From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Date: Thu, 23 Mar 2006 14:26:45 -0800 From: "Paul E. McKenney" To: David Howells Subject: Re: [PATCH] Document Linux's memory barriers [try #5] Message-ID: <20060323222645.GA1298@us.ibm.com> References: <20060316231723.GB1323@us.ibm.com> <16835.1141936162@warthog.cambridge.redhat.com> <18351.1142432599@warthog.cambridge.redhat.com> <895.1143138867@warthog.cambridge.redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: <895.1143138867@warthog.cambridge.redhat.com> Cc: akpm@osdl.org, linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org, torvalds@osdl.org, davem@redhat.com, linuxppc64-dev@ozlabs.org Reply-To: paulmck@us.ibm.com List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On Thu, Mar 23, 2006 at 06:34:27PM +0000, David Howells wrote: > Paul E. McKenney wrote: > > > smp_mb__before_atomic_dec() and friends as well? > > These seem to be something Sparc64 related; or, at least, Sparc64 seems to do > something weird with them. > > What are these meant to achieve anyway? They seems to just be barrier() on a > lot of systems, even SMP ones. On architectures such as x86 where atomic_dec() implies an smp_mb(), they do nothing. On other architectures, they supply whatever memory barrier is required. So, on x86: smp_mb(); atomic_dec(&my_atomic_counter); would result in -two- atomic instructions, but the smp_mb() would be absolutely required on CPUs with weaker memory-consistency models. So your choice is to (1) be inefficient on x86 or (2) be unsafe on weak-memory-consistency systems. What we can do instead is: smp_mb__before_atomic_dec(); atomic_dec(&my_atomic_counter); This allows x86 to generate efficient code -and- allows weak-memory machines (e.g., Alpha, MIPS, PA-RISC(!), ppc, s390, SPARC64) to generate safe code. Thanx, Paul