linux-kernel - Re: Unlock-lock questions and the Linux Kernel Memory Model

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Message-ID: <20171129195839.jqys2iwsdshpk4rk@hirez.programming.kicks-ass.net>
Date:   Wed, 29 Nov 2017 20:58:39 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Daniel Lustig <dlustig@...dia.com>
Cc:     Alan Stern <stern@...land.harvard.edu>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Andrea Parri <parri.andrea@...il.com>,
        Luc Maranget <luc.maranget@...ia.fr>,
        Jade Alglave <j.alglave@....ac.uk>,
        Boqun Feng <boqun.feng@...il.com>,
        Nicholas Piggin <npiggin@...il.com>,
        Will Deacon <will.deacon@....com>,
        David Howells <dhowells@...hat.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: Unlock-lock questions and the Linux Kernel Memory Model

On Wed, Nov 29, 2017 at 11:04:53AM -0800, Daniel Lustig wrote:
> "MP+wmb+xchg-acq" (or some such)
> 
> {}
> 
> P0(int *x, int *y)
> {
>         WRITE_ONCE(*x, 1);
>         smp_wmb();
>         WRITE_ONCE(*y, 1);
> }
> 
> P1(int *x, int *y)
> {
>         r1 = atomic_xchg_relaxed(y, 2);
>         r2 = smp_load_acquire(y);

Oh, I wasn't careful enough reading; these are both y.

And then Alan raises a good point in that RmW have dependencies.

Much tricker than I initially thought.

>         r3 = READ_ONCE(*x);
> }
> 
> exists (1:r1=1 /\ 1:r2=2 /\ 1:r3=0)