From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751833AbdHCP2W (ORCPT ); Thu, 3 Aug 2017 11:28:22 -0400 Received: from foss.arm.com ([217.140.101.70]:42146 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751144AbdHCP2V (ORCPT ); Thu, 3 Aug 2017 11:28:21 -0400 Date: Thu, 3 Aug 2017 16:28:20 +0100 From: Will Deacon To: Peter Zijlstra Cc: torvalds@linux-foundation.org, oleg@redhat.com, paulmck@linux.vnet.ibm.com, benh@kernel.crashing.org, mpe@ellerman.id.au, npiggin@gmail.com, linux-kernel@vger.kernel.org, mingo@kernel.org, stern@rowland.harvard.edu Subject: Re: [PATCH -v2 3/4] locking: Introduce smp_mb__after_spinlock(). Message-ID: <20170803152820.GD20783@arm.com> References: <20170802113837.280183420@infradead.org> <20170802114030.120586651@infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170802114030.120586651@infradead.org> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Aug 02, 2017 at 01:38:40PM +0200, Peter Zijlstra wrote: > Since its inception, our understanding of ACQUIRE, esp. as applied to > spinlocks, has changed somewhat. Also, I wonder if, with a simple > change, we cannot make it provide more. > > The problem with the comment is that the STORE done by spin_lock isn't > itself ordered by the ACQUIRE, and therefore a later LOAD can pass over > it and cross with any prior STORE, rendering the default WMB > insufficient (pointed out by Alan). > > Now, this is only really a problem on PowerPC and ARM64, both of > which already defined smp_mb__before_spinlock() as a smp_mb(). > > At the same time, we can get a much stronger construct if we place > that same barrier _inside_ the spin_lock(). In that case we upgrade > the RCpc spinlock to an RCsc. That would make all schedule() calls > fully transitive against one another. > > Cc: Alan Stern > Cc: Nicholas Piggin > Cc: Ingo Molnar > Cc: Will Deacon > Cc: Linus Torvalds > Cc: Michael Ellerman > Cc: Oleg Nesterov > Cc: Benjamin Herrenschmidt > Cc: Paul McKenney > Signed-off-by: Peter Zijlstra (Intel) > --- > arch/arm64/include/asm/spinlock.h | 2 ++ > arch/powerpc/include/asm/spinlock.h | 3 +++ > include/linux/atomic.h | 3 +++ > include/linux/spinlock.h | 36 ++++++++++++++++++++++++++++++++++++ > kernel/sched/core.c | 4 ++-- > 5 files changed, 46 insertions(+), 2 deletions(-) > > --- a/arch/arm64/include/asm/spinlock.h > +++ b/arch/arm64/include/asm/spinlock.h > @@ -367,5 +367,7 @@ static inline int arch_read_trylock(arch > * smp_mb__before_spinlock() can restore the required ordering. > */ > #define smp_mb__before_spinlock() smp_mb() > +/* See include/linux/spinlock.h */ > +#define smp_mb__after_spinlock() smp_mb() > > #endif /* __ASM_SPINLOCK_H */ Acked-by: Will Deacon Will