Re: [PATCH 1/7] Freezer: Read PF_BORROWED_MM in a nonracy way

[Oleg Nesterov <oleg@tv-sign.ru>]

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.