Re: [PATCH] sched: Avoid that __wait_on_bit_lock() hangs

[Bart Van Assche <bart.vanassche@sandisk.com>]

On 08/03/2016 02:30 PM, Oleg Nesterov wrote:
> I too can't understand the problem. Perhaps you missed the fact that
> abort_exclusive_wait() does everything under wait_queue_head_t->lock ?
>
> [ ... ]
>
> But we do not care if we race with another try_to_wake_up(), or even with
> another exclusive wake_up_nr(wq)/whatever unless wq is the same.
>
> And if this wq is the same, then wake_up_nr() will do try_to_wake_up/autoremove
> either before or after abort_exclusive_wait(), wake_up_nr() takes the same
> wq->lock.
>
> And this means that abort_exclusive_wait() can't be called "After try_to_wake_up()"
> and "before autoremove_wake_function()".

Hello Oleg,

I had noticed that abort_exclusive_wait() locks and unlocks 
wait_queue_head_t->lock.

What I had overlooked is that if try_to_wake_up() is called (indirectly) 
by __wake_up_common() that then wait_queue_head_t->lock is held. 
However, not all try_to_wake_up() callers hold that lock. Since I'm not 
a scheduler expert I would appreciate it if someone who is more familiar 
with the scheduler could explain me how the two patches that I posted in 
the context of this e-mail thread can cause a behavior difference.

Thanks,

Bart.