From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933047AbcE0JDN (ORCPT ); Fri, 27 May 2016 05:03:13 -0400 Received: from merlin.infradead.org ([205.233.59.134]:45434 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932567AbcE0JDM (ORCPT ); Fri, 27 May 2016 05:03:12 -0400 Date: Fri, 27 May 2016 11:02:21 +0200 From: Peter Zijlstra To: Martin Schwidefsky Cc: linux-kernel@vger.kernel.org, torvalds@linux-foundation.org, manfred@colorfullife.com, dave@stgolabs.net, paulmck@linux.vnet.ibm.com, will.deacon@arm.com, boqun.feng@gmail.com, Waiman.Long@hpe.com, tj@kernel.org, pablo@netfilter.org, kaber@trash.net, davem@davemloft.net, oleg@redhat.com, netfilter-devel@vger.kernel.org, sasha.levin@oracle.com, hofrat@osadl.org, rth@twiddle.net, vgupta@synopsys.com, linux@armlinux.org.uk, realmz6@gmail.com, rkuo@codeaurora.org, tony.luck@intel.com, james.hogan@imgtec.com, ralf@linux-mips.org, dhowells@redhat.com, jejb@parisc-linux.org, mpe@ellerman.id.au, ysato@users.sourceforge.jp, cmetcalf@mellanox.com, chris@zankel.net, Heiko Carstens Subject: Re: [PATCH -v2 4/6] locking, arch: Update spin_unlock_wait() Message-ID: <20160527090221.GY3193@twins.programming.kicks-ass.net> References: <20160526141922.163198062@infradead.org> <20160526142354.293350777@infradead.org> <20160527084649.065c7609@mschwide> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20160527084649.065c7609@mschwide> User-Agent: Mutt/1.5.21 (2012-12-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, May 27, 2016 at 08:46:49AM +0200, Martin Schwidefsky wrote: > > This fixes a number of spin_unlock_wait() users that (not > > unreasonably) rely on this. > > All that is missing is an smp_rmb(), no? Indeed. > > --- a/arch/s390/include/asm/spinlock.h > > +++ b/arch/s390/include/asm/spinlock.h > > @@ -95,8 +95,7 @@ static inline void arch_spin_unlock(arch > > > > static inline void arch_spin_unlock_wait(arch_spinlock_t *lock) > > { > > - while (arch_spin_is_locked(lock)) > > - arch_spin_relax(lock); > > + smp_cond_load_acquire(&lock->lock, !VAL); > > } > > > > /* > > This change adds the smp_rmb() at the end of the waiting loop, but > it also replaces arch_spin_relax() alias arch_lock_relax() with a > cpu_relax(). This is not good, these two functions do *very* different > things. cpu_relax() does an undirected yield with diagnose 0x44 but > only if the system is non-SMT. arch_lock_relax() does an additional > cpu_is_preempted() to test if the target cpu is running and does a > directed yield with diagnose 0x9c. > > Why can't we just add the smp_rmb() to the arch_spin_unlock_wait()? We can; I forgot about the special cpu_relax on s390, will fix.