From mboxrd@z Thu Jan 1 00:00:00 1970 From: paulmck@kernel.org Subject: [PATCH tip/core/rcu 01/32] tools/memory-model: Fix data race detection for unordered store and load Date: Wed, 2 Oct 2019 17:26:19 -0700 Message-ID: <20191003002650.11249-1-paulmck@kernel.org> References: <20191003001039.GA8027@paulmck-ThinkPad-P72> Return-path: In-Reply-To: <20191003001039.GA8027@paulmck-ThinkPad-P72> Sender: linux-kernel-owner@vger.kernel.org To: linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org, mingo@kernel.org Cc: stern@rowland.harvard.edu, parri.andrea@gmail.com, will@kernel.org, peterz@infradead.org, boqun.feng@gmail.com, npiggin@gmail.com, dhowells@redhat.com, j.alglave@ucl.ac.uk, luc.maranget@inria.fr, akiyks@gmail.com, "Paul E . McKenney" List-Id: linux-arch.vger.kernel.org From: Alan Stern 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 Acked-by: Andrea Parri Signed-off-by: Paul E. McKenney --- tools/memory-model/linux-kernel.cat | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/tools/memory-model/linux-kernel.cat b/tools/memory-model/linux-kernel.cat index ea2ff4b..2a9b4fe 100644 --- a/tools/memory-model/linux-kernel.cat +++ b/tools/memory-model/linux-kernel.cat @@ -197,7 +197,7 @@ empty (wr-incoh | rw-incoh | ww-incoh) as plain-coherence (* 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 -- 2.9.5 From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail.kernel.org ([198.145.29.99]:56006 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726354AbfJCA0x (ORCPT ); Wed, 2 Oct 2019 20:26:53 -0400 From: paulmck@kernel.org Subject: [PATCH tip/core/rcu 01/32] tools/memory-model: Fix data race detection for unordered store and load Date: Wed, 2 Oct 2019 17:26:19 -0700 Message-ID: <20191003002650.11249-1-paulmck@kernel.org> In-Reply-To: <20191003001039.GA8027@paulmck-ThinkPad-P72> References: <20191003001039.GA8027@paulmck-ThinkPad-P72> Sender: linux-arch-owner@vger.kernel.org List-ID: To: linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org, mingo@kernel.org Cc: stern@rowland.harvard.edu, parri.andrea@gmail.com, will@kernel.org, peterz@infradead.org, boqun.feng@gmail.com, npiggin@gmail.com, dhowells@redhat.com, j.alglave@ucl.ac.uk, luc.maranget@inria.fr, akiyks@gmail.com, "Paul E . McKenney" Message-ID: <20191003002619.jx-QSZ3cNLr1qkPu-Qqclks6C6yaSIYDwiuxQkLkKAk@z> From: Alan Stern 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 Acked-by: Andrea Parri Signed-off-by: Paul E. McKenney --- tools/memory-model/linux-kernel.cat | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/tools/memory-model/linux-kernel.cat b/tools/memory-model/linux-kernel.cat index ea2ff4b..2a9b4fe 100644 --- a/tools/memory-model/linux-kernel.cat +++ b/tools/memory-model/linux-kernel.cat @@ -197,7 +197,7 @@ empty (wr-incoh | rw-incoh | ww-incoh) as plain-coherence (* 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 -- 2.9.5