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Date:   Sat, 7 Sep 2019 08:57:06 -0700
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Alan Stern <stern@rowland.harvard.edu>
Cc:     LKMM Maintainers -- Akira Yokosawa <akiyks@gmail.com>,
        Andrea Parri <parri.andrea@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: <20190907155706.GA27893@paulmck-ThinkPad-P72>
Reply-To: paulmck@kernel.org
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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>

Queued and pushed for review and testing, thank you very much!

						Thanx, Paul

> ---
> 
>  tools/memory-model/linux-kernel.cat |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> Index: usb-devel/tools/memory-model/linux-kernel.cat
> ===================================================================
> --- usb-devel.orig/tools/memory-model/linux-kernel.cat
> +++ usb-devel/tools/memory-model/linux-kernel.cat
> @@ -197,7 +197,7 @@ empty (wr-incoh | rw-incoh | ww-incoh) a
>  (* Actual races *)
>  let ww-nonrace = ww-vis & ((Marked * W) | rw-xbstar) & ((W * Marked) | wr-vis)
>  let ww-race = (pre-race & co) \ ww-nonrace
> -let wr-race = (pre-race & (co? ; rf)) \ wr-vis
> +let wr-race = (pre-race & (co? ; rf)) \ wr-vis \ rw-xbstar^-1
>  let rw-race = (pre-race & fr) \ rw-xbstar
>  
>  flag ~empty (ww-race | wr-race | rw-race) as data-race
>