linux-kernel - Re: Linus GIT - INFO: possible circular locking dependency detected

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Message-ID: <1320868332.19727.19.camel@twins>
Date:	Wed, 09 Nov 2011 20:52:12 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Luis Henriques <henrix@...andro.org>
Cc:	Greg KH <greg@...ah.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Vasiliy Kulikov <segoon@...nwall.com>,
	Miles Lane <miles.lane@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Alexey Dobriyan <adobriyan@...il.com>, stable@...r.kernel.org
Subject: Re: Linus GIT - INFO: possible circular locking dependency detected

On Wed, 2011-11-09 at 19:11 +0000, Luis Henriques wrote:
> [   12.948038] -> #0 (&sig->cred_guard_mutex){+.+.+.}:
> [   12.948038]        [<ffffffff8108ff9f>] __lock_acquire+0x17bf/0x2020
> [   12.948038]        [<ffffffff81092e4f>] lock_acquire+0xaf/0x1f0
> [   12.948038]        [<ffffffff8135b2a5>] __mutex_lock_common+0x65/0x4d0
> [   12.948038]        [<ffffffff8135b76b>] mutex_lock_killable_nested+0x1b/0x20
> [   12.948038]        [<ffffffff811b301e>] lock_trace+0x2e/0x80
> [   12.948038]        [<ffffffff811b73ab>] proc_readfd_common+0x5b/0x4b0
> [   12.948038]        [<ffffffff811b7835>] proc_readfd+0x15/0x20
> [   12.948038]        [<ffffffff8115f8f0>] vfs_readdir+0xb0/0xd0
> [   12.948038]        [<ffffffff8115fa09>] sys_getdents+0x89/0x100
> [   12.948038]        [<ffffffff8135e8c2>] system_call_fastpath+0x16/0x1b
> 
> sb->s_type->i_mutex_key is shown as being acquired in the execve path,
> which seems to be wrong -- it was acquired in the vfs_readdir (on the 2nd
> trace).
> 
> This means that the initial analysis from Vasiliy is incorrect, as he
> assumed the execve path.  Or Am I interpreting this log incorrectly?
> (Probably I am...). 

->#0 shows where sig->cred_guard_mutex was taken, as it was the first
lock there is no nesting yet and therefore the ->i_mutex_key#6 thing
should not be associated with this stacktrace.

->#1 shows where ->i_mutex_key#6 was taken while holding
->cred_guard_mutex (but doesn't explicitly show where that was taken).

Mostly ->#0 information is useless in lockdep reports and can be safely
ignored.
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