linux-kernel - Re: [PATCH 2/2] tools/memory-model: Add write ordering by release-acquire and by locks

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Date:   Mon, 25 Jun 2018 10:19:20 +0200
From:   Andrea Parri <andrea.parri@...rulasolutions.com>
To:     Will Deacon <will.deacon@....com>
Cc:     Alan Stern <stern@...land.harvard.edu>,
        LKMM Maintainers -- Akira Yokosawa <akiyks@...il.com>,
        Boqun Feng <boqun.feng@...il.com>,
        David Howells <dhowells@...hat.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Luc Maranget <luc.maranget@...ia.fr>,
        Nicholas Piggin <npiggin@...il.com>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] tools/memory-model: Add write ordering by
 release-acquire and by locks

On Fri, Jun 22, 2018 at 07:30:08PM +0100, Will Deacon wrote:
> > > I think the second example would preclude us using LDAPR for load-acquire,

> I don't think it's a moot point. We want new architectures to implement
> acquire/release efficiently, and it's not unlikely that they will have
> acquire loads that are similar in semantics to LDAPR. This patch prevents
> them from doing so,

By this same argument, you should not be a "big fan" of rfi-rel-acq in ppo ;)
consider, e.g., the two litmus tests below: what am I missing?

  Andrea


C MP+fencewmbonceonce+pooncerelease-rfireleaseacquire-poacquireonce

{}

P0(int *x, int *y)
{
	WRITE_ONCE(*x, 1);
	smp_wmb();
	WRITE_ONCE(*y, 1);
}

P1(int *x, int *y, int *z)
{
	r0 = READ_ONCE(*y);
	smp_store_release(z, 1);
	r1 = smp_load_acquire(z);
	r2 = READ_ONCE(*x);
}

exists (1:r0=1 /\ 1:r1=1 /\ 1:r2=0)


AArch64 MP+dmb.st+popl-rfilq-poqp
"DMB.STdWW Rfe PodRWPL RfiLQ PodRRQP Fre"
Generator=diyone7 (version 7.49+02(dev))
Prefetch=0:x=F,0:y=W,1:y=F,1:x=T
Com=Rf Fr
Orig=DMB.STdWW Rfe PodRWPL RfiLQ PodRRQP Fre
{
0:X1=x; 0:X3=y;
1:X1=y; 1:X3=z; 1:X6=x;
}
 P0          | P1            ;
 MOV W0,#1   | LDR W0,[X1]   ;
 STR W0,[X1] | MOV W2,#1     ;
 DMB ST      | STLR W2,[X3]  ;
 MOV W2,#1   | LDAPR W4,[X3] ;
 STR W2,[X3] | LDR W5,[X6]   ;
exists
(1:X0=1 /\ 1:X4=1 /\ 1:X5=0)