linux-kernel - Re: [-mm][PATCH 1/2] mm owner fix race between swap and exit

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Date:	Mon, 11 Aug 2008 17:31:38 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Balbir Singh <balbir@...ux.vnet.ibm.com>
Cc:	linux-mm@...ck.org, skumar@...ux.vnet.ibm.com,
	yamamoto@...inux.co.jp, menage@...gle.com, lizf@...fujitsu.com,
	linux-kernel@...r.kernel.org, nishimura@....nes.nec.co.jp,
	xemul@...nvz.org, hugh@...itas.com, balbir@...ux.vnet.ibm.com,
	kamezawa.hiroyu@...fujitsu.com
Subject: Re: [-mm][PATCH 1/2] mm owner fix race between swap and exit

On Mon, 11 Aug 2008 15:37:33 +0530
Balbir Singh <balbir@...ux.vnet.ibm.com> wrote:

> There's a race between mm->owner assignment and try_to_unuse(). The condition
> occurs when try_to_unuse() runs in parallel with an exiting task.
> 
> The race can be visualized below. To quote Hugh
> "I don't think your careful alternation of CPU0/1 events at the end matters:
> the swapoff CPU simply dereferences mm->owner after that task has gone"
> 
> But the alteration does help understand the race better (at-least for me :))
> 
> CPU0					CPU1
> 					try_to_unuse
> task 1 stars exiting			look at mm = task1->mm
> ..					increment mm_users
> task 1 exits
> mm->owner needs to be updated, but
> no new owner is found
> (mm_users > 1, but no other task
> has task->mm = task1->mm)
> mm_update_next_owner() leaves
> 
> grace period
> 					user count drops, call mmput(mm)
> task 1 freed
> 					dereferencing mm->owner fails
> 
> The fix is to notify the subsystem (via mm_owner_changed callback), if
> no new owner is found by specifying the new task as NULL.

This patch applies to mainline, 2.6.27-rc2 and even 2.6.26.

Against which kernel/patch is it actually applicable?

(If the answer was "all of the above" then please don't go embedding
mainline bugfixes in the middle of a -mm-only patch series!)

Thanks.
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