From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1765764AbXGXHt3 (ORCPT ); Tue, 24 Jul 2007 03:49:29 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753107AbXGXHtV (ORCPT ); Tue, 24 Jul 2007 03:49:21 -0400 Received: from gw.goop.org ([64.81.55.164]:55742 "EHLO mail.goop.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752579AbXGXHtU (ORCPT ); Tue, 24 Jul 2007 03:49:20 -0400 Message-ID: <46A5AECB.3060208@goop.org> Date: Tue, 24 Jul 2007 00:48:27 -0700 From: Jeremy Fitzhardinge User-Agent: Thunderbird 2.0.0.5 (X11/20070719) MIME-Version: 1.0 To: Satyam Sharma CC: Nick Piggin , Linux Kernel Mailing List , David Howells , Andi Kleen , Andrew Morton , Linus Torvalds Subject: Re: [PATCH 8/8] i386: bitops: smp_mb__{before, after}_clear_bit() definitions References: <20070723160528.22137.84144.sendpatchset@cselinux1.cse.iitk.ac.in> <20070723160608.22137.14790.sendpatchset@cselinux1.cse.iitk.ac.in> <46A577AA.4020804@yahoo.com.au> In-Reply-To: X-Enigmail-Version: 0.95.2 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Satyam Sharma wrote: > Consider this (the above two functions exist only for clear_bit(), > the atomic variant, as you already know), the _only_ memory reference > we care about is that of the address of the passed bit-string: > > (1) The compiler must not optimize / elid it (i.e. we need to disallow > compiler optimization for that reference) -- but we've already taken > care of that with the __asm__ __volatile__ and the constraints on > the memory "addr" operand there, and, > (2) For the i386, it also includes an implicit memory (CPU) barrier > already. > > So I /think/ it makes sense to let the compiler optimize _other_ memory > references across the call to clear_bit(). There's a difference. I think > we'd be safe even if we do this, because the synchronization in callers > must be based upon the _passed bit-string_, otherwise _they_ are the > ones who're buggy. > > [ However, elsewhere Jeremy Fitzhardinge mentioned the case of > some callers, for instance, doing a memset() on an alias of > the same bit-string. But again, I think that is dodgy/buggy/ > extremely border-line usage on the caller's side itself ... > *unless* the caller is doing that inside a higher-level lock > anyway, in which case he wouldn't be needing to use the > locked variants either ... ] > You miss my point. If you have: memset(&my_bitmask, 0, sizeof(my_bitmask)); set_bit(my_bitmask, 44); Then unless the set_bit has a constraint argument which covers the whole of the (multiword) bitmask, the compiler may see fit to interleave the memset writes with the set_bit in bad ways. In other words, plain "+m" (*(long *)ptr) won't cut it. You'd need "+m" (my_bitmask), I think. J