linux-kernel - Re: [PATCH] autofs4: fix race in unhashed dentry code

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Message-ID: <461D254C.9020607@suse.com>
Date:	Wed, 11 Apr 2007 14:13:32 -0400
From:	Jeff Mahoney <jeffm@...e.com>
To:	Ian Kent <raven@...maw.net>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...l.org>
Subject: Re: [PATCH] autofs4: fix race in unhashed dentry code

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Ian Kent wrote:
> On Thu, 2007-04-12 at 02:00 +0800, Ian Kent wrote:
>> On Thu, 2007-04-12 at 01:49 +0800, Ian Kent wrote:
>>> On Wed, 2007-04-11 at 11:05 -0400, Jeff Mahoney wrote:
>>>>  Commit f50b6f8691cae2e0064c499dd3ef3f31142987f0 introduced a
>>>>  race in autofs4 between autofs_lookup_unhashed() and
>>>>  autofs_dentry_release().
>>>>
>>>>  autofs_dentry_release() ends up clearing the ->dentry and ->inode
>>>>  members of autofs_info before removing it from the rehash list. The
>>>>  list is protected by the rehash lock in both functions, but
>>>>  since autofs_dentry_release() starts tearing the autofs_info struct
>>>>  down before removing it from the list, autofs_lookup_unhashed() can
>>>>  get a autofs_info with a NULL dentry.
>>>>
>>>>  This patch moves the clearing of ->dentry and ->inode after the removal
>>>>  from the rehash list.
>>> Oh .. excellent, I had a bug report but I just couldn't see it for
>>> looking.
>> Maybe I've been a bit hasty with the celebration.
>> It looks like I've got a bigger locking problem here.
>> If autofs4_dentry_release waits on the rehash lock and
>> autofs4_lookup_unhashed reclaims it then the info struct and the dentry
>> go away unconditionally as the release is called just prior to freeing
>> the dentry memory, right?.
> 
> No I'm wrong, dentry_iput holds the dcache locks till it sets d_inode to
> NULL, the point of the d_inode check in autofs4_lookup_unhashed.
> 
> My day is still made.
> 

Ok, that was quick. I thought it was ok, but was about to dig back in to
make sure. :)

- -Jeff

- --
Jeff Mahoney
SUSE Labs
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