From mboxrd@z Thu Jan 1 00:00:00 1970 From: Benjamin Herrenschmidt Subject: Re: memory barrier question Date: Sun, 19 Sep 2010 12:47:01 +1000 Message-ID: <1284864421.30449.113.camel@pasglop> References: <3777.1284638136@redhat.com> <6383.1284647456@redhat.com> <20100916150356.GD2462@linux.vnet.ibm.com> <20100916163708.GG2462@linux.vnet.ibm.com> <1284656978.26423.11.camel@mulgrave.site> <1284760148.30449.107.camel@pasglop> <20100917231222.GA3060@linux.vnet.ibm.com> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit Return-path: Received: from gate.crashing.org ([63.228.1.57]:59006 "EHLO gate.crashing.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755231Ab0ISCrm (ORCPT ); Sat, 18 Sep 2010 22:47:42 -0400 In-Reply-To: <20100917231222.GA3060@linux.vnet.ibm.com> Sender: linux-arch-owner@vger.kernel.org List-ID: To: paulmck@linux.vnet.ibm.com Cc: Miklos Szeredi , James Bottomley , dhowells@redhat.com, linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org On Fri, 2010-09-17 at 16:12 -0700, Paul E. McKenney wrote: > On Sat, Sep 18, 2010 at 07:49:08AM +1000, Benjamin Herrenschmidt wrote: > > > > > Right but in the concrete namei example I can't see how a compiler > > > optimization can make a difference. The order of the loads is quite > > > clear: > > > > > > LOAD inode = next.dentry->inode > > > if (inode != NULL) > > > LOAD inode->f_op > > > > > > What is there the compiler can optimize? > > > > Those two loads depend on each other, I don't think any implementation > > can re-order them. In fact, such data dependency is typically what is > > used to avoid having barriers in some cases. The second load cannot be > > issued until the value from the first one is returned. > > Sufficiently sadistic compiler and CPU implementations could do value > speculation, for example, driven by profile-feedback optimization. > Then the guess might initially incorrect, but then a store by some other > CPU could make the subsequent test decide (wrongly) that the guess had > in fact been correct. > > Needless to say, I am not a fan of value speculation. But other people > do like it a lot. Well, this verges on insanity... we get to a point where nobody's going to get any code right :-) I don't think the powerpc arch allows that, that leaves us with the compiler, but so far I don't think gcc is -that- crazy. Those constructs are common enough... Cheers, Ben.