Commit 61e96496 authored by Oleg Nesterov's avatar Oleg Nesterov Committed by Linus Torvalds
Browse files

task_work: use READ_ONCE/lockless_dereference, avoid pi_lock if !task_works 

Change task_work_cancel() to use lockless_dereference(), this is what
the code really wants but we didn't have this helper when it was
written.

Also add the fast-path task->task_works == NULL check, in the likely
case this task has no pending works and we can avoid
spin_lock(task->pi_lock).

While at it, change other users of ACCESS_ONCE() to use READ_ONCE().

Link: http://lkml.kernel.org/r/20160610150042.GA13868@redhat.com


Signed-off-by: default avatarOleg Nesterov <oleg@redhat.com>
Cc: Andrea Parri <parri.andrea@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 949bed2f

kernel/task_work.c

+6 −4
Original line number Diff line number Diff line
@@ -29,7 +29,7 @@ task_work_add(struct task_struct *task, struct callback_head *work, bool notify) 
	struct callback_head *head;



	do {

		head = ACCESS_ONCE(task->task_works);

		head = READ_ONCE(task->task_works);

		if (unlikely(head == &work_exited))

			return -ESRCH;

		work->next = head;
@@ -57,6 +57,9 @@ task_work_cancel(struct task_struct *task, task_work_func_t func) 
	struct callback_head **pprev = &task->task_works;

	struct callback_head *work;

	unsigned long flags;



	if (likely(!task->task_works))

		return NULL;

	/*

	 * If cmpxchg() fails we continue without updating pprev.

	 * Either we raced with task_work_add() which added the
@@ -64,8 +67,7 @@ task_work_cancel(struct task_struct *task, task_work_func_t func) 
	 * we raced with task_work_run(), *pprev == NULL/exited.

	 */

	raw_spin_lock_irqsave(&task->pi_lock, flags);

	while ((work = ACCESS_ONCE(*pprev))) {

		smp_read_barrier_depends();

	while ((work = lockless_dereference(*pprev))) {

		if (work->func != func)

			pprev = &work->next;

		else if (cmpxchg(pprev, work, work->next) == work)
@@ -95,7 +97,7 @@ void task_work_run(void) 
		 * work_exited unless the list is empty.

		 */

		do {

			work = ACCESS_ONCE(task->task_works);

			work = READ_ONCE(task->task_works);

			head = !work && (task->flags & PF_EXITING) ?

				&work_exited : NULL;

		} while (cmpxchg(&task->task_works, work, head) != work);

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