Re: [RFC PATCH(experimental) 2/2] Fix freezer-kthread_stop race

Oleg Nesterov
2007-04-20T21:21:28Z

On 04/20, Gautham R Shenoy wrote:
>
> On Fri, Apr 20, 2007 at 10:54:36AM +0200, Rafael J. Wysocki wrote:
> >
> > Hmm, can't we do something like this instead:
> >
> > ---
> > kernel/kthread.c | 10 ++++++++++
> > 1 file changed, 10 insertions(+)
> >
> > Index: linux-2.6.21-rc7/kernel/kthread.c
> > ===================================================================
> > --- linux-2.6.21-rc7.orig/kernel/kthread.c
> > +++ linux-2.6.21-rc7/kernel/kthread.c
> > @@ -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 +233,15 @@ int kthread_stop(struct task_struct *k)
> >
> > /* Now set kthread_should_stop() to true, and wake it up. */
> > kthread_stop_info.k = k;
> > + if (!(current->flags & PF_NOFREEZE)) {
> > + /* If we are freezable, the freezer will wait for us */
> > + task_lock(k);
> > + k->flags |= PF_NOFREEZE;
> > + if (frozen(k))
> > + k->flags &= ~PF_FROZEN;
> > +
> > + task_unlock(k);
> > + }
>
> Yes, we can do this for now since the tasks have only two freeze states,
> namely Freezeable and Non Freezeable.

No, we can't change k->flags, k owns its ->flags, and it is not atomic.

Rafael, may I suggest you to document task_lock() in thaw_process() ? This
looks really confusing, as if task_lock() protects "p->flags &= ~PF_FROZEN".

Actually, task_lock() is needed to prevent the race with refrigerator()
when the freezing fails, but this is not obvious.

Oleg.