linux-kernel - Re: mpol_to

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Message-ID: <CAHGf_=rLjQbtWQLDcbsaq5=zcZgjdveaOVdGtBgBwZFt78py4Q@mail.gmail.com>
Date:	Tue, 16 Oct 2012 01:10:34 -0400
From:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
To:	David Rientjes <rientjes@...gle.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, Oct 15, 2012 at 11:58 PM, David Rientjes <rientjes@...gle.com> wrote:
> 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.

Even though 80de7c3138ee9fd86a98696fd2cf7ad89b995d0a itself is simple. It bring
to caller complex. That's not good and have no worth.

>> 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.

I agree that's obviously a bug and we should fix it.
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