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Date: Wed, 10 Aug 2016 13:03:57 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Bart Van Assche <bart.vanassche@sandisk.com>,
        Bart Van Assche <bvanassche@acm.org>,
        "mingo@kernel.org" <mingo@kernel.org>,
        Andrew Morton <akpm@linux-foundation.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
Message-ID: <20160810110357.GL30192@twins.programming.kicks-ass.net>
References: <11007730-3fa5-139a-8091-655743894ae8@sandisk.com>
 <20160803213006.GA11712@redhat.com>
 <17b65ff9-215f-ab74-9f5f-15dbd308d054@sandisk.com>
 <20160804140938.GB24652@twins.programming.kicks-ass.net>
 <16207b90-2e6c-fe23-1b4b-3763e5cf0384@sandisk.com>
 <20160808102213.GA6879@twins.programming.kicks-ass.net>
 <4091e252-18d9-1795-de63-9fbc678aa6b1@acm.org>
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On Wed, Aug 10, 2016 at 12:57:25PM +0200, Oleg Nesterov wrote:
> 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.

I suspect the same. Removing that else generates 'spurious' wakeups,
which can unstick the situation, hiding the real source of the problem.