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

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Message-ID: <20070326115046.GK11794@in.ibm.com>
Date:	Mon, 26 Mar 2007 17:20:46 +0530
From:	Srivatsa Vaddagiri <vatsa@...ibm.com>
To:	Balbir Singh <balbir@...ibm.com>
Cc:	pj@....com, akpm@...ux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Fix race between attach_task and cpuset_exit

On Sun, Mar 25, 2007 at 11:22:15PM +0530, Balbir Singh wrote:
> >+	struct cpuset *oldcs_tobe_released = NULL;
> 
> How about oldcs_to_be_released?

Yes, I wanted to use that, but my typo I guess.

> >@@ -2242,19 +2241,20 @@ void cpuset_exit(struct task_struct *tsk
> > {
> > 	struct cpuset *cs;
> >
> >+	task_lock(tsk);
> > 	cs = tsk->cpuset;
> > 	tsk->cpuset = &top_cpuset;	/* the_top_cpuset_hack - see above */
> >+	atomic_dec(&cs->count);
> 
> How about using a local variable like ref_count and using
> 
> ref_count = atomic_dec_and_test(&cs->count); This will avoid the two
> atomic operations, atomic_dec() and atomic_read() below.

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. But I notice that check_for_release() has that
atomic_read-and-check-for-zero-refcount inbuilt into it, which means we can 
blindly call it. Modified patch in another mail.

-- 
Regards,
vatsa
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