linux-kernel - Re: [PATCH] sysfs: Add lockdep annotations for the sysfs active reference

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Date:	Sun, 03 Jan 2010 09:02:24 +0900
From:	Tejun Heo <tj@...nel.org>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
CC:	Greg KH <greg@...ah.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Borislav Petkov <petkovbb@...glemail.com>,
	David Airlie <airlied@...ux.ie>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Al Viro <viro@...IV.linux.org.uk>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH] sysfs: Add lockdep annotations for the sysfs active reference

On 01/03/2010 06:37 AM, Eric W. Biederman wrote:
> 
> Holding locks over device_del -> kobject_del -> sysfs_deactivate can
> cause deadlocks if those same locks are grabbed in sysfs show or store
> methods.
> 
> The I model s_active count + completion as a sleeping read/write lock.
> I describe to lockdep sysfs_get_active as a read_trylock,
> sysfs_put_active as a read_unlock, and sysfs_deactivate as a
> write_lock and write_unlock pair.  This seems to capture the essence
> for purposes of finding deadlocks, and in my testing gives finds real
> issues and ignores non-issues.
> 
> This brings us back to holding locks over kobject_del is a problem
> that ideally we should find a way of addressing, but at least lockdep
> can tell us about the problems instead of requiring developers to debug
> rare strange system deadlocks, that happen when sysfs files are removed
> while being written to.
> 
> Signed-off-by: Eric W. Biederman <ebiederm@...ssion.com>

Looks good to me.

Acked-by: Tejun Heo <tj@...nel.org>

Thanks for doing this.

-- 
tejun
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