linux-kernel - Re: [patch 09/12] futex: Provide state handling for exec() as well

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Message-ID: <alpine.DEB.2.21.1911071053400.4256@nanos.tec.linutronix.de>
Date:   Thu, 7 Nov 2019 10:54:40 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>
cc:     LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Darren Hart <darren@...art.com>,
        Yi Wang <wang.yi59@....com.cn>,
        Yang Tao <yang.tao172@....com.cn>,
        Oleg Nesterov <oleg@...hat.com>,
        Florian Weimer <fweimer@...hat.com>,
        Carlos O'Donell <carlos@...hat.com>,
        Alexander Viro <viro@...iv.linux.org.uk>
Subject: Re: [patch 09/12] futex: Provide state handling for exec() as well

On Thu, 7 Nov 2019, Peter Zijlstra wrote:
> On Wed, Nov 06, 2019 at 10:55:43PM +0100, Thomas Gleixner wrote:
> 
> > +static void futex_cleanup_end(struct task_struct *tsk, int state)
> > +{
> > +	/*
> > +	 * Lockless store. The only side effect is that an observer might
> > +	 * take another loop until it becomes visible.
> > +	 */
> > +	tsk->futex_state = state;
> 
> As I mentioned yesterday, paranoia would've made me write this as
> smp_store_release(), also to avoid it creaping back into the locked
> region.
> 
> That is, the comment above deals with it being visible late, but it
> could be visible early.
> 
> At the same time, if this is a release, what does it pair with. The
> obvious place would be the load in handle_exit_race() but that didn't
> want to make sense last night -- and I'm not sure it wants to make more
> sense today.

Right. I couldn't come up with any sensible argument either.

Thanks,

	tglx