From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1161339AbXBPDP2 (ORCPT ); Thu, 15 Feb 2007 22:15:28 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1161374AbXBPDP2 (ORCPT ); Thu, 15 Feb 2007 22:15:28 -0500 Received: from x35.xmailserver.org ([64.71.152.41]:1899 "EHLO x35.xmailserver.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1161339AbXBPDP1 (ORCPT ); Thu, 15 Feb 2007 22:15:27 -0500 X-AuthUser: davidel@xmailserver.org Date: Thu, 15 Feb 2007 19:15:24 -0800 (PST) From: Davide Libenzi X-X-Sender: davide@alien.or.mcafeemobile.com To: Ingo Molnar cc: Linux Kernel Mailing List , Linus Torvalds , Arjan van de Ven , Christoph Hellwig , Andrew Morton , Alan Cox , Ulrich Drepper , Zach Brown , Evgeniy Polyakov , "David S. Miller" , Suparna Bhattacharya , Thomas Gleixner Subject: Re: [patch 09/14] syslets: x86, add move_user_context() method In-Reply-To: Message-ID: References: <20070215165242.GJ4285@elte.hu> X-GPG-FINGRPRINT: CFAE 5BEE FD36 F65E E640 56FE 0974 BF23 270F 474E X-GPG-PUBLIC_KEY: http://www.xmailserver.org/davidel.asc MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 15 Feb 2007, Davide Libenzi wrote: > On Thu, 15 Feb 2007, Ingo Molnar wrote: > > > /* > > + * Move user-space context from one kernel thread to another. > > + * This includes registers and FPU state. Callers must make > > + * sure that neither task is running user context at the moment: > > + */ > > +void > > +move_user_context(struct task_struct *new_task, struct task_struct *old_task) > > +{ > > + struct pt_regs *old_regs = task_pt_regs(old_task); > > + struct pt_regs *new_regs = task_pt_regs(new_task); > > + union i387_union *tmp; > > + > > + *new_regs = *old_regs; > > + /* > > + * Flip around the FPU state too: > > + */ > > + tmp = new_task->thread.i387; > > + new_task->thread.i387 = old_task->thread.i387; > > + old_task->thread.i387 = tmp; > > +} > > Let's say that old_task ("prev" at the incoming schedule) has TS_USEDFPU > set. Its context gets moved to the new_task (the one returning to > userspace) *before* the __unlazy_fpu() done in __switch_to(). The > __unlazy_fpu() at the following schedule will save the state to the old > new_task context, and that fine as far as the going-to-sleep task goes. > The next fault happening in new_task (return to userspace one) will reload > a non up2date context (the one we got from old_task, but never hit by > the __unlazy_fpu() flush). Right? Yeah. Given TS_USEDFPU set, before move_user_context(): CPU => FPUc NTSK => FPUn OTSK => FPUo After move_user_context(): CPU => FPUc NTSK => FPUo OTSK => FPUn After the incoming __unlazy_fpu() in __switch_to(): CPU => FPUc NTSK => FPUo OTSK => FPUc After the first fault in NTSK: CPU => FPUo NTSK => FPUo OTSK => FPUc So NTSK loads a non up2date FPUo, instead of the FPUc that was the "dirty" context to migrate (since TS_USEDFPU was set). I think you need an early __unlazy_fpu() in that case. - Davide