linux-kernel - Re: [PATCH 2/2] sched/wait: avoid abort_exclusive_wait() in __wait_on_bit

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Date:   Fri, 2 Sep 2016 14:06:43 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...nel.org>, Al Viro <viro@...IV.linux.org.uk>,
        Bart Van Assche <bvanassche@....org>,
        Johannes Weiner <hannes@...xchg.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Neil Brown <neilb@...e.de>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] sched/wait: avoid abort_exclusive_wait() in
        __wait_on_bit_lock()

On 09/02, Peter Zijlstra wrote:
>
> FWIW, the way the mutex code avoids this issue is by doing the
> signal_pending test while holding the q->lock, that way its exclusive
> with wakeup.

And __wait_event_interruptible_locked() too.

BTW it is buggy anyway, it needs the

	-	__add_wait_queue_tail(&(wq), &__wait);
	+	if (exclusive)
	+		__add_wait_queue_tail(&(wq), &__wait);
	+	else
	+		__add_wait_queue((&(wq), &__wait);

and in fact it should use __add_wait_queue_exclusive() so that we
can remove another "if (exclusive)" but this is off-topic.

Yes, I considered this option, but to me the addtional finish_wait()
looks simpler.

And, if you agree with this change I will try to change __wait_event()
as well and kill abort_exclusive_wait().

And in this case we certainly do not want to check the "condition" with
q->lock held, because this would mean that "condition" won't be able to
take this lock.

Oleg.