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

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

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.

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