linux-kernel - Re: [PATCH] Fix race between attach_task and cpuset

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Message-ID: <20070327023559.y7gvoty7ksg4kw44@imap.linux.ibm.com>
Date:	Tue, 27 Mar 2007 02:35:59 -0400
From:	Srivatsa Vaddagiri <vatsa@...ibm.com>
To:	Paul Jackson <pj@....com>
Cc:	balbir@...ibm.com, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Fix race between attach_task and cpuset_exit

Quoting Paul Jackson <pj@....com>:

> vatsa wrote:
>> Well, someone may have attached to this cpuset while we were waiting on the
>> mutex_lock(). So we need to do a atomic_read again to ensure it is still
>> unused.
>
> I don't see how this could happen. If we hold the task lock that now
> (thanks to your good work) guards this pointer, and if we decrement to
> zero the reference count on the cpuset to which it points and then
> -overwrite- this last remaining visible pointer to that cpuset with a
> pointer to a different cpuset, then aren't we guaranteed to be holding
> the last remaining reference to the old cpuset in our local variable,
> making it impossible for anyone else to attach to it in any way?

Yes, but the cpuset is not made invisible to userspace (in filesystem)  
yet. So as cpuset_exit() discovers that cpuset B has zero refcount now  
and blocks on mutex_lock(&manage_mutex) [ to do a check_for_release  
later ], someone could have done a attach_task to that cpuset.

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