Re: [PATCH 1/8] Enhance process freezer interface for usage beyond software suspend

[Gautham R Shenoy <ego@in.ibm.com>]

On Thu, Apr 05, 2007 at 01:46:33PM +0400, Oleg Nesterov wrote:
> On 04/02, Gautham R Shenoy wrote:
> >
> > +/*
> > + * Exempt the current process from being frozen for a certain event
> > + */
> > +static inline void freezer_exempt(unsigned long exempt_freeze_event)
> > +{
> > +	if (exempt_freeze_event == FE_NONE)
> > +		current->flags &= ~PF_FE_ALL;
> > +	else
> > +		current->flags |= exempt_freeze_event;
> > +}
> 
> So, a kernel_thread should call freezer_exempt(FE_XXX) somewhere at the
> beginning if it doesn't want to be considered as freezeable. But what if
> the freezing is already in progress? In that case freezer_exempt() should
> somehow clear TIF_FREEZE (if exempt_freeze_event doesn't match freeze_event
> parameter of freeze_processes()), otherwise we may hit a nasty bug, much
> worse than a freezing failure (which could be restarted).
>

That's a nice catch! The problem still exists in the current -mm with
tasks marking themselves PF_NOFREEZE and then calling try_to_freeze_tasks().

> try_to_freeze_tasks() succeeds because the task is !freezeable(), the
> task goes to refrigerator (while it should not), thaw_tasks() ignores
> process and it stays frozen.

Yup! That's really nasty.
> 
> Alternatively, we can do a re-check in refrigerator() to fix this race.
> In any case, it looks like freeze_event should be stored in a global var.

In the patch 2/8, I am maintaining a global system_freeze_event_mask.
The reads and writes to this mask are serialized by freezer_mutex.
But the mutex is held only in the freeze_processes() and
thaw_processes() path.

So the check in refrigerator() which is not in the freeze_processes()/
thaw_processes() path might need some more thought. Will cook up
something.

> 
> > --- linux-2.6.21-rc5.orig/kernel/sched.c
> > +++ linux-2.6.21-rc5/kernel/sched.c
> > @@ -5057,6 +5057,7 @@ static int migration_thread(void *data)
> >  	BUG_ON(rq->migration_thread != current);
> >
> >  	set_current_state(TASK_INTERRUPTIBLE);
> > +	freezer_exempt(FE_ALL);
> 
> This is a real nitpick, but it was hard to me to understand this change.
> Because it looks as if we have a subtle reason to set TASK_INTERRUPTIBLE
> before freezer_exempt(). Unless I missed something, I'd suggest to move
> freezer_exempt() up, before set_current_state().
> 
> The same for apm_mainloop().

Ok, no subtle reasons for freezer_exempt()ing after set_current_state().
So no problems changing the order. But (just curious), is there any specific
problem with this particular order ?

> 
> Oleg.
> 

Thanks for the review.
Regards
gautham.
-- 
Gautham R Shenoy
Linux Technology Center
IBM India.
"Freedom comes with a price tag of responsibility, which is still a bargain,
because Freedom is priceless!"