Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S261647AbVBDBei (ORCPT ); Thu, 3 Feb 2005 20:34:38 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S262501AbVBDBbf (ORCPT ); Thu, 3 Feb 2005 20:31:35 -0500 Received: from adsl-63-197-226-105.dsl.snfc21.pacbell.net ([63.197.226.105]:60060 "EHLO cheetah.davemloft.net") by vger.kernel.org with ESMTP id S263223AbVBDBbN (ORCPT ); Thu, 3 Feb 2005 20:31:13 -0500 Date: Thu, 3 Feb 2005 17:23:57 -0800 From: "David S. Miller" To: Herbert Xu Cc: anton@samba.org, okir@suse.de, netdev@oss.sgi.com, linux-kernel@vger.kernel.org Subject: Re: [PATCH] arp_queue: serializing unlink + kfree_skb Message-Id: <20050203172357.670c3402.davem@davemloft.net> In-Reply-To: <20050204012053.GA8949@gondor.apana.org.au> References: <20050131102920.GC4170@suse.de> <20050203142705.GA11318@krispykreme.ozlabs.ibm.com> <20050203203010.GA7081@gondor.apana.org.au> <20050203141901.5ce04c92.davem@davemloft.net> <20050203235044.GA8422@gondor.apana.org.au> <20050203164922.2627a112.davem@davemloft.net> <20050204012053.GA8949@gondor.apana.org.au> X-Mailer: Sylpheed version 1.0.0 (GTK+ 1.2.10; sparc-unknown-linux-gnu) X-Face: "_;p5u5aPsO,_Vsx"^v-pEq09'CU4&Dc1$fQExov$62l60cgCc%FnIwD=.UF^a>?5'9Kn[;433QFVV9M..2eN.@4ZWPGbdi<=?[:T>y?SD(R*-3It"Vj:)"dP Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1236 Lines: 31 On Fri, 4 Feb 2005 12:20:53 +1100 Herbert Xu wrote: > This is true if CPU 0 reads the count before reading skb->list. > Without a memory barrier, atomic_read and reading skb->list can > be reordered. Put it another way, reading skb->list could return > a cached value that was read from the main memory prior to the > atomic_read. > > So in order for CPU 0 to always see an up-to-date value of skb->list, > it needs to do an smp_rmb() between the atomic_read and reading > skb->list. You're absolutely right. Ok, so we do need to change kfree_skb(). I believe even with the memory barrier, the atomic_read() optimization is still worth it. atomic ops on sparc64 take a minimum of 40 some odd cycles on UltraSPARC-III and later, whereas the memory barrier will take up a single cycle most of the time. So it'll look something like: if (atomic_read(&skb->users) == 1) smb_rmb(); else if (!atomic_dec_and_test(&skb->users)) return; __kfree_skb(skb); - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/