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 10:33:15 +0200
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Peter Zijlstra <peterz@...radead.org>,
	Sasha Levin <sasha.levin@...cle.com>
CC:	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 10:25 AM, Peter Zijlstra wrote:
> On Thu, Jul 10, 2014 at 03:02:29PM -0400, Sasha Levin wrote:
>> What if we move lockdep's acquisition point to after it actually got the
>> lock?
>
> NAK, you want to do deadlock detection _before_ you're stuck in a
> deadlock.
>
>> We'd miss deadlocks, but we don't care about them right now. Anyways, doesn't
>> lockdep have anything built in to allow us to separate between locks which
>> we attempt to acquire and locks that are actually acquired?
>>
>> (cc PeterZ)
>>
>> We can treat locks that are in the process of being acquired the same as
>> acquired locks to avoid races, but when we print something out it would
>> be nice to have annotation of the read state of the lock.
>
> I'm missing the problem here I think.

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.



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