linux-kernel - Re: [PATCH] sched: Avoid that __wait_on_bit

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Date:	Wed, 3 Aug 2016 23:30:06 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Bart Van Assche <bart.vanassche@...disk.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	"mingo@...nel.org" <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Neil Brown <neilb@...e.de>,
	Michael Shaver <jmshaver@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sched: Avoid that __wait_on_bit_lock() hangs

Hi Bart,

I too can't understand the problem. Perhaps you missed the fact that
abort_exclusive_wait() does everything under wait_queue_head_t->lock ?

On 08/03, Bart Van Assche wrote:
>
> try_to_wake_up() locks task_struct.pi_lock but abort_exclusive_wait() not.
> My assumption is that the following sequence of events leads to the lockup
> that I had mentioned in the description of my patch:
> * try_to_wake_up() is called for the task that will execute
>   abort_exclusive_wait().
> * After try_to_wake_up() has checked task_struct.state and before
>   autoremove_wake_function() has tried to remove the task from the wait
>   queue, abort_exclusive_wait() is executed for the same task.

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()".

Oleg.