task_work: Make task_work_add() lockless
Change task_work's to use llist-like code to avoid pi_lock in task_work_add(), this makes it useable under rq->lock. task_work_cancel() and task_work_run() still use pi_lock to synchronize with each other. (This is in preparation for a deadlock fix.) Suggested-by: Peter Zijlstra <peterz@infradead.org> Signed-off-by: Oleg Nesterov <oleg@redhat.com> Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl> Cc: Al Viro <viro@zeniv.linux.org.uk> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Thomas Gleixner <tglx@linutronix.de> Link: http://lkml.kernel.org/r/20120826191209.GA4221@redhat.com Signed-off-by: Ingo Molnar <mingo@kernel.org>
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@ -3,25 +3,18 @@
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#include <linux/tracehook.h>
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int
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task_work_add(struct task_struct *task, struct callback_head *twork, bool notify)
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task_work_add(struct task_struct *task, struct callback_head *work, bool notify)
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{
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struct callback_head *last, *first;
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unsigned long flags;
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struct callback_head *head;
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/*
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* Not inserting the new work if the task has already passed
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* exit_task_work() is the responisbility of callers.
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*/
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raw_spin_lock_irqsave(&task->pi_lock, flags);
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last = task->task_works;
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first = last ? last->next : twork;
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twork->next = first;
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if (last)
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last->next = twork;
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task->task_works = twork;
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raw_spin_unlock_irqrestore(&task->pi_lock, flags);
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do {
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head = ACCESS_ONCE(task->task_works);
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work->next = head;
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} while (cmpxchg(&task->task_works, head, work) != head);
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/* test_and_set_bit() implies mb(), see tracehook_notify_resume(). */
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if (notify)
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set_notify_resume(task);
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return 0;
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@ -30,52 +23,60 @@ task_work_add(struct task_struct *task, struct callback_head *twork, bool notify
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struct callback_head *
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task_work_cancel(struct task_struct *task, task_work_func_t func)
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{
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struct callback_head **pprev = &task->task_works;
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struct callback_head *work = NULL;
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unsigned long flags;
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struct callback_head *last, *res = NULL;
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/*
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* If cmpxchg() fails we continue without updating pprev.
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* Either we raced with task_work_add() which added the
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* new entry before this work, we will find it again. Or
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* we raced with task_work_run(), *pprev == NULL.
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*/
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raw_spin_lock_irqsave(&task->pi_lock, flags);
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last = task->task_works;
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if (last) {
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struct callback_head *q = last, *p = q->next;
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while (1) {
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if (p->func == func) {
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q->next = p->next;
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if (p == last)
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task->task_works = q == p ? NULL : q;
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res = p;
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break;
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}
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if (p == last)
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break;
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q = p;
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p = q->next;
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}
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while ((work = ACCESS_ONCE(*pprev))) {
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read_barrier_depends();
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if (work->func != func)
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pprev = &work->next;
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else if (cmpxchg(pprev, work, work->next) == work)
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break;
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}
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raw_spin_unlock_irqrestore(&task->pi_lock, flags);
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return res;
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return work;
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}
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void task_work_run(void)
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{
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struct task_struct *task = current;
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struct callback_head *p, *q;
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struct callback_head *work, *head, *next;
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while (1) {
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raw_spin_lock_irq(&task->pi_lock);
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p = task->task_works;
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task->task_works = NULL;
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raw_spin_unlock_irq(&task->pi_lock);
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for (;;) {
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work = xchg(&task->task_works, NULL);
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if (!work)
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break;
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/*
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* Synchronize with task_work_cancel(). It can't remove
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* the first entry == work, cmpxchg(task_works) should
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* fail, but it can play with *work and other entries.
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*/
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raw_spin_unlock_wait(&task->pi_lock);
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smp_mb();
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if (unlikely(!p))
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return;
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/* Reverse the list to run the works in fifo order */
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head = NULL;
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do {
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next = work->next;
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work->next = head;
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head = work;
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work = next;
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} while (work);
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q = p->next; /* head */
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p->next = NULL; /* cut it */
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while (q) {
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p = q->next;
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q->func(q);
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q = p;
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work = head;
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do {
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next = work->next;
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work->func(work);
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work = next;
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cond_resched();
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}
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} while (work);
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}
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}
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