Commit 741a76b3 authored Apr 30, 2018 by Peter Zijlstra Committed by Ingo Molnar May 03, 2018

kthread, sched/wait: Fix kthread_parkme() wait-loop



Gaurav reported a problem with __kthread_parkme() where a concurrent
try_to_wake_up() could result in competing stores to ->state which,
when the TASK_PARKED store got lost bad things would happen.

The comment near set_current_state() actually mentions this competing
store, but only mentions the case against TASK_RUNNING. This same
store, with different timing, can happen against a subsequent !RUNNING
store.

This normally is not a problem, because as per that same comment, the
!RUNNING state store is inside a condition based wait-loop:

  for (;;) {
    set_current_state(TASK_UNINTERRUPTIBLE);
    if (!need_sleep)
      break;
    schedule();
  }
  __set_current_state(TASK_RUNNING);

If we loose the (first) TASK_UNINTERRUPTIBLE store to a previous
(concurrent) wakeup, the schedule() will NO-OP and we'll go around the
loop once more.

The problem here is that the TASK_PARKED store is not inside the
KTHREAD_SHOULD_PARK condition wait-loop.

There is a genuine issue with sleeps that do not have a condition;
this is addressed in a subsequent patch.

Reported-by: Gaurav Kohli <gkohli@codeaurora.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>

parent 457be908

kernel/kthread.c

+4 −3

Original line number	Diff line number	Diff line
		@@ -177,12 +177,13 @@ void kthread_probe_data(struct task_struct task)

		static void __kthread_parkme(struct kthread *self)
		{
		__set_current_state(TASK_PARKED);
		while (test_bit(KTHREAD_SHOULD_PARK, &self->flags)) {
		for (;;) {
		set_current_state(TASK_PARKED);
		if (!test_bit(KTHREAD_SHOULD_PARK, &self->flags))
		break;
		if (!test_and_set_bit(KTHREAD_IS_PARKED, &self->flags))
		complete(&self->parked);
		schedule();
		__set_current_state(TASK_PARKED);
		}
		clear_bit(KTHREAD_IS_PARKED, &self->flags);
		__set_current_state(TASK_RUNNING);

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