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McKenney" To: linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org Cc: davidtgoldblatt@gmail.com, stern@rowland.harvard.edu, 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 Subject: Interrupts, smp_load_acquire(), smp_store_release(), etc. Reply-To: paulmck@linux.ibm.com MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 18102016-0040-0000-0000-00000483D3FD X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00009907; HX=3.00000242; KW=3.00000007; PH=3.00000004; SC=3.00000268; SDB=6.01105477; UDB=6.00572377; IPR=6.00885527; MB=3.00023839; MTD=3.00000008; XFM=3.00000015; UTC=2018-10-20 16:10:53 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 18102016-0041-0000-0000-0000088BEC54 Message-Id: <20181020161049.GA13756@linux.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2018-10-20_08:,, 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=437 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1807170000 definitions=main-1810200151 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hello! David Goldblatt (CCed) came up with an interesting pair of C++ litmus tests involving POSIX signals that have Linux-kernel counterparts involving interrupts. These litmus tests can (in paranoid theory, anyway) produce counter-intuitive results on architectures that use explicit fences to enforce ordering as part of a larger primitive, which in the specific case of smp_store_release() includes all architectures other than arm64, ia64, s390, SPARC, x86, and of course any UP-only architecture. David's first litmus test made use of the C11 sequentially consistent store, which in the Linux kernel would require two separate statements anyway (a WRITE_ONCE() either preceded or followed by smp_mb()), so the outcome that is counter-intuitive in C11 should be expected in the Linux kernel. (Yes, there are similar but more complicated examples that would have more interesting outcomes in the Linux kernel, but let's keep it simple for the moment.) The second (informal) litmus test has a more interesting Linux-kernel counterpart: void t1_interrupt(void) { r0 = READ_ONCE(y); smp_store_release(&x, 1); } void t1(void) { smp_store_release(&y, 1); } void t2(void) { r1 = smp_load_acquire(&x); r2 = smp_load_acquire(&y); } On store-reordering architectures that implement smp_store_release() as a memory-barrier instruction followed by a store, the interrupt could arrive betweentimes in t1(), so that there would be no ordering between t1_interrupt()'s store to x and t1()'s store to y. This could (again, in paranoid theory) result in the outcome r0==0 && r1==0 && r2==1. In practice, we analyzed exception paths in the sys_membarrier() review, and ended up with this function: static void ipi_mb(void *info) { smp_mb(); /* IPIs should be serializing but paranoid. */ } So how paranoid should we be with respect to interrupt handlers for smp_store_release(), smp_load_acquire(), and the various RMW atomic operations that are sometimes implemented with separate memory-barrier instructions? ;-) Thanx, Paul