From mboxrd@z Thu Jan 1 00:00:00 1970 From: Will Deacon Subject: [PATCH 10/13] tools/memory-model: Remove smp_read_barrier_depends() from informal doc Date: Fri, 8 Nov 2019 17:01:17 +0000 Message-ID: <20191108170120.22331-11-will@kernel.org> References: <20191108170120.22331-1-will@kernel.org> Mime-Version: 1.0 Content-Transfer-Encoding: 8bit Return-path: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1573232524; bh=wCL903ScuiGofElDf7EX9mpRBQ2yI6oD2F0VBzIhMiI=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=MAKtRC+RFnOPFfR4QyCGaTOzj4F+z/r+5QEkX66W3MQdu1xPBVYek4UJmHra0op04 e4o7aS6/9iO9gOqLrnywDYUUkfJppi3EUPo5JXIRfHpargTMHQjCYN2EdNOsQO/+3u NJ8X4Q6h7xFJrtdqq5EciOWZizRB/aadRDMwGttk= In-Reply-To: <20191108170120.22331-1-will@kernel.org> Sender: linux-kernel-owner@vger.kernel.org List-ID: Content-Type: text/plain; charset="us-ascii" To: linux-kernel@vger.kernel.org Cc: Will Deacon , Yunjae Lee , SeongJae Park , "Paul E. McKenney" , Josh Triplett , Matt Turner , Ivan Kokshaysky , Richard Henderson , Peter Zijlstra , Alan Stern , Michael Ellerman , "Michael S. Tsirkin" , Jason Wang , Arnd Bergmann , Joe Perches , Boqun Feng , linux-alpha@vger.kernel.org, virtualization@lists.linux-foundation.org 'smp_read_barrier_depends()' has gone the way of mmiowb() and so many esoteric memory barriers before it. Drop the two mentions of this deceased barrier from the LKMM informal explanation document. Signed-off-by: Will Deacon --- .../Documentation/explanation.txt | 26 +++++++++---------- 1 file changed, 12 insertions(+), 14 deletions(-) diff --git a/tools/memory-model/Documentation/explanation.txt b/tools/memory-model/Documentation/explanation.txt index 488f11f6c588..3050bf67b8d0 100644 --- a/tools/memory-model/Documentation/explanation.txt +++ b/tools/memory-model/Documentation/explanation.txt @@ -1118,12 +1118,10 @@ maintain at least the appearance of FIFO order. In practice, this difficulty is solved by inserting a special fence between P1's two loads when the kernel is compiled for the Alpha architecture. In fact, as of version 4.15, the kernel automatically -adds this fence (called smp_read_barrier_depends() and defined as -nothing at all on non-Alpha builds) after every READ_ONCE() and atomic -load. The effect of the fence is to cause the CPU not to execute any -po-later instructions until after the local cache has finished -processing all the stores it has already received. Thus, if the code -was changed to: +adds this fence after every READ_ONCE() and atomic load on Alpha. The +effect of the fence is to cause the CPU not to execute any po-later +instructions until after the local cache has finished processing all +the stores it has already received. Thus, if the code was changed to: P1() { @@ -1142,14 +1140,14 @@ READ_ONCE() or another synchronization primitive rather than accessed directly. The LKMM requires that smp_rmb(), acquire fences, and strong fences -share this property with smp_read_barrier_depends(): They do not allow -the CPU to execute any po-later instructions (or po-later loads in the -case of smp_rmb()) until all outstanding stores have been processed by -the local cache. In the case of a strong fence, the CPU first has to -wait for all of its po-earlier stores to propagate to every other CPU -in the system; then it has to wait for the local cache to process all -the stores received as of that time -- not just the stores received -when the strong fence began. +share this property: They do not allow the CPU to execute any po-later +instructions (or po-later loads in the case of smp_rmb()) until all +outstanding stores have been processed by the local cache. In the +case of a strong fence, the CPU first has to wait for all of its +po-earlier stores to propagate to every other CPU in the system; then +it has to wait for the local cache to process all the stores received +as of that time -- not just the stores received when the strong fence +began. And of course, none of this matters for any architecture other than Alpha. -- 2.24.0.rc1.363.gb1bccd3e3d-goog