linux-kernel - Re: [RFC PATCH] freezer: revert 27920651fe "PM / Freezer: Make fake_signal_wake_up() wake TASK

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Date:	Tue, 1 Nov 2011 11:27:53 -0700
From:	Tejun Heo <tj@...nel.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Jeff Layton <jlayton@...hat.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Steve French <sfrench@...ba.org>, linux-kernel@...r.kernel.org,
	linux-pm@...r.kernel.org, linux-cifs@...r.kernel.org,
	"J. Bruce Fields" <bfields@...ldses.org>,
	Neil Brown <neilb@...e.de>, trond.myklebust@...app.com,
	linux-nfs@...r.kernel.org
Subject: Re: [RFC PATCH] freezer: revert 27920651fe "PM / Freezer: Make
 fake_signal_wake_up() wake TASK_KILLABLE tasks too"

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
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