Re: wake_up_process implied memory barrier clarification

Michal Hocko
2015-08-28T14:51:25Z

On Thu 27-08-15 20:26:54, Oleg Nesterov wrote:
> On 08/27, Michal Hocko wrote:
> >
> > --- a/Documentation/memory-barriers.txt
> > +++ b/Documentation/memory-barriers.txt
> > @@ -2031,6 +2031,9 @@ something up. The barrier occurs before the task state is cleared, and so sits
> > <general barrier> STORE current->state
> > LOAD event_indicated
> >
> > +Please note that wake_up_process is an exception here because it implies
> > +the write memory barrier unconditionally.
> > +
>
> I simply can't understand (can't even parse) this part of memory-barriers.txt.

Do you mean the added text or the example above it?

> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -1967,8 +1967,7 @@ static void try_to_wake_up_local(struct task_struct *p)
> > *
> > * Return: 1 if the process was woken up, 0 if it was already running.
> > *
> > - * It may be assumed that this function implies a write memory barrier before
> > - * changing the task state if and only if any tasks are woken up.
> > + * It may be assumed that this function implies a write memory barrier.
> > */
>
> I won't argue, technically this is correct of course. And I agree that
> the old comment is misleading.

Well the reason I've noticed is the following race in the scsi code
CPU0 CPU1
scsi_error_handler scsi_host_dev_release
kthread_stop()
while (!kthread_should_stop()) {
set_bit(KTHREAD_SHOULD_STOP)
wake_up_process()
wait_for_completion()

set_current_state(TASK_INTERRUPTIBLE)
schedule()
[...]
}

I have read the comment for wake_up_process and was wondering that
moving set_current_state before kthread_should_stop wouldn't be enough
because the the task at CPU0 might be TASK_RUNNIG and so wake_up_process
wouldn't wake up it and the missing write barrier could lead to a missed
KTHREAD_SHOULD_STOP. A look into ttwu made my worry void.

> But the new comment looks as if it is fine to avoid wmb() if you do
> wake_up_process(). Say,
>
> void w(void)
> {
> A = 1;
> wake_up_process(something_unrelated);
> // we know that it implies wmb().
> B = 1;
> }
>
> void r(void)
> {
> int a, b;
>
> b = B;
> rmb();
> a = A;
>
> BUG_ON(b && !a);
> }
>
> Perhaps this part of the comment should be simply removed, the unconditional
> wmb() in ttwu() is just implementation detail. And note that initially the
> documented behaviour of smp_mb__before_spinlock() was only the STORE - LOAD
> serialization. Then people noticed that it actually does wmb() and started
> to rely on this fact.
>
> To me, this comment should just explain that this function implies a barrier
> but only in a sense that you do not need another one after CONDITION = T and
> before wake_up_process().

I have no objection against more precise wording here but what we have is just
misleading.
--
Michal Hocko
SUSE Labs