Date: Wed, 21 Feb 2007 23:03:14 +0300
From: Oleg Nesterov <oleg@tv-sign.ru>
To: "Rafael J. Wysocki" <rjw@sisk.pl>
Cc: paulmck@linux.vnet.ibm.com, ego@in.ibm.com, akpm@osdl.org,
       paulmck@us.ibm.com, mingo@elte.hu, vatsa@in.ibm.com,
       dipankar@in.ibm.com, venkatesh.pallipadi@intel.com,
       linux-kernel@vger.kernel.org, Pavel Machek <pavel@ucw.cz>
Subject: Re: freezer problems
Message-ID: <20070221200314.GA91@tv-sign.ru>
References: <20070214144031.GA15257@in.ibm.com> <200702201929.03776.rjw@sisk.pl> <20070221181404.GC7063@linux.vnet.ibm.com> <200702211913.41883.rjw@sisk.pl>
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On 02/21, Rafael J. Wysocki wrote:
>
> On Wednesday, 21 February 2007 19:14, Paul E. McKenney wrote:
> > On Tue, Feb 20, 2007 at 07:29:01PM +0100, Rafael J. Wysocki wrote:
> > > On Tuesday, 20 February 2007 01:32, Rafael J. Wysocki wrote:
> > > > On Tuesday, 20 February 2007 01:12, Oleg Nesterov wrote:
> > > > Hm.  In the case discussed above we have a task that's right before calling
> > > > frozen_process(), so we can't thaw it, because it's not frozen.  It will be
> > > > frozen just in a while, but try_to_freeze_tasks() and thaw_tasks() have no
> > > > way to check this.
> > > > 
> > > > I think to close this race the refrigerator should check TIF_FREEZE and set
> > > > PF_FROZEN _and_ reset TIF_FREEZE under a lock

I personally think this is good. Not only this allows us to close the race,
I think we can do more.

>                                                      that would also have to be
> > > > taken by try_to_freeze_tasks() in the beginning of the error path.  This will
> > > > ensure that all tasks either freeze themselves before the error path in
> > > > try_to_freeze_tasks() is executed, or remain unfrozen.

How about take this lock in thaw_tasks() instead/too ?

Currently we need a separate loop in thaw_tasks() to handle PF_FREEZER_SKIP. This
means that PF_FREEZER_SKIP is not so generic: thaw_tasks() can't tolerate if such
a task was woken in between. What if we change thaw_process() to clear TIF_FREEZE ?

Note also that we can use task_lock() instead of global refrigerator_lock. This
means that thaw_process() should take it too, probably this is slowdown, but I
think not too much because thaw_process() is going to write to p->flags anyway.
In this case thaw_process() works perfectly as cancel_freezing_and_thaw() and
can be used to fix exec/coredump in future.

Thoughts?

Oleg.

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