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

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Date:	Sat, 12 May 2007 04:05:07 +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

I hope Rafael will correct me if I am wrong,

On 05/12, Oleg Nesterov wrote:
>
> 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().

Probably there is some misunderstanding. This patch doesn't claim it solves
all problems. Before this patch we have

	static inline int is_user_space(struct task_struct *p)
	{
		return p->mm && !(p->flags & PF_BORROWED_MM);
	}

and this is clearly racy wrt to use_mm() which sets this PF_BORROWED_MM bit.
So this is just a little improvement, nothing more.

Oleg.

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