Re: [RFC][PATCH -mm 3/3] freezer: Fix problem with kthread_stop

Rafael J. Wysocki
2007-04-23T20:03:09Z

On Monday, 23 April 2007 14:35, Gautham R Shenoy wrote:
> On Sun, Apr 22, 2007 at 09:40:59PM +0200, Rafael J. Wysocki wrote:
> > From: Rafael J. Wysocki <rjw@sisk.pl>
> >
> > Fix the problem with kthread_stop() that causes the freezer to fail if a
> > freezable thread is attempting to stop a frozen one and that may cause the
> > freezer to fail if the thread being stopped is freezable and
> > try_to_freeze_tasks() is running concurrently with kthread_stop().
> >
> > Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
> > ---
> > kernel/kthread.c | 9 +++++++++
> > 1 file changed, 9 insertions(+)
> >
> > Index: linux-2.6.21-rc6-mm1/kernel/kthread.c
> > ===================================================================
> > --- linux-2.6.21-rc6-mm1.orig/kernel/kthread.c 2007-04-09 15:23:48.000000000 +0200
> > +++ linux-2.6.21-rc6-mm1/kernel/kthread.c 2007-04-22 19:05:29.000000000 +0200
> > @@ -13,6 +13,7 @@
> > #include <linux/file.h>
> > #include <linux/module.h>
> > #include <linux/mutex.h>
> > +#include <linux/freezer.h>
> > #include <asm/semaphore.h>
> >
> > /*
> > @@ -232,6 +233,14 @@ int kthread_stop(struct task_struct *k)
> >
> > /* Now set kthread_should_stop() to true, and wake it up. */
> > kthread_stop_info.k = k;
> > + if (!freezer_should_exempt(current)) {
> > + /* We are freezable, so we must make sure that the thread being
> > + * stopped is not frozen and will not be frozen until it dies
> > + */
> > + freezer_exempt(k);
> > + if (frozen(k))
> > + clear_frozen_flag(k);
> > + }
>
> I'm trying hard to convince myself that this will work. May be I am
> missing something here, but I find a potential race window (very small though)
> when k is entering the refrigerator.
>
> Here's how.
>
> kthread_stop(k) k->refrigerator()
> ---------------------------------------------------------------------
> task_lock(k);
> 1) check_if_exempted();
> /* not exempted. So
> * we will freeze
> * ourself.
> */
> 2) freezer_exempt(k);
>
> 3) if(frozen(k))
> /* No, we're not yet frozen. So we
> * don't clear_frozen_flag(k) here
> */
> 4) frozen_process(k);
> task_unlock(k);
>
> 5) for(;;) {
> set_current_state(UNINTERRUPTIBLE);
> if(!frozen_process(k))
> /* k is frozen. We
> * don't break :(
> */
>
> schedule();
> }
>
> > wake_up_process(k);
> > put_task_struct(k);
> >
>
> Thus the freezer can still fail, no?
> IMO, we need the to take the task_lock for k here. Something like
>
> > + if (!freezer_should_exempt(current)) {
> task_lock(k);
> > + /* We are freezable, so we must make sure that the thread being
> > + * stopped is not frozen and will not be frozen until it dies
> > + */
> > + freezer_exempt(k);
> > + if (frozen(k))
> > + clear_frozen_flag(k);
> task_unlock(k);
> > + }

Yes, that's correct. We need to take task_lock() to avoid the race with
refrigerator().

I'll fix the patch.

BTW, I think I should rediff the entire series against -mm with your patch from
http://lkml.org/lkml/2007/4/23/98 applied.

Greetings,
Rafael