linux-kernel - Re: [PATCH] fix group stop with exit race

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Date:	Thu, 6 Dec 2007 19:22:07 +0300
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Davide Libenzi <davidel@...ilserver.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Roland McGrath <roland@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] fix group stop with exit race

On 12/05, Davide Libenzi wrote:
>
> On Wed, 5 Dec 2007, Oleg Nesterov wrote:
>
> > do_signal_stop() counts all sub-thread and sets ->group_stop_count accordingly.
> > Every thread should decrement ->group_stop_count and stop, the last one should
> > notify the parent.
> >
> > However a sub-thread can exit before it notices the signal_pending(), or it may
> > be somewhere in do_exit() already. In that case the group stop never finishes
> > properly.
> >
> > Note: this is a minimal fix, we can add some optimizations later. Say we can
> > return quickly if thread_group_empty(). Also, we can move some signal related
> > code from exit_notify() to exit_signals().
> >
> > Signed-off-by: Oleg Nesterov <oleg@...sign.ru>
>
> Looks OK for me, even though we're doing more work on the exit path. OTOH
> I don't see a non-racy way of doing it w/out grabbing the lock. Did you
> try to bench how much this change costs?

Yes, you are right, this unconditional spin_lock() is not good, especially for
exit_group/exec.

But please look at the next patch I am sending, it removes the pessimization
almost completely.

The only difference: when there is no group exit in progress, we are doing

	spin_lock_irq(siglock);
	if (!signal_pending()) {
		unlock and return
	}

while the current code does

	if (!signal_pending())
		return;
	spin_lock_irq(siglock);
	...

It would be nice to measure the difference, but I can't invent the test-case.

I tested (just in case) 100000 fork+exit 's

	perl -e 'fork ? wait : exit for 1 .. 100_000'

with and without the patch, and didn't notice any difference as expected.

> Acked-by: Davide Libenzi <davidel@...ilserver.org>

Thanks for looking at this!

Oleg.

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