linux-kernel - Re: [tip:sched/core] sched: Fix ancient race in do

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Message-ID: <CA+55aFx4nJwxWab4x8pKM+NEHPx_3kHbwf3xzQOLEpaXMks-CA@mail.gmail.com>
Date:	Sun, 29 Jan 2012 10:28:06 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	mingo@...hat.com, hpa@...or.com, linux-kernel@...r.kernel.org,
	a.p.zijlstra@...llo.nl, y-goto@...fujitsu.com,
	akpm@...ux-foundation.org, tglx@...utronix.de, mingo@...e.hu,
	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:sched/core] sched: Fix ancient race in do_exit()

On Sun, Jan 29, 2012 at 9:44 AM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> So it may be completely and utterly broken for some subtle reason, but
> I don't see what it would be. It seems to clean up and simplify the
> logic, and remove all the bogus workarounds for the fact that we used
> to do things stupidly.
>
> But maybe there's some reason for those "stupid" things. I just don't see it.

Hmm. Ok, so I see one reason for it. The silly extraneous "set task to
TASK_UNINTERRUPTIBLE" shouldn't matter normally - even if there are
spurious wakeups (say, disk IO while taking a page fault - not that I
see why we'd be on any wait queues yet), we'll just schedule a bit
more than we need in the extremely unlikely case that they hit us.

But for RT tasks with higher priorities, looping - even if we call
schedule() all the time - can cause livelocks. Damn. So while I don't
think the spurious wakeup is a big issue (I don't think it happens in
practice), it could lead to problems.

I think we could possibly use the "flags" field to do that "are we
just about to get woken up" logic, and set TASK_UNINTERRUPTIBLE in the
loop - and just clear "flags" before doing the wakeup (the same way we
used to clear "task"). Dunno. Ideas?

                         Linus
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