linux-kernel - Re: mpol_to

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Message-ID: <alpine.DEB.2.00.1210152055150.5400@chino.kir.corp.google.com>
Date:	Mon, 15 Oct 2012 20:58:33 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
cc:	Dave Jones <davej@...hat.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	bhutchings@...arflare.com, linux-mm@...ck.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: mpol_to_str revisited.

On Mon, 15 Oct 2012, KOSAKI Motohiro wrote:

> I don't think 80de7c3138ee9fd86a98696fd2cf7ad89b995d0a is right fix.

It's certainly not a complete fix, but I think it's a much better result 
of the race, i.e. we don't panic anymore, we simply fail the read() 
instead.

> we should
> close a race (or kill remain ref count leak) if we still have.

As I mentioned earlier in the thread, the read() is done here on a task 
while only a reference to the task_struct is taken and we do not hold 
task_lock() which is required for task->mempolicy.  Once that is fixed, 
mpol_to_str() should never be called for !task->mempolicy so it will never 
need to return -EINVAL in such a condition.
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