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 11:02:26 -0800
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Mel Gorman <mgorman@...e.de>
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

Mel Gorman <mgorman@...e.de> writes:

> 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 expect we see the race in sysfs because of uevents that get triggered
on hotplug.  So a lot is occurring around the time of the race.  You can
get to shrink_dcache_parent with fork/exit in proc which is a lot easier
to trigger.  But usually in fork/exec you don't have the dentries cached...

> 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.

Sounds good.

Eric
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