From mboxrd@z Thu Jan 1 00:00:00 1970 From: Roland Dreier Subject: Re: MMIO and gcc re-ordering issue Date: Tue, 27 May 2008 14:33:48 -0700 Message-ID: References: <1211852026.3286.36.camel@pasglop> <20080526.184047.88207142.davem@davemloft.net> <1211854540.3286.42.camel@pasglop> <20080526.192812.184590464.davem@davemloft.net> <20080526204233.75b71bb8@infradead.org> <1211872130.3286.64.camel@pasglop> <1211922696.3286.82.camel@pasglop> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Received: from sj-iport-6.cisco.com ([171.71.176.117]:32612 "EHLO sj-iport-6.cisco.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758467AbYE0VfY (ORCPT ); Tue, 27 May 2008 17:35:24 -0400 In-Reply-To: <1211922696.3286.82.camel@pasglop> (Benjamin Herrenschmidt's message of "Wed, 28 May 2008 07:11:36 +1000") Sender: linux-arch-owner@vger.kernel.org List-ID: To: benh@kernel.crashing.org Cc: Arjan van de Ven , linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org, tpiepho@freescale.com, linuxppc-dev@ozlabs.org, scottwood@freescale.com, torvalds@linux-foundation.org, David Miller , alan@lxorguk.ukuu.org.uk > This is a different issue. We deal with it on powerpc by having writel > set a per-cpu flag and spin_unlock() test it, and do the barrier if > needed there. Cool... I assume you do this for mutex_unlock() etc? Is there any reason why ia64 can't do this too so we can kill mmiowb and save everyone a lot of hassle? (mips, sh and frv have non-empty mmiowb() definitions too but I'd guess that these are all bugs based on misunderstandings of the mmiowb() semantics...) > However, drivers such as e1000 -also- have a wmb() between filling the > ring buffer and kicking the DMA with MMIO, with a comment about this > being needed for ia64 relaxed ordering. I put these barriers into mthca, mlx4 etc, although it came from my possible misunderstanding of the memory ordering rules in the kernel more than any experience of problems (as opposed the the mmiowb()s, which all came from real world bugs). - R.