linux-kernel - Re: a competition when some threads acquire futex

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Message-ID: <20170906090631.tcer7olhogwxfrlh@hirez.programming.kicks-ass.net>
Date:   Wed, 6 Sep 2017 11:06:31 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     "chengjian (D)" <cj.chengjian@...wei.com>, huawei.libin@...wei.com,
        mingo@...hat.com, dvhart@...radead.org,
        linux-kernel@...r.kernel.org
Subject: Re: a competition when some threads acquire futex

On Wed, Sep 06, 2017 at 10:56:08AM +0200, Peter Zijlstra wrote:

> Right, but even if it was a coherent patch, I'm not sure it makes sense.
> 
> futex_wait() / futex_wake() don't make ordering guarantees and in
> general you don't get to have wakeup preemption if you don't run a
> PREEMPT kernel.
> 
> So what makes this wakeup so special? Any changelog would need to have a
> convincing argument.

Also, even on !PREEMPT, if that wakeup sets NEED_RESCHED, the return to
userspace after futex_wake() should reschedule.


So I'm really not getting it.