linux-kernel - Re: + shmem-fix-faulting-into-a-hole-while-its-punched-take-2.patch added to -mm tree

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Date:	Fri, 11 Jul 2014 10:51:32 +0200
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Sasha Levin <sasha.levin@...cle.com>,
	Hugh Dickins <hughd@...gle.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	akpm@...ux-foundation.org, davej@...hat.com, koct9i@...il.com,
	lczerner@...hat.com, stable@...r.kernel.org,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: + shmem-fix-faulting-into-a-hole-while-its-punched-take-2.patch
 added to -mm tree

On 07/11/2014 10:38 AM, Peter Zijlstra wrote:
> On Fri, Jul 11, 2014 at 10:33:15AM +0200, Vlastimil Babka wrote:
>> Quoting Hugh from previous mail in this thread:
>>
>>>>>
>>>>> [  363.600969] INFO: task trinity-c327:9203 blocked for more than 120 seconds.
>>>>> [  363.605359]       Not tainted 3.16.0-rc4-next-20140708-sasha-00022-g94c7290-dirty #772
>>>>> [  363.609730] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
>>>>> [  363.615861] trinity-c327    D 000000000000000b 13496  9203   8559 0x10000004
>>>>> [  363.620284]  ffff8800b857bce8 0000000000000002 ffffffff9dc11b10 0000000000000001
>>>>> [  363.624468]  ffff880104860000 ffff8800b857bfd8 00000000001d7740 00000000001d7740
>>>>> [  363.629118]  ffff880104863000 ffff880104860000 ffff8800b857bcd8 ffff8801eaed8868
>>>>> [  363.633879] Call Trace:
>>>>> [  363.635442]  [<ffffffff9a4dc535>] schedule+0x65/0x70
>>>>> [  363.638638]  [<ffffffff9a4dc948>] schedule_preempt_disabled+0x18/0x30
>>>>> [  363.642833]  [<ffffffff9a4df0a5>] mutex_lock_nested+0x2e5/0x550
>>>>> [  363.646599]  [<ffffffff972a4d7c>] ? shmem_fallocate+0x6c/0x350
>>>>> [  363.651319]  [<ffffffff9719b721>] ? get_parent_ip+0x11/0x50
>>>>> [  363.654683]  [<ffffffff972a4d7c>] ? shmem_fallocate+0x6c/0x350
>>>>> [  363.658264]  [<ffffffff972a4d7c>] shmem_fallocate+0x6c/0x350
>>>
>>> So it's trying to acquire i_mutex at shmem_fallocate+0x6c...
>>>
>>>>> [  363.662010]  [<ffffffff971bd96e>] ? put_lock_stats.isra.12+0xe/0x30
>>>>> [  363.665866]  [<ffffffff9730c043>] do_fallocate+0x153/0x1d0
>>>>> [  363.669381]  [<ffffffff972b472f>] SyS_madvise+0x33f/0x970
>>>>> [  363.672906]  [<ffffffff9a4e3f13>] tracesys+0xe1/0xe6
>>>>> [  363.682900] 2 locks held by trinity-c327/9203:
>>>>> [  363.684928]  #0:  (sb_writers#12){.+.+.+}, at: [<ffffffff9730c02d>] do_fallocate+0x13d/0x1d0
>>>>> [  363.715102]  #1:  (&sb->s_type->i_mutex_key#16){+.+.+.}, at: [<ffffffff972a4d7c>] shmem_fallocate+0x6c/0x350
>>>
>>> ...but it already holds i_mutex, acquired at shmem_fallocate+0x6c.
>>> Am I reading that correctly?
>>
>> The output looks like mutex #1 is already taken, but actually the process is
>> sleeping when trying to take it. It appears that the output has taken
>> mutex_acquire_nest() action into account, but doesn't distinguish if
>> lock_acquired() already happened or not.
>
> The call trace is very clear on it that its not. I've never found this
> to be a problem in practise. You need to engage your brain anyhow, this
> little bit extra isn't going to make a difference or not.

OK, but what about the case of "Showing all locks held in the system:"
output where you don't have the stacktraces? Wouldn't it be better if that
distinguished locks already taken and locks being taken?

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