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

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Message-ID: <20160810105724.GA9389@redhat.com>
Date:	Wed, 10 Aug 2016 12:57:25 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Bart Van Assche <bart.vanassche@...disk.com>
Cc:	Bart Van Assche <bvanassche@....org>,
	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

On 08/09, Bart Van Assche wrote:
>
> Hello Oleg,
>
> Something that puzzles me is that removing the "else" keyword from
> abort_exclusive_wait() is sufficient to avoid the hang.

Yes, we need to understand this.

> If there would
> be code that clears PG_locked without calling wake_up() this hang
> probably would also be triggered by workloads that do not wake up
> lock_page_killable() with a signal.

Yes, and I already have another debugging patch to test this... it simply turns
lock_page_killable() into lock_page(). But lets check __ClearPageLocked() first
(the patch I sent a minute ago).

> BTW, the
> WARN_ONCE(!list_empty(&wait->task_list) && waitqueue_active(q), "mode =
> %#x\n", mode) statement that I added in abort_exclusive_wait() just
> produced the following call stack:

This condition is fine, and the trace is clear. This means that lock_page_killable()
was interrupted and wake_bit_function() was not called. We do not need another wakeup
in this case but somehow it helps. Again, I think because the necessary wakeup was
already lost/missed.

Oleg.