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Date: Wed, 10 Aug 2016 12:57:25 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: Bart Van Assche <bart.vanassche@sandisk.com>
Cc: Bart Van Assche <bvanassche@acm.org>,
        Peter Zijlstra <peterz@infradead.org>,
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        Michael Shaver <jmshaver@gmail.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] sched: Avoid that __wait_on_bit_lock() hangs
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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.