Re: [RFC PATCH] freezer: revert 27920651fe "PM / Freezer: Make fake_signal_wake_up() wake TASK_KILLABLE tasks too"

[Tejun Heo <tj@kernel.org>]

Hello,

On Tue, Nov 01, 2011 at 07:13:29PM +0100, Oleg Nesterov wrote:
> On 11/01, Tejun Heo wrote:
> >
> > Yeah yeah, Trond already pointed it out.  I forgot about the
> > sigpending special case in schedule(), which I think is rather odd,
> 
> I disagree with "rather odd" ;)
> 
> We have a lot of examples of
> 
> 	current->state = TASK_INTERRUPTIBLE;
> 	...
> 	if (signal_pending())
> 		break;
> 	schedule();
> 
> Without that special case in schedule() the code above becomes racy.
> Just consider __wait_event_interruptible().

But __wait_event_interruptible() does proper set-TASK_*, check
sigpending and schedule() sequence.  As long as the waker performs
seg-sigpending, wakeup sequence in the correct order, nothing is
broken (as w/ any other wakeup conditions).  The special case deals
with callers which don't check sigpending between set-TASK_* and
schedule() and that's the part I think is a bit odd.  Whether I feel
odd or not is irrelevant tho - it's already there.

> > Any better ideas?
> 
> Well. As a simple (probably temporary) fix, I'd suggest
> 
> 	#define wait_event_freezekillable(wq, condition)
> 	{
> 		freezer_do_not_count();
> 		__retval = wait_event_killable(condition);
> 		freezer_count();
> 		__retval;
> 	}
> 
> Do you think it can work?

Yeah, probably.  I was hoping to remove count/do_not_count tho.
Hmmm... maybe we can just flip PF_NOFREEZE instead with a bit of
modification, I think.

Thanks.

-- 
tejun