linux-kernel - Re: [PATCH 1/7] Freezer: Read PF_BORROWED

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Date:	Sat, 12 May 2007 03:48:19 +0400
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	"Rafael J. Wysocki" <rjw@...k.pl>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Gautham R Shenoy <ego@...ibm.com>,
	LKML <linux-kernel@...r.kernel.org>, Pavel Machek <pavel@....cz>,
	"Eric W. Biederman" <ebiederm@...ssion.com>
Subject: Re: [PATCH 1/7] Freezer: Read PF_BORROWED_MM in a nonracy way

On 05/11, Linus Torvalds wrote:
>
> On Sat, 12 May 2007, Oleg Nesterov wrote:
> > 
> > without task_lock() we can see "p->mm != NULL" but not PF_BORROWED_MM.
> 
> Let me explain it one more time:
>  - shouldn't the *caller* protect this?
>
> Afaik, there's two situations:
>  - either things don't change (in which case you don't need locking at 
>    all, since things are statically one way or the other)
>  - or things change (in which case the caller can't rely on the return 
>    value anyway, since they might change *after* you release the lock)

things change, ->mm is not stable if the kernel thread does use_mm/unuse_mm.

However, the return value == 0 does not change in that particular case,
exactly because is_user_space() takes task_lock().

Oleg.

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