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Date: Thu, 30 Nov 2017 10:02:20 +0000
From: Will Deacon <will.deacon@arm.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Daniel Lustig <dlustig@nvidia.com>,
        Alan Stern <stern@rowland.harvard.edu>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Andrea Parri <parri.andrea@gmail.com>,
        Luc Maranget <luc.maranget@inria.fr>,
        Jade Alglave <j.alglave@ucl.ac.uk>, Boqun Feng <boqun.feng@gmail.com>,
        Nicholas Piggin <npiggin@gmail.com>,
        David Howells <dhowells@redhat.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Kernel development list <linux-kernel@vger.kernel.org>
Subject: Re: Unlock-lock questions and the Linux Kernel Memory Model
Message-ID: <20171130100219.GA21983@arm.com>
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On Wed, Nov 29, 2017 at 08:46:02PM +0100, Peter Zijlstra wrote:
> On Wed, Nov 29, 2017 at 11:04:53AM -0800, Daniel Lustig wrote:
> 
> > While we're here, let me ask about another test which isn't directly
> > about unlock/lock but which is still somewhat related to this
> > discussion:
> > 
> > "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);
> >         r3 = READ_ONCE(*x);
> > }
> > 
> > exists (1:r1=1 /\ 1:r2=2 /\ 1:r3=0)
> > 
> > C/C++ would call the atomic_xchg_relaxed part of a release sequence
> > and hence would forbid this outcome.
> 
> That's just weird. Either its _relaxed, or its _release. Making _relaxed
> mean _release is just daft.

I don't think it's actually that weird. If, for example, the write to *y in
P0 was part of an UNLOCK operation and the load_acquire of y in P1 was a
LOCK operation, then the xchg could just be setting some waiting bit in
other bits of the lock word. C/C++ also requires order here if the xchg is
done on its own thread.

Will