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

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Message-ID: <CA+55aFyO0KxGQxYYUqmVrNqG6qVFnRT2g-kYYKtH5QG3vTnO=A@mail.gmail.com>
Date:	Sat, 28 Jan 2012 13:12:02 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	mingo@...hat.com, hpa@...or.com, linux-kernel@...r.kernel.org,
	a.p.zijlstra@...llo.nl, torvalds@...ux-foundation.org,
	y-goto@...fujitsu.com, akpm@...ux-foundation.org,
	tglx@...utronix.de, oleg@...hat.com, mingo@...e.hu
Cc:	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:sched/core] sched: Fix ancient race in do_exit()

On Sat, Jan 28, 2012 at 4:03 AM, tip-bot for Yasunori Goto
<y-goto@...fujitsu.com> wrote:
>
> sched: Fix ancient race in do_exit()

Ugh.

It would be much nicer to just clear the rwsem waiter->task thing
*after* waking the task up, which would avoid this race entirely,
afaik.

Tell me, why wouldn't that work? rwsem_down_failed_common() does

        /* wait to be given the lock */
        for (;;) {
                if (!waiter.task)
                        break;
               ...

so then we wouldn't need the task refcount crap in rwsem either etc,
and we'd get rid of all races with wakeup.

I wonder why we're clearing that whole waiter->task so early.

                       Linus
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