From mboxrd@z Thu Jan 1 00:00:00 1970 From: Peter Zijlstra Subject: Re: [PATCH] padata: use smp_mb in padata_reorder to avoid orphaned padata jobs Date: Tue, 16 Jul 2019 15:13:50 +0200 Message-ID: <20190716131350.GA3402@hirez.programming.kicks-ass.net> References: <20190711221205.29889-1-daniel.m.jordan@oracle.com> <20190712100636.mqdr567p7ozanlyl@gondor.apana.org.au> <20190712101012.GW14601@gauss3.secunet.de> <20190712160737.iniaaxlsnhs6azg5@ca-dmjordan1.us.oracle.com> <20190716125309.GA10672@andrea> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <20190716125309.GA10672@andrea> Sender: linux-kernel-owner@vger.kernel.org To: Andrea Parri Cc: Daniel Jordan , Steffen Klassert , Herbert Xu , boqun.feng@gmail.com, paulmck@linux.ibm.com, linux-arch@vger.kernel.org, linux-crypto@vger.kernel.org, linux-kernel@vger.kernel.org, will@kernel.org List-Id: linux-arch.vger.kernel.org On Tue, Jul 16, 2019 at 02:53:09PM +0200, Andrea Parri wrote: > C daniel-padata > > { } > > P0(atomic_t *reorder_objects, spinlock_t *pd_lock) > { > int r0; > > spin_lock(pd_lock); > spin_unlock(pd_lock); > smp_mb(); > r0 = atomic_read(reorder_objects); > } > > P1(atomic_t *reorder_objects, spinlock_t *pd_lock, spinlock_t *reorder_lock) > { > int r1; > > spin_lock(reorder_lock); > atomic_inc(reorder_objects); > spin_unlock(reorder_lock); > //smp_mb(); > r1 = spin_trylock(pd_lock); > } > > exists (0:r0=0 /\ 1:r1=0) > > It seems worth noticing that this test's "exists" clause is satisfiable > according to the (current) memory consistency model. (Informally, this > can be explained by noticing that the RELEASE from the spin_unlock() in > P1 does not provide any order between the atomic increment and the read > part of the spin_trylock() operation.) FWIW, uncommenting the smp_mb() > in P1 would suffice to prevent this clause from being satisfiable; I am > not sure, however, whether this approach is feasible or ideal... (sorry, > I'm definitely not too familiar with this code... ;/) Urgh, that one again. Yes, you need the smp_mb(); although a whole bunch of architectures can live without it. IIRC it is part of the eternal RCsc/RCpc debate. Paul/RCU have their smp_mb__after_unlock_lock() that is about something similar, although we've so far confinsed that to the RCU code, because of how confusing that all is. From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from bombadil.infradead.org ([198.137.202.133]:58972 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726997AbfGPNOE (ORCPT ); Tue, 16 Jul 2019 09:14:04 -0400 Date: Tue, 16 Jul 2019 15:13:50 +0200 From: Peter Zijlstra Subject: Re: [PATCH] padata: use smp_mb in padata_reorder to avoid orphaned padata jobs Message-ID: <20190716131350.GA3402@hirez.programming.kicks-ass.net> References: <20190711221205.29889-1-daniel.m.jordan@oracle.com> <20190712100636.mqdr567p7ozanlyl@gondor.apana.org.au> <20190712101012.GW14601@gauss3.secunet.de> <20190712160737.iniaaxlsnhs6azg5@ca-dmjordan1.us.oracle.com> <20190716125309.GA10672@andrea> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190716125309.GA10672@andrea> Sender: linux-arch-owner@vger.kernel.org List-ID: To: Andrea Parri Cc: Daniel Jordan , Steffen Klassert , Herbert Xu , boqun.feng@gmail.com, paulmck@linux.ibm.com, linux-arch@vger.kernel.org, linux-crypto@vger.kernel.org, linux-kernel@vger.kernel.org, will@kernel.org Message-ID: <20190716131350.aha1Z7JKneOQLfM6BDBHGyxGFeWcm2c5yAtL96oF-1k@z> On Tue, Jul 16, 2019 at 02:53:09PM +0200, Andrea Parri wrote: > C daniel-padata > > { } > > P0(atomic_t *reorder_objects, spinlock_t *pd_lock) > { > int r0; > > spin_lock(pd_lock); > spin_unlock(pd_lock); > smp_mb(); > r0 = atomic_read(reorder_objects); > } > > P1(atomic_t *reorder_objects, spinlock_t *pd_lock, spinlock_t *reorder_lock) > { > int r1; > > spin_lock(reorder_lock); > atomic_inc(reorder_objects); > spin_unlock(reorder_lock); > //smp_mb(); > r1 = spin_trylock(pd_lock); > } > > exists (0:r0=0 /\ 1:r1=0) > > It seems worth noticing that this test's "exists" clause is satisfiable > according to the (current) memory consistency model. (Informally, this > can be explained by noticing that the RELEASE from the spin_unlock() in > P1 does not provide any order between the atomic increment and the read > part of the spin_trylock() operation.) FWIW, uncommenting the smp_mb() > in P1 would suffice to prevent this clause from being satisfiable; I am > not sure, however, whether this approach is feasible or ideal... (sorry, > I'm definitely not too familiar with this code... ;/) Urgh, that one again. Yes, you need the smp_mb(); although a whole bunch of architectures can live without it. IIRC it is part of the eternal RCsc/RCpc debate. Paul/RCU have their smp_mb__after_unlock_lock() that is about something similar, although we've so far confinsed that to the RCU code, because of how confusing that all is.