linux-kernel - Re: [PATCH 1/2] fs: sysfs: Do dcache-related updates to sysfs dentries under sysfs

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Date:	Wed, 11 Jan 2012 18:07:23 +0000
From:	Mel Gorman <mgorman@...e.de>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Linux-MM <linux-mm@...ck.org>,
	Linux-FSDevel <linux-fsdevel@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	Gilad Ben-Yossef <gilad@...yossef.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Miklos Szeredi <mszeredi@...ell.com>, Greg KH <gregkh@...e.de>,
	Gong Chen <gong.chen@...el.com>
Subject: Re: [PATCH 1/2] fs: sysfs: Do dcache-related updates to sysfs
 dentries under sysfs_mutex

On Wed, Jan 11, 2012 at 09:11:27AM -0800, Eric W. Biederman wrote:
> > In Miklos's case, the problem is with the bonding driver but during
> > CPU online or offline, a number of dentries are being created and
> > deleted and this deadlock is also being hit. Looking at sysfs, there
> > is a global sysfs_mutex that protects the sysfs directory tree from
> > concurrent reclaims. Almost all operations involving directory inodes
> > and dentries take place under the sysfs_mutex - linking, unlinking,
> > patch searching lookup, renames and readdir. d_invalidate is slightly
> > different. It is mostly under the mutex but if the dentry has to be
> > removed from the dcache, the mutex is dropped.
> 
> The sysfs_mutex protects the sysfs data structures not the vfs.
> 

Ok.

> > Where as Miklos' patch changes dcache, this patch changes sysfs to
> > consistently hold the mutex for dentry-related operations. Once
> > applied, this particular bug with CPU hotadd/hotremove no longer
> > occurs.
> 
> After taking a quick skim over the code to reacquaint myself with
> it appears that the usage in sysfs is idiomatic.  That is sysfs
> uses shrink_dcache_parent without a lock and in a context where
> the right race could trigger this deadlock.
> 

Yes.

> And in particular I expect you could trigger the same deadlock in
> proc, nfs, and gfs2 with if you can get the timing right.
> 

Agreed. When the dcache-specific fix was being discussed on an external
bugzilla, this came up. It's probably easiest to race in sysfs because
it's possible to create/delete directories faster than is possible
for proc, nfs or gfs2.

> I don't think adding a work-around for the bug in shrink_dcache_parent
> is going to do anything except hide the bug in the VFS, and
> unnecessarily increase the sysfs_mutex hold times.
> 

Ok.

> I may be blind but I don't see a reason at this point to rush out an
> incomplete work-around for the bug in shrink_dcahce_parent instead of
> actually fixing shrink_dcache_parent.
> 

Since I wrote this patch, the dcache specific fix was finished, merged
and I expect it'll make it to stable. Assuming that happens, this patch
will no longer be required.

Thanks Eric.

-- 
Mel Gorman
SUSE Labs
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