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

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Date:	Tue, 01 Nov 2011 12:49:31 -0400
From:	Trond Myklebust <Trond.Myklebust@...app.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Jeff Layton <jlayton@...hat.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Steve French <sfrench@...ba.org>, linux-kernel@...r.kernel.org,
	Oleg Nesterov <oleg@...hat.com>, linux-pm@...r.kernel.org,
	linux-cifs@...r.kernel.org,
	"J. Bruce Fields" <bfields@...ldses.org>,
	Neil Brown <neilb@...e.de>, linux-nfs@...r.kernel.org
Subject: Re: [RFC PATCH] freezer: revert 27920651fe "PM / Freezer: Make
 fake_signal_wake_up() wake TASK_KILLABLE tasks too"

On Tue, 2011-11-01 at 09:30 -0700, Tejun Heo wrote: 
> Hello, Jeff.
> 
> On Tue, Nov 01, 2011 at 06:59:58AM -0400, Jeff Layton wrote:
> > > I suppose we could look at going back to the world of complicated
> > > signal handling and TASK_INTERRUPTIBLE, but that's not a trivial change
> > > either. The TASK_WAKE_FREEZABLE flag you mention might make more sense
> > > than doing that.
> 
> I see.
> 
> > Also, since task state and signal handling clearly isn't my forte...can
> > you clarify what the main difference is in using a TASK_KILLABLE sleep
> > vs. TASK_INTERRUPTIBLE with all signals but SIGKILL blocked?
> >
> > I know that the process state would end up different (S vs. D state).
> > Is there anything else we'd need to be concerned with if we converted
> > all these call sites to use such a scheme in lieu of a new
> > TASK_WAKE_FREEZABLE flag or similar?
> 
> Manipulating sigmask would work in most cases but there are corner
> cases (e.g. debug signals will force the mask off) and diddling with
> sigmask in kernel generally isn't a good idea.
> 
> If TASK_KILLABLE is only used for killable && freezable, that probably
> would be the cleanest solution but given the name and the fact that
> people are in general much more aware of SIGKILL's then freezer, I
> think adding such assumption is likely to cause very subtle bugs.  For
> example, mem_cgroup_handle_oom() seems to assume that if it's waken
> from TASK_KILLABLE either the condition it was waiting for is true or
> it is dying.
> 
> If there are only a handful sites which need this type of behavior,
> wouldn't something like the following work?
> 
> #define wait_event_freezekillable(wq, condition)			\
> do {									\
> 	DEFINE_WAIT(__wait);						\
> 	for (;;) {							\
> 		prepare_to_wait(&wq, &__wait, TASK_INTERRUPTIBLE);	\
> 		if (condition || fatal_signal_pending(current))		\
> 			break;						\
> 		schedule();						\
> 		try_to_freeze();					\
> 	}								\
> 	finish_wait(&wq, &__wait);					\
> } while (0)

Err... Won't this end up busy-waiting if a non-fatal signal is received?

-- 
Trond Myklebust
Linux NFS client maintainer

NetApp
Trond.Myklebust@...app.com
www.netapp.com

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