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Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Sat, 20 Oct 2018 17:04:14 -0400 Received: from b01ledav003.gho.pok.ibm.com (b01ledav003.gho.pok.ibm.com [9.57.199.108]) by b01cxnp22034.gho.pok.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id w9KL4Dks32833762 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=FAIL); Sat, 20 Oct 2018 21:04:13 GMT Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 222A6B205F; Sat, 20 Oct 2018 21:04:13 +0000 (GMT) Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 09E6FB2070; Sat, 20 Oct 2018 21:04:13 +0000 (GMT) Received: from paulmck-ThinkPad-W541 (unknown [9.80.226.122]) by b01ledav003.gho.pok.ibm.com (Postfix) with ESMTP; Sat, 20 Oct 2018 21:04:12 +0000 (GMT) Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id 8ED2416C3598; Sat, 20 Oct 2018 14:04:13 -0700 (PDT) Date: Sat, 20 Oct 2018 14:04:13 -0700 From: "Paul E. McKenney" To: Alan Stern Cc: linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org, davidtgoldblatt@gmail.com, 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: Re: Interrupts, smp_load_acquire(), smp_store_release(), etc. Reply-To: paulmck@linux.ibm.com References: <20181020161049.GA13756@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: 18102021-2213-0000-0000-00000306F18E X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00009909; HX=3.00000242; KW=3.00000007; PH=3.00000004; SC=3.00000268; SDB=6.01105575; UDB=6.00572436; IPR=6.00885625; MB=3.00023841; MTD=3.00000008; XFM=3.00000015; UTC=2018-10-20 21:04:17 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 18102021-2214-0000-0000-00005BF7403B Message-Id: <20181020210413.GB2674@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=647 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1807170000 definitions=main-1810200196 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, Oct 20, 2018 at 04:18:37PM -0400, Alan Stern wrote: > On Sat, 20 Oct 2018, Paul E. McKenney wrote: > > > 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. > > This is disconcerting only if we assume that t1_interrupt() has to be > executed by the same CPU as t1(). If the interrupt could be fielded by > a different CPU then the paranoid outcome is perfectly understandable, > even in an SC context. > > So the question really should be limited to situations where a handler > is forced to execute in the context of a particular thread. While > POSIX does allow such restrictions for user programs, I'm not aware of > any similar mechanism in the kernel. Good point, and I was in fact assuming that t1() and t1_interrupt() were executing on the same CPU. This sort of thing happens naturally in the kernel when both t1() and t1_interrupt() are accessing per-CPU variables. Thanx, Paul