linux-kernel - Re: freezer problems

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Date:	Tue, 20 Feb 2007 03:12:09 +0300
From:	Oleg Nesterov <oleg@...sign.ru>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
Cc:	ego@...ibm.com, akpm@...l.org, paulmck@...ibm.com, mingo@...e.hu,
	vatsa@...ibm.com, dipankar@...ibm.com,
	venkatesh.pallipadi@...el.com, linux-kernel@...r.kernel.org,
	Pavel Machek <pavel@....cz>
Subject: Re: freezer problems

On 02/20, Rafael J. Wysocki wrote:
>
> On Monday, 19 February 2007 23:41, Oleg Nesterov wrote:
> > On 02/19, Rafael J. Wysocki wrote:
> > >
> > > On Monday, 19 February 2007 21:23, Oleg Nesterov wrote:
> > > 
> > > > > @@ -199,6 +189,10 @@ static void thaw_tasks(int thaw_user_spa
> > > > >
> > > > >         do_each_thread(g, p) {
> > > > > +               if (freezer_should_skip(p))
> > > > > +                       cancel_freezing(p);
> > > > > +       } while_each_thread(g, p);
> > > > > +       do_each_thread(g, p) {
> > > > >                 if (!freezeable(p))
> > > > >                         continue;
> > > > 
> > > > Any reason for 2 separate do_each_thread() loops ?
> > > 
> > > Yes.  If there is a "freeze" request pending for the vfork parent (TIF_FREEZE
> > > set), we have to cancel it before the child is unfrozen, since otherwise the
> > > parent may go freezing after we try to reset PF_FROZEN for it.
> > 
> > I see, thanks... thaw_process() doesn't take TIF_FREEZE into account.
> > 
> > But doesn't this mean we have a race?
> > 
> > Suppose that try_to_freeze_tasks() failed. It does cancel_freezing() for each
> > process before return, but what if some thread already checked TIF_FREEZE and
> > (for simplicity) it is preempted before frozen_process() in refrigerator().
> > 
> > thaw_tasks() runs, ignores this task (P), returns. P gets CPU, and becomes
> > frozen, but nobody will thaw it.
> > 
> > No?
> 
> Well, I think this is highly theoretical.  Namely, try_to_freeze_tasks() only
> fails after the timeout that's currently set to 20 sec., and it yields the CPU
> in each iteration of the main loop.  The task in question would have to refuse
> being frozen for 20 sec. and then suddenly decide to freeze itself right before
> try_to_freeze_tasks() checks the timeout for the very last time.  Then, it
> would have to get preempted at this very moment and stay unfrozen at least
> until thaw_tasks() starts running and in fact even longer.

Yes, yes, it is pure theroretical,

> I think we may avoid this by making try_to_freeze_tasks() sleep for some time
> after it has reset TIF_FREEZE for all tasks in the error path, if anyone is
> ever able to trigger it.

This makes this race  (pure theroretical) ** 2  :)

Still. May be it make sense to introduce cancel_freezing_and_thaw() function
(not right now) which stops the task from sleeping in refrigirator reliably.
I didn't think much about this, but it looks like we can fix coredump/exec
problems. Of course, this is not so important, we can ignore them at least
for now (->vfork_done is different, should be imho solved, because any user
can block freezer forever).

The fix:

	refrigerator:

	+	// we are going to call do_exit() really soon,
	+	// we have a pending SIGKILL
	+	if (current->signal->flags & SIGNAL_GROUP_EXIT)
	+		return;

		frozen_process(current);
		...


	zap_other_threads:

		for_each_subthread() {
			...

	+		// ---- SIGNAL_GROUP_EXIT is set ------
	+		// we can check sig->group_exit_task to detect de_thread,
	+		// but perhaps it doesn't hurt if the caller is do_group_exit
	+		cancel_freezing_and_thaw(p);
			sigaddset(&t->pending.signal, SIGKILL);
			signal_wake_up(t, 1);
		}

This way execer reliably kills all sub-threads and proceeds without blocking
try_to_freeze_tasks(). The same change could be done for zap_process() to fix
coredump.

Oleg.

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