From mboxrd@z Thu Jan 1 00:00:00 1970 From: Oleg Nesterov Subject: Re: schedule() && prev/current (Was: [PATCH 3/3] Make get_current() __attribute__((const))) Date: Wed, 19 May 2010 12:27:43 +0200 Message-ID: <20100519102743.GA10040@redhat.com> 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> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Received: from mx1.redhat.com ([209.132.183.28]:38363 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750787Ab0ESKaQ (ORCPT ); Wed, 19 May 2010 06:30:16 -0400 Content-Disposition: inline In-Reply-To: <1274250079.5605.9967.camel@twins> Sender: linux-arch-owner@vger.kernel.org List-ID: To: Peter Zijlstra Cc: David Howells , Ingo Molnar , Yong Zhang , linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org 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 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 Oleg.