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

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Message-ID: <11007730-3fa5-139a-8091-655743894ae8@sandisk.com>
Date:	Wed, 3 Aug 2016 11:56:16 -0700
From:	Bart Van Assche <bart.vanassche@...disk.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	"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>,
	Oleg Nesterov <oleg@...hat.com>
Subject: Re: [PATCH] sched: Avoid that __wait_on_bit_lock() hangs

On 08/03/2016 11:11 AM, Peter Zijlstra wrote:
> That seems to do the right thing, so clearly I misunderstand. Please
> clarify.

Hello Peter,

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.

Please note that the call stack I had mentioned in my e-mail had been 
reported before. See e.g.
* Michael Shaver, Kernel deadlock during mdadm reshape, July 2016 
(http://www.spinics.net/lists/raid/msg53056.html).
* Bart Van Assche, Kernel hangs in truncate_inode_pages(), August 2012 
(https://lkml.org/lkml/2012/8/24/185).

Bart.