From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Paul E. McKenney" Subject: Re: [RFC PATCH v2] memory-barriers: remove smp_mb__after_unlock_lock() Date: Mon, 20 Jul 2015 06:48:49 -0700 Message-ID: <20150720134849.GC3717@linux.vnet.ibm.com> References: <1436929578.10956.3.camel@ellerman.id.au> <20150715104420.GD1005@arm.com> <1437012028.28475.2.camel@ellerman.id.au> <1437023004.28088.27.camel@kernel.crashing.org> <1437023695.28088.29.camel@kernel.crashing.org> <20150716151142.GR3717@linux.vnet.ibm.com> <1437087265.28088.53.camel@kernel.crashing.org> <20150717093221.GB18994@arm.com> <1437171254.28088.87.camel@kernel.crashing.org> <20150720133906.GG9908@arm.com> Reply-To: paulmck@linux.vnet.ibm.com Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Received: from e37.co.us.ibm.com ([32.97.110.158]:57279 "EHLO e37.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753024AbbGTNs5 (ORCPT ); Mon, 20 Jul 2015 09:48:57 -0400 Received: from /spool/local by e37.co.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Mon, 20 Jul 2015 07:48:56 -0600 Content-Disposition: inline In-Reply-To: <20150720133906.GG9908@arm.com> Sender: linux-arch-owner@vger.kernel.org List-ID: To: Will Deacon Cc: Benjamin Herrenschmidt , Michael Ellerman , "linux-arch@vger.kernel.org" , "linux-kernel@vger.kernel.org" , Peter Zijlstra On Mon, Jul 20, 2015 at 02:39:06PM +0100, Will Deacon wrote: > On Fri, Jul 17, 2015 at 11:14:14PM +0100, Benjamin Herrenschmidt wrote: > > On Fri, 2015-07-17 at 10:32 +0100, Will Deacon wrote: > > > static inline void arch_spin_unlock(arch_spinlock_t *lock) > > > { > > > - SYNC_IO; > > > - __asm__ __volatile__("# arch_spin_unlock\n\t" > > > - PPC_RELEASE_BARRIER: : :"memory"); > > > + smp_mb(); > > > lock->slock = 0; > > > } > > > > That probably needs to be mb() in case somebody has the expectation that > > it does a barrier vs. DMA on UP. > > Hmm, but on !SMP doesn't arch_spin_unlock effectively expand to barrier() > in the core code? Yes, to barrier(), but that doesn't generate any code. In contrast, the mb() that Ben is asking for puts out a sync instruction. Without that sync instruction, MMIO accesses can be reordered with the spin_unlock(), even on single-CPU systems. So the bm() is really needed if unlock is to order against MMIO (and thus DMA) on UP. Thanx, Paul