linux/kernel/task_work.c
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   1#include <linux/spinlock.h>
   2#include <linux/task_work.h>
   3#include <linux/tracehook.h>
   4
   5static struct callback_head work_exited; /* all we need is ->next == NULL */
   6
   7/**
   8 * task_work_add - ask the @task to execute @work->func()
   9 * @task: the task which should run the callback
  10 * @work: the callback to run
  11 * @notify: send the notification if true
  12 *
  13 * Queue @work for task_work_run() below and notify the @task if @notify.
  14 * Fails if the @task is exiting/exited and thus it can't process this @work.
  15 * Otherwise @work->func() will be called when the @task returns from kernel
  16 * mode or exits.
  17 *
  18 * This is like the signal handler which runs in kernel mode, but it doesn't
  19 * try to wake up the @task.
  20 *
  21 * RETURNS:
  22 * 0 if succeeds or -ESRCH.
  23 */
  24int
  25task_work_add(struct task_struct *task, struct callback_head *work, bool notify)
  26{
  27        struct callback_head *head;
  28
  29        do {
  30                head = ACCESS_ONCE(task->task_works);
  31                if (unlikely(head == &work_exited))
  32                        return -ESRCH;
  33                work->next = head;
  34        } while (cmpxchg(&task->task_works, head, work) != head);
  35
  36        if (notify)
  37                set_notify_resume(task);
  38        return 0;
  39}
  40
  41/**
  42 * task_work_cancel - cancel a pending work added by task_work_add()
  43 * @task: the task which should execute the work
  44 * @func: identifies the work to remove
  45 *
  46 * Find the last queued pending work with ->func == @func and remove
  47 * it from queue.
  48 *
  49 * RETURNS:
  50 * The found work or NULL if not found.
  51 */
  52struct callback_head *
  53task_work_cancel(struct task_struct *task, task_work_func_t func)
  54{
  55        struct callback_head **pprev = &task->task_works;
  56        struct callback_head *work;
  57        unsigned long flags;
  58        /*
  59         * If cmpxchg() fails we continue without updating pprev.
  60         * Either we raced with task_work_add() which added the
  61         * new entry before this work, we will find it again. Or
  62         * we raced with task_work_run(), *pprev == NULL/exited.
  63         */
  64        raw_spin_lock_irqsave(&task->pi_lock, flags);
  65        while ((work = ACCESS_ONCE(*pprev))) {
  66                smp_read_barrier_depends();
  67                if (work->func != func)
  68                        pprev = &work->next;
  69                else if (cmpxchg(pprev, work, work->next) == work)
  70                        break;
  71        }
  72        raw_spin_unlock_irqrestore(&task->pi_lock, flags);
  73
  74        return work;
  75}
  76
  77/**
  78 * task_work_run - execute the works added by task_work_add()
  79 *
  80 * Flush the pending works. Should be used by the core kernel code.
  81 * Called before the task returns to the user-mode or stops, or when
  82 * it exits. In the latter case task_work_add() can no longer add the
  83 * new work after task_work_run() returns.
  84 */
  85void task_work_run(void)
  86{
  87        struct task_struct *task = current;
  88        struct callback_head *work, *head, *next;
  89
  90        for (;;) {
  91                /*
  92                 * work->func() can do task_work_add(), do not set
  93                 * work_exited unless the list is empty.
  94                 */
  95                do {
  96                        work = ACCESS_ONCE(task->task_works);
  97                        head = !work && (task->flags & PF_EXITING) ?
  98                                &work_exited : NULL;
  99                } while (cmpxchg(&task->task_works, work, head) != work);
 100
 101                if (!work)
 102                        break;
 103                /*
 104                 * Synchronize with task_work_cancel(). It can't remove
 105                 * the first entry == work, cmpxchg(task_works) should
 106                 * fail, but it can play with *work and other entries.
 107                 */
 108                raw_spin_unlock_wait(&task->pi_lock);
 109                smp_mb();
 110
 111                /* Reverse the list to run the works in fifo order */
 112                head = NULL;
 113                do {
 114                        next = work->next;
 115                        work->next = head;
 116                        head = work;
 117                        work = next;
 118                } while (work);
 119
 120                work = head;
 121                do {
 122                        next = work->next;
 123                        work->func(work);
 124                        work = next;
 125                        cond_resched();
 126                } while (work);
 127        }
 128}
 129
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