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Date:   Wed, 18 Sep 2019 10:24:10 -0700
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Andrea Parri <parri.andrea@gmail.com>
Cc:     Alan Stern <stern@rowland.harvard.edu>,
        LKMM Maintainers -- Akira Yokosawa <akiyks@gmail.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        Daniel Lustig <dlustig@nvidia.com>,
        David Howells <dhowells@redhat.com>,
        Jade Alglave <j.alglave@ucl.ac.uk>,
        Luc Maranget <luc.maranget@inria.fr>,
        Nicholas Piggin <npiggin@gmail.com>,
        "Paul E. McKenney" <paulmck@linux.ibm.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Will Deacon <will@kernel.org>,
        Kernel development list <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH RFC] tools/memory-model: Fix data race detection for
 unordered store and load
Message-ID: <20190918172410.GJ30224@paulmck-ThinkPad-P72>
Reply-To: paulmck@kernel.org
References: <Pine.LNX.4.44L0.1909061649430.1627-100000@iolanthe.rowland.org>
 <20190917113959.GA19404@andrea.guest.corp.microsoft.com>
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On Tue, Sep 17, 2019 at 01:39:59PM +0200, Andrea Parri wrote:
> On Fri, Sep 06, 2019 at 04:57:22PM -0400, Alan Stern wrote:
> > Currently the Linux Kernel Memory Model gives an incorrect response
> > for the following litmus test:
> > 
> > C plain-WWC
> > 
> > {}
> > 
> > P0(int *x)
> > {
> > 	WRITE_ONCE(*x, 2);
> > }
> > 
> > P1(int *x, int *y)
> > {
> > 	int r1;
> > 	int r2;
> > 	int r3;
> > 
> > 	r1 = READ_ONCE(*x);
> > 	if (r1 == 2) {
> > 		smp_rmb();
> > 		r2 = *x;
> > 	}
> > 	smp_rmb();
> > 	r3 = READ_ONCE(*x);
> > 	WRITE_ONCE(*y, r3 - 1);
> > }
> > 
> > P2(int *x, int *y)
> > {
> > 	int r4;
> > 
> > 	r4 = READ_ONCE(*y);
> > 	if (r4 > 0)
> > 		WRITE_ONCE(*x, 1);
> > }
> > 
> > exists (x=2 /\ 1:r2=2 /\ 2:r4=1)
> > 
> > The memory model says that the plain read of *x in P1 races with the
> > WRITE_ONCE(*x) in P2.
> > 
> > The problem is that we have a write W and a read R related by neither
> > fre or rfe, but rather W ->coe W' ->rfe R, where W' is an intermediate
> > write (the WRITE_ONCE() in P0).  In this situation there is no
> > particular ordering between W and R, so either a wr-vis link from W to
> > R or an rw-xbstar link from R to W would prove that the accesses
> > aren't concurrent.
> > 
> > But the LKMM only looks for a wr-vis link, which is equivalent to
> > assuming that W must execute before R.  This is not necessarily true
> > on non-multicopy-atomic systems, as the WWC pattern demonstrates.
> > 
> > This patch changes the LKMM to accept either a wr-vis or a reverse
> > rw-xbstar link as a proof of non-concurrency.
> > 
> > Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
> 
> Acked-by: Andrea Parri <parri.andrea@gmail.com>

Applied, thank you both!

							Thanx, Paul