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

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Message-Id: <200705112240.54304.rjw@sisk.pl>
Date:	Fri, 11 May 2007 22:40:53 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Gautham R Shenoy <ego@...ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Oleg Nesterov <oleg@...sign.ru>, 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 Friday, 11 May 2007 21:39, Andrew Morton wrote:
> On Fri, 11 May 2007 00:36:25 +0200
> "Rafael J. Wysocki" <rjw@...k.pl> wrote:
> 
> > The reading of PF_BORROWED_MM in is_user_space() without task_lock() is racy. 
> > Fix it.
> > 
> > Signed-off-by: Rafael J. Wysocki <rjw@...k.pl>
> > Acked-by: Pavel Machek <pavel@....cz>
> > ---
> >  kernel/power/process.c |    8 +++++++-
> >  1 file changed, 7 insertions(+), 1 deletion(-)
> > 
> > Index: linux-2.6/kernel/power/process.c
> > ===================================================================
> > --- linux-2.6.orig/kernel/power/process.c	2007-05-10 21:44:23.000000000 +0200
> > +++ linux-2.6/kernel/power/process.c	2007-05-10 21:44:28.000000000 +0200
> > @@ -8,6 +8,7 @@
> >  
> >  #undef DEBUG
> >  
> > +#include <linux/sched.h>
> >  #include <linux/interrupt.h>
> >  #include <linux/suspend.h>
> >  #include <linux/module.h>
> > @@ -88,7 +89,12 @@ static void cancel_freezing(struct task_
> >  
> >  static inline int is_user_space(struct task_struct *p)
> >  {
> > -	return p->mm && !(p->flags & PF_BORROWED_MM);
> > +	int ret;
> > +
> > +	task_lock(p);
> > +	ret = p->mm && !(p->flags & PF_BORROWED_MM);
> > +	task_unlock(p);
> > +	return ret;
> >  }
> 
> The whole function is racy, isn't it?  I mean, the condition which it is
> testing can go from true->false or false->true at any instant after this
> function returns its now-wrong value.
> 
> iow, callers of this function need to to something to prevent the expression
> `p->mm && !(p->flags & PF_BORROWED_MM);' from changing value _anyway_.  In
> which case the new locking is not needed?

For user space processes this condition is always true.

For kernel threads:
(1) the change of tsk->mm from NULL to a nonzero value is only made in
fs/aio.c:use_mm() along with the setting of PF_BORROWED_MM under
the task_lock(),
(2) the change of tsk->mm from a nonzero value to NULL is only made in
fs/aio.c:unuse_mm() along with the resetting of PF_BORROWED_MM
under the task_lock().
Therefore, by taking the task_lock() here we make sure that the condition
is alyways false when we check it for kernel threads.

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