From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751459Ab0ESNHZ (ORCPT ); Wed, 19 May 2010 09:07:25 -0400 Received: from mail-pv0-f174.google.com ([74.125.83.174]:54558 "EHLO mail-pv0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751189Ab0ESNHT (ORCPT ); Wed, 19 May 2010 09:07:19 -0400 DomainKey-Signature: a=rsa-sha1; c=nofws; d=gmail.com; s=gamma; h=date:from:to:cc:subject:message-id:reply-to:references:mime-version :content-type:content-disposition:in-reply-to:user-agent; b=azItP83KmwhFdlpJaL3NlF8lHmYrByiiqR8TDm2rWlVCiJSDYTk3tf3SA37JMj9m57 AWIQixhioRylmd+7/y2TWIiwVS6qdld0Lfrjfhtkds7YHySBLtMy+SwghRYqybE4TDix se1HmGkcqIhi00oupKXvkeUTgRWbVGhBzAk7g= Date: Wed, 19 May 2010 21:07:09 +0800 From: Yong Zhang To: Oleg Nesterov Cc: Peter Zijlstra , David Howells , Ingo Molnar , linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: schedule() && prev/current (Was: [PATCH 3/3] Make get_current() __attribute__((const))) Message-ID: <20100519130709.GA2216@zhy> Reply-To: Yong Zhang References: <20100518164537.6194.73366.stgit@warthog.procyon.org.uk> <20100518164547.6194.94193.stgit@warthog.procyon.org.uk> <20100518212220.GA5092@redhat.com> <1274250079.5605.9967.camel@twins> <20100519102743.GA10040@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20100519102743.GA10040@redhat.com> User-Agent: Mutt/1.5.20 (2009-06-14) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, May 19, 2010 at 12:27:43PM +0200, Oleg Nesterov wrote: > On 05/19, Peter Zijlstra wrote: > > > > On Tue, 2010-05-18 at 23:22 +0200, Oleg Nesterov wrote: > > > > > And, looking at this patch I think that schedule() can be simplified > > > a little bit. > > > > > > "sched: Reassign prev and switch_count when reacquire_kernel_lock() fail" > > > commit 6d558c3ac9b6508d26fd5cadccce51fc9d726b1c says: > > > > > > Assume A->B schedule is processing, > > > ... > > > Then on B's context, > > > ... > > > prev and switch_count are related to A > > > > > > How so? switch_count - yes, we should change it. But prev must be > > > equal to B, and it must be equal to current. When we return from > > > switch_to() registers/stack should be restored correctly, so we > > > can do > > > > What if schedule() got called at a different stack depth than we are > > now? > > > > I don't think we can assume anything about the stack context we just > > switched to. > > Not sure I understand... > > OK. Firstly, we shouldn't worry about the freshly forked tasks, they > never "return" from switch_to() but call ret_from_fork()->schedule_tail(), > right? > > Now suppose that A calls schedule() and we switch to B. When switch_to() > returns on B's context, this context (register/stack) matches the previous > context which was used by B when it in turn called schedule(), correct? > > IOW. B calls schedule, prev == B. schedule() picks another task, prev > is saved on B's stck after switch_to(). A calls schedule(), prev == A > before context_switch(A, B), but after that switch_to() switches to > B's stack and prev == B. > > No? I think you are right. > > > I am looking into the git history now... and I guess I understand why > reacquire_kernel_lock() uses current. Because schedule() did something > like > > prev = context_switch(prev, next); // prev == last > > finish_task_switch(prev); > > reacquire_kernel_lock(current); // prev != current This is what I think when I wrote that patch. Now the task switch is entirely finished in context_switch(). So commit log in 6d558c3a has some flaw in it. The "prev" is also a churn. Thanks, Yong