Date: Sat, 12 May 2007 00:21:02 +0400
From: Oleg Nesterov <oleg@tv-sign.ru>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: "Rafael J. Wysocki" <rjw@sisk.pl>, Gautham R Shenoy <ego@in.ibm.com>,
       Linus Torvalds <torvalds@linux-foundation.org>,
       LKML <linux-kernel@vger.kernel.org>, Pavel Machek <pavel@ucw.cz>,
       "Eric W. Biederman" <ebiederm@xmission.com>
Subject: Re: [PATCH 1/7] Freezer: Read PF_BORROWED_MM in a nonracy way
Message-ID: <20070511202102.GA155@tv-sign.ru>
References: <200705110035.32229.rjw@sisk.pl> <200705110036.26617.rjw@sisk.pl> <20070511123959.190adfaf.akpm@linux-foundation.org>
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On 05/11, Andrew Morton wrote:
>
> On Fri, 11 May 2007 00:36:25 +0200
> "Rafael J. Wysocki" <rjw@sisk.pl> wrote:
> 
> >  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?

freeze_processes() first freezes user-space tasks only, then kernel threads.

Without task_lock() we can miss PF_BORROWED_MM and count the kernel thread
(which is doing use_mm()) as a user space process. This means it will be
frozen prematurely.

true->false means daemonize() or do_exit(), seems harmless.

false->true means exec from kernel space. That is why FREEZER_KERNEL_THREADS
in fact means all tasks, not only kernel threads.

Oleg.

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