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McKenney" To: Alan Stern Cc: David Goldblatt , mathieu.desnoyers@efficios.com, Florian Weimer , triegel@redhat.com, libc-alpha@sourceware.org, andrea.parri@amarulasolutions.com, will.deacon@arm.com, 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, dlustig@nvidia.com, linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] Linux: Implement membarrier function Reply-To: paulmck@linux.ibm.com References: <20181212194225.GB4170@linux.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 18121221-0052-0000-0000-00000366358A X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00010216; HX=3.00000242; KW=3.00000007; PH=3.00000004; SC=3.00000270; SDB=6.01130865; UDB=6.00587662; IPR=6.00910998; MB=3.00024673; MTD=3.00000008; XFM=3.00000015; UTC=2018-12-12 21:52:49 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 18121221-0053-0000-0000-00005F15216E Message-Id: <20181212215245.GC4170@linux.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2018-12-12_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=606 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1810050000 definitions=main-1812120185 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Dec 12, 2018 at 04:32:50PM -0500, Alan Stern wrote: > On Wed, 12 Dec 2018, Paul E. McKenney wrote: > > > OK. How about this one? > > > > P0 P1 P2 P3 > > Wa=2 rcu_read_lock() Wc=2 Wd=2 > > memb Wb=2 Rd=0 synchronize_rcu(); > > Rb=0 Rc=0 Ra=0 > > rcu_read_unlock() > > > > The model should say that it is allowed. Taking a look... > > > > P0 P1 P2 P3 > > Rd=0 > > Wd=2 > > synchronize_rcu(); > > Ra=0 > > Wa=2 > > membs > > rcu_read_lock() > > [m01] > > Rc=0 > > Wc=2 > > [m02] [m03] > > membe > > Rb=0 > > Wb=2 > > rcu_read_unlock() > > > > Looks allowed to me. If the synchronization of P1 and P2 were > > interchanged, it should be forbidden: > > > > P0 P1 P2 P3 > > Wa=2 Wb=2 rcu_read_lock() Wd=2 > > memb Rc=0 Wc=2 synchronize_rcu(); > > Rb=0 Rd=0 Ra=0 > > rcu_read_unlock() > > > > Taking a look... > > > > P0 P1 P2 P3 > > rcu_read_lock() > > Rd=0 > > Wa=2 Wb=2 Wd=2 > > membs synchronize_rcu(); > > [m01] > > Rc=0 > > Wc=2 > > rcu_read_unlock() > > [m02] Ra=0 [Forbidden?] > > membe > > Rb=0 For one thing, Wb=2 needs to be down here, apologies! Which then ... > Have you tried writing these as real litmus tests and running them > through herd? That comes later, but yes, I will do that. > > I believe that this ordering forbids the cycle: > > > > Wa=1 > membs -> [m01] -> Rc=0 -> Wc=2 -> rcu_read_unlock() -> > > return from synchronize_rcu() -> Ra > > > > Does this make sense, or am I missing something? > > It's hard to tell. What you have written here isn't justified by the > litmus test source code, since the position of m01 in P1's program > order is undetermined. How do you justify m01 -> Rc, for example? ... justifies Rc=0 following [m01]. > Write it this way instead, using the relations defined in the > sys_membarrier patch for linux-kernel.cat: > > memb ->memb-gp memb ->rcu-link Rc ->memb-rscsi Rc ->rcu-link > > rcu_read_unlock ->rcu-rscsi rcu_read_lock ->rcu-link > > synchronize_rcu ->rcu-gp synchronize_rcu ->rcu-link memb > > Recall that: > > memb-gp is the identity relation on sys_membarrier events, > > rcu-link includes (po? ; fre ; po), > > memb-rscsi is the identity relation on all events, > > rcu-rscsi links unlocks to their corresponding locks, and > > rcu-gp is the identity relation on synchronize_rcu events. > > These facts justify the cycle above. > > Leaving off the final rcu-link step, the sequence matches the > definition of rcu-fence (the relations are memb-gp, memb-rscsi, > rcu-rscsi, rcu-gp with rcu-links in between). Therefore the cycle is > forbidden. Understood, but that would be using the model to check the model. ;-) Thanx, Paul