_online()) return 0; if (debug_locks) lockdep_opinion = lock_is_held(&rcu_sched_lock_map); return lockdep_opinion || !preemptible(); } EXPORT_SYMBOL(rcu_read_lock_sched_held); #endif #ifndef CONFIG_TINY_RCU /* * Should expedited grace-period primitives always fall back to their * non-expedited counterparts? Intended for use within RCU. Note * that if the user specifies both rcu_expedited and rcu_normal, then * rcu_normal wins. (Except during the time period during boot from * when the first task is spawned until the rcu_set_runtime_mode() * core_initcall() is invoked, at which point everything is expedited.) */ bool rcu_gp_is_normal(void) { return READ_ONCE(rcu_normal) && rcu_scheduler_active != RCU_SCHEDULER_INIT; } EXPORT_SYMBOL_GPL(rcu_gp_is_normal); static atomic_t rcu_expedited_nesting = ATOMIC_INIT(1); /* * Should normal grace-period primitives be expedited? Intended for * use within RCU. Note that this function takes the rcu_expedited * sysfs/boot variable and rcu_scheduler_active into account as well * as the rcu_expedite_gp() nesting. So looping on rcu_unexpedite_gp() * until rcu_gp_is_expedited() returns false is a -really- bad idea. */ bool rcu_gp_is_expedited(void) { return rcu_expedited || atomic_read(&rcu_expedited_nesting) || rcu_scheduler_active == RCU_SCHEDULER_INIT; } EXPORT_SYMBOL_GPL(rcu_gp_is_expedited); /** * rcu_expedite_gp - Expedite future RCU grace periods * * After a call to this function, future calls to synchronize_rcu() and * friends act as the corresponding synchronize_rcu_expedited() function * had instead been called. */ void rcu_expedite_gp(void) { atomic_inc(&rcu_expedited_nesting); } EXPORT_SYMBOL_GPL(rcu_expedite_gp); /** * rcu_unexpedite_gp - Cancel prior rcu_expedite_gp() invocation * * Undo a prior call to rcu_expedite_gp(). If all prior calls to * rcu_expedite_gp() are undone by a subsequent call to rcu_unexpedite_gp(), * and if the rcu_expedited sysfs/boot parameter is not set, then all * subsequent calls to synchronize_rcu() and friends will return to * their normal non-expedited behavior. */ void rcu_unexpedite_gp(void) { atomic_dec(&rcu_expedited_nesting); } EXPORT_SYMBOL_GPL(rcu_unexpedite_gp); /* * Inform RCU of the end of the in-kernel boot sequence. */ void rcu_end_inkernel_boot(void) { rcu_unexpedite_gp(); if (rcu_normal_after_boot) WRITE_ONCE(rcu_normal, 1); } #endif /* #ifndef CONFIG_TINY_RCU */ /* * Test each non-SRCU synchronous grace-period wait API. This is * useful just after a change in mode for these primitives, and * during early boot. */ void rcu_test_sync_prims(void) { if (!IS_ENABLED(CONFIG_PROVE_RCU)) return; synchronize_rcu(); synchronize_rcu_bh(); synchronize_sched(); synchronize_rcu_expedited(); synchronize_rcu_bh_expedited(); synchronize_sched_expedited(); } #if !defined(CONFIG_TINY_RCU) || defined(CONFIG_SRCU) /* * Switch to run-time mode once RCU has fully initialized. */ static int __init rcu_set_runtime_mode(void) { rcu_test_sync_prims(); rcu_scheduler_active = RCU_SCHEDULER_RUNNING; rcu_test_sync_prims(); return 0; } core_initcall(rcu_set_runtime_mode); #endif /* #if !defined(CONFIG_TINY_RCU) || defined(CONFIG_SRCU) */ #ifdef CONFIG_DEBUG_LOCK_ALLOC static struct lock_class_key rcu_lock_key; struct lockdep_map rcu_lock_map = STATIC_LOCKDEP_MAP_INIT("rcu_read_lock", &rcu_lock_key); EXPORT_SYMBOL_GPL(rcu_lock_map); static struct lock_class_key rcu_bh_lock_key; struct lockdep_map rcu_bh_lock_map = STATIC_LOCKDEP_MAP_INIT("rcu_read_lock_bh", &rcu_bh_lock_key); EXPORT_SYMBOL_GPL(rcu_bh_lock_map); static struct lock_class_key rcu_sched_lock_key; struct lockdep_map rcu_sched_lock_map = STATIC_LOCKDEP_MAP_INIT("rcu_read_lock_sched", &rcu_sched_lock_key); EXPORT_SYMBOL_GPL(rcu_sched_lock_map); static struct lock_class_key rcu_callback_key; struct lockdep_map rcu_callback_map = STATIC_LOCKDEP_MAP_INIT("rcu_callback", &rcu_callback_key); EXPORT_SYMBOL_GPL(rcu_callback_map); int notrace debug_lockdep_rcu_enabled(void) { return rcu_scheduler_active != RCU_SCHEDULER_INACTIVE && debug_locks && current->lockdep_recursion == 0; } EXPORT_SYMBOL_GPL(debug_lockdep_rcu_enabled); NOKPROBE_SYMBOL(debug_lockdep_rcu_enabled); /** * rcu_read_lock_held() - might we be in RCU read-side critical section? * * If CONFIG_DEBUG_LOCK_ALLOC is selected, returns nonzero iff in an RCU * read-side critical section. In absence of CONFIG_DEBUG_LOCK_ALLOC, * this assumes we are in an RCU read-side critical section unless it can * prove otherwise. This is useful for debug checks in functions that * require that they be called within an RCU read-side critical section. * * Checks debug_lockdep_rcu_enabled() to prevent false positives during boot * and while lockdep is disabled. * * Note that rcu_read_lock() and the matching rcu_read_unlock() must * occur in the same context, for example, it is illegal to invoke * rcu_read_unlock() in process context if the matching rcu_read_lock() * was invoked from within an irq handler. * * Note that rcu_read_lock() is disallowed if the CPU is either idle or * offline from an RCU perspective, so check for those as well. */ int rcu_read_lock_held(void) { if (!debug_lockdep_rcu_enabled()) return 1; if (!rcu_is_watching()) return 0; if (!rcu_lockdep_current_cpu_online()) return 0; return lock_is_held(&rcu_lock_map); } EXPORT_SYMBOL_GPL(rcu_read_lock_held); /** * rcu_read_lock_bh_held() - might we be in RCU-bh read-side critical section? * * Check for bottom half being disabled, which covers both the * CONFIG_PROVE_RCU and not cases. Note that if someone uses * rcu_read_lock_bh(), but then later enables BH, lockdep (if enabled) * will show the situation. This is useful for debug checks in functions * that require that they be called within an RCU read-side critical * section. * * Check debug_lockdep_rcu_enabled() to prevent false positives during boot. * * Note that rcu_read_lock() is disallowed if the CPU is either idle or * offline from an RCU perspective, so check for those as well. */ int rcu_read_lock_bh_held(void) { if (!debug_lockdep_rcu_enabled()) return 1; if (!rcu_is_watching()) return 0; if (!rcu_lockdep_current_cpu_online()) return 0; return in_softirq() || irqs_disabled(); } EXPORT_SYMBOL_GPL(rcu_read_lock_bh_held); #endif /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */ /** * wakeme_after_rcu() - Callback function to awaken a task after grace period * @head: Pointer to rcu_head member within rcu_synchronize structure * * Awaken the corresponding task now that a grace period has elapsed. */ void wakeme_after_rcu(struct rcu_head *head) { struct rcu_synchronize *rcu; rcu = container_of(head, struct rcu_synchronize, head); complete(&rcu->completion); } EXPORT_SYMBOL_GPL(wakeme_after_rcu); void __wait_rcu_gp(bool checktiny, int n, call_rcu_func_t *crcu_array, struct rcu_synchronize *rs_array) { int i; int j; /* Initialize and register callbacks for each flavor specified. */ for (i = 0; i < n; i++) { if (checktiny && (crcu_array[i] == call_rcu || crcu_array[i] == call_rcu_bh)) { might_sleep(); continue; } init_rcu_head_on_stack(&rs_array[i].head); init_completion(&rs_array[i].completion); for (j = 0; j < i; j++) if (crcu_array[j] == crcu_array[i]) break; if (j == i) (crcu_array[i])(&rs_array[i].head, wakeme_after_rcu); } /* Wait for all callbacks to be invoked. */ for (i = 0; i < n; i++) { if (checktiny && (crcu_array[i] == call_rcu || crcu_array[i] == call_rcu_bh)) continue; for (j = 0; j < i; j++) if (crcu_array[j] == crcu_array[i]) break; if (j == i) wait_for_completion(&rs_array[i].completion); destroy_rcu_head_on_stack(&rs_array[i].head); } } EXPORT_SYMBOL_GPL(__wait_rcu_gp); #ifdef CONFIG_DEBUG_OBJECTS_RCU_HEAD void init_rcu_head(struct rcu_head *head) { debug_object_init(head, &rcuhead_debug_descr); } EXPORT_SYMBOL_GPL(init_rcu_head); void destroy_rcu_head(struct rcu_head *head) { debug_object_free(head, &rcuhead_debug_descr); } EXPORT_SYMBOL_GPL(destroy_rcu_head); static bool rcuhead_is_static_object(void *addr) { return true; } /** * init_rcu_head_on_stack() - initialize on-stack rcu_head for debugobjects * @head: pointer to rcu_head structure to be initialized * * This function informs debugobjects of a new rcu_head structure that * has been allocated as an auto variable on the stack. This function * is not required for rcu_head structures that are statically defined or * that are dynamically allocated on the heap. This function has no * effect for !CONFIG_DEBUG_OBJECTS_RCU_HEAD kernel builds. */ void init_rcu_head_on_stack(struct rcu_head *head) { debug_object_init_on_stack(head, &rcuhead_debug_descr); } EXPORT_SYMBOL_GPL(init_rcu_head_on_stack); /** * destroy_rcu_head_on_stack() - destroy on-stack rcu_head for debugobjects * @head: pointer to rcu_head structure to be initialized * * This function informs debugobjects that an on-stack rcu_head structure * is about to go out of scope. As with init_rcu_head_on_stack(), this * function is not required for rcu_head structures that are statically * defined or that are dynamically allocated on the heap. Also as with * init_rcu_head_on_stack(), this function has no effect for * !CONFIG_DEBUG_OBJECTS_RCU_HEAD kernel builds. */ void destroy_rcu_head_on_stack(struct rcu_head *head) { debug_object_free(head, &rcuhead_debug_descr); } EXPORT_SYMBOL_GPL(destroy_rcu_head_on_stack); struct debug_obj_descr rcuhead_debug_descr = { .name = "rcu_head", .is_static_object = rcuhead_is_static_object, }; EXPORT_SYMBOL_GPL(rcuhead_debug_descr); #endif /* #ifdef CONFIG_DEBUG_OBJECTS_RCU_HEAD */ #if defined(CONFIG_TREE_RCU) || defined(CONFIG_PREEMPT_RCU) || defined(CONFIG_RCU_TRACE) void do_trace_rcu_torture_read(const char *rcutorturename, struct rcu_head *rhp, unsigned long secs, unsigned long c_old, unsigned long c) { trace_rcu_torture_read(rcutorturename, rhp, secs, c_old, c); } EXPORT_SYMBOL_GPL(do_trace_rcu_torture_read); #else #define do_trace_rcu_torture_read(rcutorturename, rhp, secs, c_old, c) \ do { } while (0) #endif #ifdef CONFIG_RCU_STALL_COMMON #ifdef CONFIG_PROVE_RCU #define RCU_STALL_DELAY_DELTA (5 * HZ) #else #define RCU_STALL_DELAY_DELTA 0 #endif int rcu_cpu_stall_suppress __read_mostly; /* 1 = suppress stall warnings. */ EXPORT_SYMBOL_GPL(rcu_cpu_stall_suppress); static int rcu_cpu_stall_timeout __read_mostly = CONFIG_RCU_CPU_STALL_TIMEOUT; module_param(rcu_cpu_stall_suppress, int, 0644); module_param(rcu_cpu_stall_timeout, int, 0644); int rcu_jiffies_till_stall_check(void) { int till_stall_check = READ_ONCE(rcu_cpu_stall_timeout); /* * Limit check must be consistent with the Kconfig limits * for CONFIG_RCU_CPU_STALL_TIMEOUT. */ if (till_stall_check < 3) { WRITE_ONCE(rcu_cpu_stall_timeout, 3); till_stall_check = 3; } else if (till_stall_check > 300) { WRITE_ONCE(rcu_cpu_stall_timeout, 300); till_stall_check = 300; } return till_stall_check * HZ + RCU_STALL_DELAY_DELTA; } void rcu_sysrq_start(void) { if (!rcu_cpu_stall_suppress) rcu_cpu_stall_suppress = 2; } void rcu_sysrq_end(void) { if (rcu_cpu_stall_suppress == 2) rcu_cpu_stall_suppress = 0; } static int rcu_panic(struct notifier_block *this, unsigned long ev, void *ptr) { rcu_cpu_stall_suppress = 1; return NOTIFY_DONE; } static struct notifier_block rcu_panic_block = { .notifier_call = rcu_panic, }; static int __init check_cpu_stall_init(void) { atomic_notifier_chain_register(&panic_notifier_list, &rcu_panic_block); return 0; } early_initcall(check_cpu_stall_init); #endif /* #ifdef CONFIG_RCU_STALL_COMMON */ #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. */ /* Global list of callbacks and associated lock. */ static struct rcu_head *rcu_tasks_cbs_head; static struct rcu_head **rcu_tasks_cbs_tail = &rcu_tasks_cbs_head; static DECLARE_WAIT_QUEUE_HEAD(rcu_tasks_cbs_wq); static DEFINE_RAW_SPINLOCK(rcu_tasks_cbs_lock); /* Track exiting tasks in order to allow them to be waited for. */ DEFINE_STATIC_SRCU(tasks_rcu_exit_srcu); /* 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); static struct task_struct *rcu_tasks_kthread_ptr; /** * 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) { unsigned long flags; bool needwake; rhp->next = NULL; rhp->func = func; raw_spin_lock_irqsave(&rcu_tasks_cbs_lock, flags); needwake = !rcu_tasks_cbs_head; *rcu_tasks_cbs_tail = rhp; rcu_tasks_cbs_tail = &rhp->next; raw_spin_unlock_irqrestore(&rcu_tasks_cbs_lock, flags); /* We can't create the thread unless interrupts are enabled. */ if (needwake && READ_ONCE(rcu_tasks_kthread_ptr)) wake_up(&rcu_tasks_cbs_wq); } 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. * * Note that this guarantee implies further memory-ordering guarantees. * On systems with more than one CPU, when synchronize_rcu_tasks() returns, * each CPU is guaranteed to have executed a full memory barrier since the * end of its last RCU-tasks read-side critical section whose beginning * preceded the call to synchronize_rcu_tasks(). In addition, each CPU * having an RCU-tasks read-side critical section that extends beyond * the return from synchronize_rcu_tasks() is guaranteed to have executed * a full memory barrier after the beginning of synchronize_rcu_tasks() * and before the beginning of that RCU-tasks 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_rcu_tasks(), 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_rcu_tasks() -- even if CPU A and CPU B are the same CPU * (but again only if the system has more than one CPU). */ void synchronize_rcu_tasks(void) { /* Complain if the scheduler has not started. */ RCU_LOCKDEP_WARN(rcu_scheduler_active == RCU_SCHEDULER_INACTIVE, "synchronize_rcu_tasks called too soon"); /* Wait for the grace period. */ wait_rcu_gp(call_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); /* 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); } /* RCU-tasks kthread that detects grace periods and invokes callbacks. */ static int __noreturn rcu_tasks_kthread(void *arg) { unsigned long flags; struct task_struct *g, *t; unsigned long lastreport; struct rcu_head *list; struct rcu_head *next; LIST_HEAD(rcu_tasks_holdouts); int fract; /* Run on housekeeping CPUs by default. Sysadm can move if desired. */ housekeeping_affine(current, HK_FLAG_RCU); /* * 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 (;;) { /* Pick up any new callbacks. */ raw_spin_lock_irqsave(&rcu_tasks_cbs_lock, flags); list = rcu_tasks_cbs_head; rcu_tasks_cbs_head = NULL; rcu_tasks_cbs_tail = &rcu_tasks_cbs_head; raw_spin_unlock_irqrestore(&rcu_tasks_cbs_lock, flags); /* If there were none, wait a bit and start over. */ if (!list) { wait_event_interruptible(rcu_tasks_cbs_wq, rcu_tasks_cbs_head); if (!rcu_tasks_cbs_head) { WARN_ON(signal_pending(current)); schedule_timeout_interruptible(HZ/10); } continue; } /* * Wait for all pre-existing t->on_rq and t->nvcsw * transitions to complete. Invoking synchronize_sched() * suffices because all these transitions occur with * interrupts disabled. Without this synchronize_sched(), * a read-side critical section that started before the * grace period might be incorrectly seen as having started * after the grace period. * * This synchronize_sched() also dispenses with the * need for a memory barrier on the first store to * ->rcu_tasks_holdout, as it forces the store to happen * after the beginning of the grace period. */ synchronize_sched(); /* * 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 rcu_tasks_holdouts. */ rcu_read_lock(); for_each_process_thread(g, t) { 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, &rcu_tasks_holdouts); } } rcu_read_unlock(); /* * Wait for tasks that are in the process of exiting. * This does only part of the job, ensuring that all * tasks that were previously exiting reach the point * where they have disabled preemption, allowing the * later synchronize_sched() to finish the job. */ synchronize_srcu(&tasks_rcu_exit_srcu); /* * 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 HZ/10 wait and slowly back off to 1 HZ wait*/ fract = 10; for (;;) { bool firstreport; bool needreport; int rtst; struct task_struct *t1; if (list_empty(&rcu_tasks_holdouts)) break; /* Slowly back off waiting for holdouts */ schedule_timeout_interruptible(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)); list_for_each_entry_safe(t, t1, &rcu_tasks_holdouts, rcu_tasks_holdout_list) { check_holdout_task(t, needreport, &firstreport); cond_resched(); } } /* * 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_sched() * to force the needed ordering on all such CPUs. * * This synchronize_sched() 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_sched() waits for exiting * tasks to complete their final preempt_disable() region * of execution, cleaning up after the synchronize_srcu() * above. */ synchronize_sched(); /* Invoke the callbacks. */ 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_uninterruptible(HZ/10); } } /* Spawn rcu_tasks_kthread() at core_initcall() time. */ static int __init rcu_spawn_tasks_kthread(void) { struct task_struct *t; t = kthread_run(rcu_tasks_kthread, NULL, "rcu_tasks_kthread"); BUG_ON(IS_ERR(t)); smp_mb(); /* Ensure others see full kthread. */ WRITE_ONCE(rcu_tasks_kthread_ptr, t); return 0; } core_initcall(rcu_spawn_tasks_kthread); /* Do the srcu_read_lock() for the above synchronize_srcu(). */ void exit_tasks_rcu_start(void) { preempt_disable(); current->rcu_tasks_idx = __srcu_read_lock(&tasks_rcu_exit_srcu); preempt_enable(); } /* Do the srcu_read_unlock() for the above synchronize_srcu(). */ void exit_tasks_rcu_finish(void) { preempt_disable(); __srcu_read_unlock(&tasks_rcu_exit_srcu, current->rcu_tasks_idx); preempt_enable(); } #endif /* #ifdef CONFIG_TASKS_RCU */ #ifndef CONFIG_TINY_RCU /* * Print any non-default Tasks RCU settings. */ static void __init rcu_tasks_bootup_oddness(void) { #ifdef CONFIG_TASKS_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); else pr_info("\tTasks RCU enabled.\n"); #endif /* #ifdef CONFIG_TASKS_RCU */ } #endif /* #ifndef CONFIG_TINY_RCU */ #ifdef CONFIG_PROVE_RCU /* * Early boot self test parameters, one for each flavor */ static bool rcu_self_test; static bool rcu_self_test_bh; static bool rcu_self_test_sched; module_param(rcu_self_test, bool, 0444); module_param(rcu_self_test_bh, bool, 0444); module_param(rcu_self_test_sched, bool, 0444); static int rcu_self_test_counter; static void test_callback(struct rcu_head *r) { rcu_self_test_counter++; pr_info("RCU test callback executed %d\n", rcu_self_test_counter); } static void early_boot_test_call_rcu(void) { static struct rcu_head head; call_rcu(&head, test_callback); } static void early_boot_test_call_rcu_bh(void) { static struct rcu_head head; call_rcu_bh(&head, test_callback); } static void early_boot_test_call_rcu_sched(void) { static struct rcu_head head; call_rcu_sched(&head, test_callback); } void rcu_early_boot_tests(void) { pr_info("Running RCU self tests\n"); if (rcu_self_test) early_boot_test_call_rcu(); if (rcu_self_test_bh) early_boot_test_call_rcu_bh(); if (rcu_self_test_sched) early_boot_test_call_rcu_sched(); rcu_test_sync_prims(); } static int rcu_verify_early_boot_tests(void) { int ret = 0; int early_boot_test_counter = 0; if (rcu_self_test) { early_boot_test_counter++; rcu_barrier(); } if (rcu_self_test_bh) { early_boot_test_counter++; rcu_barrier_bh(); } if (rcu_self_test_sched) { early_boot_test_counter++; rcu_barrier_sched(); } if (rcu_self_test_counter != early_boot_test_counter) { WARN_ON(1); ret = -1; } return ret; } late_initcall(rcu_verify_early_boot_tests); #else void rcu_early_boot_tests(void) {} #endif /* CONFIG_PROVE_RCU */ #ifndef CONFIG_TINY_RCU /* * Print any significant non-default boot-time settings. */ void __init rcupdate_announce_bootup_oddness(void) { if (rcu_normal) pr_info("\tNo expedited grace period (rcu_normal).\n"); else if (rcu_normal_after_boot) pr_info("\tNo expedited grace period (rcu_normal_after_boot).\n"); else if (rcu_expedited) pr_info("\tAll grace periods are expedited (rcu_expedited).\n"); if (rcu_cpu_stall_suppress) pr_info("\tRCU CPU stall warnings suppressed (rcu_cpu_stall_suppress).\n"); if (rcu_cpu_stall_timeout != CONFIG_RCU_CPU_STALL_TIMEOUT) pr_info("\tRCU CPU stall warnings timeout set to %d (rcu_cpu_stall_timeout).\n", rcu_cpu_stall_timeout); rcu_tasks_bootup_oddness(); } #endif /* #ifndef CONFIG_TINY_RCU */