linux-kernel - Re: Fwd: Fw: [2.6.20.4] BUG: dentry xattrs still in use in shrink_dcache_for

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Date:	Sat, 21 Apr 2007 10:34:47 -0400
From:	Jeff Mahoney <jeffm@...e.de>
To:	Jeff Mahoney <jeffm@...e.com>
Cc:	a.righi@...eca.it, Andrew Morton <akpm@...ux-foundation.org>,
	"Vladimir V. Saveliev" <vs@...esys.com>,
	linux-kernel@...r.kernel.org, reiserfs-dev@...esys.com,
	Edward Shishkin <edward@...esys.com>, zam@...sterfs.com,
	ak@...e.de
Subject: Re: Fwd: Fw: [2.6.20.4] BUG: dentry xattrs still in use in shrink_dcache_for_umount()
 with reiserfs

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Jeff Mahoney wrote:
> Andrea Righi wrote:
>> Jeff Mahoney wrote:
>>> I have the patchset that I mentioned, but I'm not proposing it for 2.6.21.
>>> It's much too invasive to be introduced in an -rc7, but it does include
>>> locking changes that I believe avoid this bug.
>>>
>>> Vladimir was right in his analysis that sometimes get_xa_root() takes the
>>> reference once and other times twice, but not for the right reasons.  I save
>>> a reference to the xattr dir to avoid a lookup later, but when there are multiple
>>> getxattrs or listxattrs as the first xattr operation on the file system, we can end
>>> up taking a second reference when we shouldn't. This is because those operations
>>> are protected by read locks and the ->xattr_root pointer isn't protected by anything
>>> else. A quick fix would be to just extend the protection of the priv root's i_mutex
>>> around the assignment, and test first. The right fix involves a complete rework of
>>> the locking, and I have code to do that, it's just too late to include it in 2.6.21.
>>>
>>> I'd love to know what Andrea (and now Andi Kleen) are doing to hit this now. There
>>> haven't been any changes in this code in a while, and the shrink_dcache_for_umount()
>>> has been around since October. I'm unable to reproduce locally so far, so if Andrea
>>> or Andi could see if this fixes it for them, I'd appreciate it.
>> Jeff, your patch doesn't resolve, but you hit the problem. Actually, I
>> think the xattr_root pointer must be protected also in get_xa_root()
>> using the same private root i_mutex.
>>
>> I've tested the following patch and it resolves in my case. What do you
>> think about it? could it be a reasonable fix?
> 
> Your patch fixes the problem. That'll teach me to spin a patch up while I'm headed
> out the door. I think a cleaner solution for now is just to refactor get_xa_root,
> __get_xa_root, and create_xa_root into one function to simplify the lookup and the
> locking. I think I'll add another step to my patch set that removes the caching of
> xattr_root entirely or creates in on mount, as the priv root is created. The privroot
> must be cached to avoid a deadlock on the fs-root directory's i_mutex, but xattr_root
> was cached as an optimzation. If we need to take privroot->i_mutex every time, there's
> no point in caching it since it will probably be in the dcache anyway.
> 
> -Jeff
> 

Ignore this patch. I missed a spot.

- -Jeff

- --
Jeff Mahoney
SUSE Labs
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