Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932107AbWCOJJy (ORCPT ); Wed, 15 Mar 2006 04:09:54 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S932133AbWCOJJy (ORCPT ); Wed, 15 Mar 2006 04:09:54 -0500 Received: from javad.com ([216.122.176.236]:25363 "EHLO javad.com") by vger.kernel.org with ESMTP id S932107AbWCOJJx (ORCPT ); Wed, 15 Mar 2006 04:09:53 -0500 From: Sergei Organov To: "linux-os \(Dick Johnson\)" Cc: "David Howells" , Subject: Re: [PATCH] Document Linux's memory barriers [try #4] References: <878xrecypp.fsf@javad.com> <16835.1141936162@warthog.cambridge.redhat.com> <31016.1142368317@warthog.cambridge.redhat.com> Date: Wed, 15 Mar 2006 12:09:43 +0300 In-Reply-To: (linux-os@analogic.com's message of "Tue, 14 Mar 2006 16:11:36 -0500") Message-ID: <87bqw8axbc.fsf@javad.com> User-Agent: Gnus/5.110004 (No Gnus v0.4) XEmacs/21.4.18 (linux) MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1914 Lines: 49 "linux-os \(Dick Johnson\)" writes: > On Tue, 14 Mar 2006, David Howells wrote: > >> Sergei Organov wrote: >> >>> "You can prevent an `asm' instruction from being deleted by writing the >>> keyword `volatile' after the `asm'. [...] >>> The `volatile' keyword indicates that the instruction has important >>> side-effects. GCC will not delete a volatile `asm' if it is reachable. >>> (The instruction can still be deleted if GCC can prove that >>> control-flow will never reach the location of the instruction.) *Note >>> that even a volatile `asm' instruction can be moved relative to other >>> code, including across jump instructions.*" >> >> Ummm... If "asm volatile" statements don't form compiler barriers, then how do >> you specify a compiler barrier? Or is that what the "memory" bit in: >> >> #define barrier() __asm__ __volatile__("": : :"memory") >> >> does? >> >> David > > Yeh. This is the problem (restated) that I mentioned the other > day when you must do a dummy read of the PCI/Bus to flush all > the writes, to some variable that gcc can't decide isn't > important. That's why (void)readl(PCI_STATUS) won't work > (with gcc 3.3.3 anyway). If it indeed doesn't, then it's a bug in GCC. GCC shouldn't throw away volatile accesses. I've already quoted the GCC manual for you: " Less obvious expressions are where something which looks like an access is used in a void context. An example would be, volatile int *src = SOMEVALUE; *src; With C, such expressions are rvalues, and as rvalues cause a read of the object, GCC interprets this as a read of the volatile being pointed to. " -- Sergei. - 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/