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Date: Fri, 24 Feb 2012 13:37:42 -0800
From: Dave Hansen <dave@linux.vnet.ibm.com>
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To: Christoph Lameter <cl@linux.com>
CC: "Eric W. Biederman" <ebiederm@xmission.com>, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org
Subject: Re: [RFC][PATCH] fix move/migrate_pages() race on task struct
References: <20120223180740.C4EC4156@kernel> <alpine.DEB.2.00.1202231240590.9878@router.home> <4F468F09.5050200@linux.vnet.ibm.com> <alpine.DEB.2.00.1202231334290.10914@router.home> <4F469BC7.50705@linux.vnet.ibm.com> <alpine.DEB.2.00.1202231536240.13554@router.home> <m1ehtkapn9.fsf@fess.ebiederm.org> <alpine.DEB.2.00.1202240859340.2621@router.home> <4F47BF56.6010602@linux.vnet.ibm.com> <alpine.DEB.2.00.1202241053220.3726@router.home> <alpine.DEB.2.00.1202241105280.3726@router.home> <4F47C800.4090903@linux.vnet.ibm.com> <alpine.DEB.2.00.1202241131400.3726@router.home>
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On 02/24/2012 09:32 AM, Christoph Lameter wrote:
> @@ -1318,10 +1318,10 @@ SYSCALL_DEFINE4(migrate_pages, pid_t, pi
>  	rcu_read_lock();
>  	task = pid ? find_task_by_vpid(pid) : current;
>  	if (!task) {
> -		rcu_read_unlock();
>  		err = -ESRCH;
>  		goto out;
>  	}
...
> +	put_task_struct(task);
> +	task = NULL;
>  	err = do_migrate_pages(mm, old, new,
>  		capable(CAP_SYS_NICE) ? MPOL_MF_MOVE_ALL : MPOL_MF_MOVE);
>  out:
> +	if (task)
> +		put_task_struct(task);
> +
>  	if (mm)
>  		mmput(mm);
>  	NODEMASK_SCRATCH_FREE(scratch);

Man, patch did not like this for some reason.  I kept throwing most of
the mempolicy.c hunks away.  I've never seen anything like it.

Anyway...  This looks fine except I think that rcu_read_unlock() need to
stay.  There's currently no release of it after out:.

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