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

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Message-Id: <1176315189.3377.35.camel@raven.themaw.net>
Date:	Thu, 12 Apr 2007 02:13:09 +0800
From:	Ian Kent <raven@...maw.net>
To:	Jeff Mahoney <jeffm@...e.com>
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

On Wed, 2007-04-11 at 14:13 -0400, Jeff Mahoney wrote:
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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. :)

Ya .. I'll think about it some more but it looks good.
Thanks again.

> 
> - -Jeff
> 
> - --
> Jeff Mahoney
> SUSE Labs
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