linux-kernel - Re: ksm/memory hotplug: lockdep warning for ksm_thread_mutex vs. (memory

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Message-ID: <4F2BFF4C.5050905@de.ibm.com>
Date:	Fri, 03 Feb 2012 16:37:48 +0100
From:	Gerald Schaefer <gerald.schaefer@...ibm.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
CC:	Hugh Dickins <hughd@...gle.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Martin Schwidefsky <martin.schwidefsky@...ibm.com>,
	Heiko Carstens <h.carstens@...ibm.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Chris Wright <chrisw@...s-sol.org>,
	Izik Eidus <izik.eidus@...ellosystems.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Subject: Re: ksm/memory hotplug: lockdep warning for ksm_thread_mutex vs.
 (memory_chain).rwsem

On 03.02.2012 00:00, KOSAKI Motohiro wrote:
> 2012/2/2 Gerald Schaefer<gerald.schaefer@...ibm.com>:
>> Setting a memory block offline triggers the following lockdep warning. This
>> looks exactly like the issue reported by Kosaki Motohiro in
>> https://lkml.org/lkml/2010/10/25/110. Seems like the resulting commit a0b0f58cdd
>> did not fix the lockdep warning. I'm able to reproduce it with current 3.3.0-rc2
>> as well as 2.6.37-rc4-00147-ga0b0f58.
>>
>> I'm not familiar with lockdep annotations, but I tried using down_read_nested()
>> for (memory_chain).rwsem, similar to the mutex_lock_nested() which was
>> introduced for ksm_thread_mutex, but that didn't help.
> 
> Heh, interesting. Simple question, do you have any user visible buggy
> behavior? or just false positive warn issue?
> 
> *_nested() is just hacky trick. so, any change may break their lie.
> Anyway I'd like to dig this one. thanks for reporting.

There is no real deadlock and no user visible buggy behaviour, the memory is
being offlined as requested. I think your conclusion from last time is still
valid, that both locks are inside mem_hotplug_mutex and there can't be a
deadlock. Question is how to convince lockdep of this.

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