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McKenney" To: Peter Zijlstra Cc: Will Deacon , Alan Stern , LKMM Maintainers -- Akira Yokosawa , Andrea Parri , Boqun Feng , Daniel Lustig , David Howells , Jade Alglave , Luc Maranget , Nicholas Piggin , Kernel development list Subject: Re: [PATCH v3] tools/memory-model: Add extra ordering for locks and remove it for ordinary release/acquire Reply-To: paulmck@linux.vnet.ibm.com References: <20180710162555.GV3593@linux.vnet.ibm.com> <20180711094344.GE13963@arm.com> <20180711163456.GL2476@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180711163456.GL2476@hirez.programming.kicks-ass.net> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 18071118-2213-0000-0000-000002C93E99 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00009352; HX=3.00000241; KW=3.00000007; PH=3.00000004; SC=3.00000266; SDB=6.01059888; UDB=6.00544000; IPR=6.00837808; MB=3.00022105; MTD=3.00000008; XFM=3.00000015; UTC=2018-07-11 18:08:26 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 18071118-2214-0000-0000-00005ACC3D0F Message-Id: <20180711181041.GZ3593@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2018-07-11_04:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=0 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 mlxscore=0 impostorscore=0 mlxlogscore=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1806210000 definitions=main-1807110191 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jul 11, 2018 at 06:34:56PM +0200, Peter Zijlstra wrote: > On Wed, Jul 11, 2018 at 10:43:45AM +0100, Will Deacon wrote: > > Hi Alan, > > > > On Tue, Jul 10, 2018 at 02:18:13PM -0400, Alan Stern wrote: > > > More than one kernel developer has expressed the opinion that the LKMM > > > should enforce ordering of writes by locking. In other words, given > > > the following code: > > > > > > WRITE_ONCE(x, 1); > > > spin_unlock(&s): > > > spin_lock(&s); > > > WRITE_ONCE(y, 1); > > > > > > the stores to x and y should be propagated in order to all other CPUs, > > > even though those other CPUs might not access the lock s. In terms of > > > the memory model, this means expanding the cumul-fence relation. > > > > > > Locks should also provide read-read (and read-write) ordering in a > > > similar way. Given: > > > > > > READ_ONCE(x); > > > spin_unlock(&s); > > > spin_lock(&s); > > > READ_ONCE(y); // or WRITE_ONCE(y, 1); > > > > > > the load of x should be executed before the load of (or store to) y. > > > The LKMM already provides this ordering, but it provides it even in > > > the case where the two accesses are separated by a release/acquire > > > pair of fences rather than unlock/lock. This would prevent > > > architectures from using weakly ordered implementations of release and > > > acquire, which seems like an unnecessary restriction. The patch > > > therefore removes the ordering requirement from the LKMM for that > > > case. > > > > > > All the architectures supported by the Linux kernel (including RISC-V) > > > do provide this ordering for locks, albeit for varying reasons. > > > Therefore this patch changes the model in accordance with the > > > developers' wishes. > > > > > > Signed-off-by: Alan Stern > > > > Thanks, I'm happy with this version of the patch: > > > > Reviewed-by: Will Deacon > > Me too! Thanks Alan. > > Acked-by: Peter Zijlstra (Intel) And I applies you ask as well, thank you! Thanx, Paul