Re: [PATCH] slow_work_thread() should do the exclusive wait

[Oleg Nesterov <oleg@redhat.com>]

On 04/16, David Howells wrote:
>
> Andrew Morton <akpm@linux-foundation.org> wrote:
>
> > I wonder if slow_work_cull_timeout() should have some sort of barrier,
> > so the write is suitably visible to the woken thread.
>
> That's an interesting question.  Should wake_up() imply a barrier of any sort,
> I wonder.  Well, __wake_up() does impose a barrier as it uses a spinlock, but
> I wonder if that's sufficient.

wake_up() does imply the barrier. Note the smp_wmb() in try_to_wake_up().
And in fact this wmb() implies mb(), because spin_lock() itself is STORE,
and the futher LOADs can't leak up before spin_lock().

But afaics, this doesn't matter? prepare_to_wait() sets task->state under
wait_queue_head_t->lock and wake_up() takes this look too, so we can't miss
the event.

Or I completely misunderstood the issue...

Oleg.