linux-kernel - Re: fs: lockup on rename

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Message-ID: <20141026030608.GN7996@ZenIV.linux.org.uk>
Date:	Sun, 26 Oct 2014 03:06:08 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Sasha Levin <sasha.levin@...cle.com>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Dave Jones <davej@...hat.com>
Subject: Re: fs: lockup on rename_mutex in fs/dcache.c:1035

On Sat, Oct 25, 2014 at 08:01:41PM -0700, Eric W. Biederman wrote:
> Sasha Levin <sasha.levin@...cle.com> writes:
> 
> > Hi all,
> >
> > While fuzzing with trinity inside a KVM tools guest running the latest -next
> > kernel, I've stumbled on the following spew:
> 
> Weird.  I took a quick look and I don't see any changes in d_walk that
> in Al's tree or in Linus's kernel for years.
> 
> Has read_seqbegin_or_lock changed somewhere?
> 
> >From a quick reading of the code it simply isn't possible for d_walk to
> take the lock twice short of memory corruption.  And the fact that the
> code has not changed in years seems to suggest it isn't the obvious
> cause of d_walk talking the rename_lock twice.

It is a fairly obvious case of d_walk() forgetting to drop rename_lock.
See upthread for analysis and (hopefully) a fix.
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