From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1030319AbXCENlE (ORCPT ); Mon, 5 Mar 2007 08:41:04 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1030426AbXCENlE (ORCPT ); Mon, 5 Mar 2007 08:41:04 -0500 Received: from mx2.suse.de ([195.135.220.15]:53478 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1030319AbXCENlB (ORCPT ); Mon, 5 Mar 2007 08:41:01 -0500 Date: Mon, 5 Mar 2007 15:40:33 +0100 From: Andi Kleen To: Andrew Morton Cc: Mathieu Desnoyers , "Martin J. Bligh" , linux-kernel@vger.kernel.org, Andi Kleen , "David S. Miller" , Paul Mackerras , "Luck, Tony" , Haavard Skinnemoen Subject: Re: Thread flags modified without set_thread_flag() (non atomically) Message-ID: <20070305144033.GG22829@bingen.suse.de> References: <45E33EBD.6020603@google.com> <20070228220349.b42bf571.akpm@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20070228220349.b42bf571.akpm@linux-foundation.org> Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org > It does seem risky. Perhaps it is a micro-optimisation which utilises > knowledge that this thread_struct cannot be looked up via any path in this > context. > > Or perhaps it is a bug. Andi, can you please comment? On flush_thread nobody else can mess with the thread, so yes it's a micro optimization. > > > And about this specific flush_thread, I am puzzled about the t->flags ^= > > (_TIF_ABI_PENDING | _TIF_IA32); line. The XOR will clearly flip the > > _TIF_ABI_PENDING bit to 0, and very likely set _TIF_IA32 to the opposite > > of its current value. Why does this change need to be written atomically > > (can other threads play with these flags ?) ? > > > > Don't know. iirc it came from DaveM originally. He just likes to write things in comp^wclever ways :0) It's just a little shorter. > No, I don't immediately see anything in the flush_old_exec() code path > which tells us that nobody else can look up this thread_info (or be holding > a ref to it) in this context. Normally the process flags atomicity should only matter with signals; i don't think you can send a signal to a process being in exec this way. -Andi