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[mirror_ubuntu-bionic-kernel.git] / kernel / rcu / update.c

/*
 * 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 <dipankar@in.ibm.com>
 *          Manfred Spraul <manfred@colorfullife.com>
 *
 * Based on the original work by Paul McKenney <paulmck@us.ibm.com>
 * 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 <linux/types.h>
#include <linux/kernel.h>
#include <linux/init.h>
#include <linux/spinlock.h>
#include <linux/smp.h>
#include <linux/interrupt.h>
#include <linux/sched/signal.h>
#include <linux/sched/debug.h>
#include <linux/atomic.h>
#include <linux/bitops.h>
#include <linux/percpu.h>
#include <linux/notifier.h>
#include <linux/cpu.h>
#include <linux/mutex.h>
#include <linux/export.h>
#include <linux/hardirq.h>
#include <linux/delay.h>
#include <linux/moduleparam.h>
#include <linux/kthread.h>
#include <linux/tick.h>
#include <linux/rcupdate_wait.h>
#include <linux/sched/isolation.h>

#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_PREEMPT_RCU

/*
 * Preemptible RCU implementation for rcu_read_lock().
 * Just increment ->rcu_read_lock_nesting, shared state will be updated
 * if we block.
 */
void __rcu_read_lock(void)
{
        current->rcu_read_lock_nesting++;
        barrier();  /* critical section after entry code. */
}
EXPORT_SYMBOL_GPL(__rcu_read_lock);

/*
 * Preemptible RCU implementation for rcu_read_unlock().
 * Decrement ->rcu_read_lock_nesting.  If the result is zero (outermost
 * rcu_read_unlock()) and ->rcu_read_unlock_special is non-zero, then
 * invoke rcu_read_unlock_special() to clean up after a context switch
 * in an RCU read-side critical section and other special cases.
 */
void __rcu_read_unlock(void)
{
        struct task_struct *t = current;

        if (t->rcu_read_lock_nesting != 1) {
                --t->rcu_read_lock_nesting;
        } else {
                barrier();  /* critical section before exit code. */
                t->rcu_read_lock_nesting = INT_MIN;
                barrier();  /* assign before ->rcu_read_unlock_special load */
                if (unlikely(READ_ONCE(t->rcu_read_unlock_special.s)))
                        rcu_read_unlock_special(t);
                barrier();  /* ->rcu_read_unlock_special load before assign */
                t->rcu_read_lock_nesting = 0;
        }
#ifdef CONFIG_PROVE_LOCKING
        {
                int rrln = READ_ONCE(t->rcu_read_lock_nesting);

                WARN_ON_ONCE(rrln < 0 && rrln > INT_MIN / 2);
        }
#endif /* #ifdef CONFIG_PROVE_LOCKING */
}
EXPORT_SYMBOL_GPL(__rcu_read_unlock);

#endif /* #ifdef CONFIG_PREEMPT_RCU */

#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);

/**
 * 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,
 * 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!), 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_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);

        /* 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;
                while (!list_empty(&rcu_tasks_holdouts)) {
                        bool firstreport;
                        bool needreport;
                        int rtst;
                        struct task_struct *t1;

                        schedule_timeout_interruptible(HZ);
                        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();
                }
                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 */