From mboxrd@z Thu Jan 1 00:00:00 1970 From: Nicholas Mc Guire Subject: Re: allow preemption in check_task_state Date: Mon, 10 Feb 2014 19:45:14 +0100 Message-ID: <20140210184514.GA13362@opentech.at> References: <20140210153856.GD20017@opentech.at> <20140210111106.67438d1e@gandalf.local.home> <20140210171712.GA17517@opentech.at> <20140210173833.GQ9987@twins.programming.kicks-ass.net> <20140210181203.GA17914@opentech.at> <20140210181608.GE27965@twins.programming.kicks-ass.net> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Steven Rostedt , linux-rt-users@vger.kernel.org, LKML , Sebastian Andrzej Siewior , Carsten Emde , Thomas Gleixner , Andreas Platschek To: Peter Zijlstra Return-path: Received: from hofr.at ([212.69.189.236]:46225 "EHLO mail.hofr.at" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752295AbaBJSpP (ORCPT ); Mon, 10 Feb 2014 13:45:15 -0500 Content-Disposition: inline In-Reply-To: <20140210181608.GE27965@twins.programming.kicks-ass.net> Sender: linux-rt-users-owner@vger.kernel.org List-ID: On Mon, 10 Feb 2014, Peter Zijlstra wrote: > On Mon, Feb 10, 2014 at 07:12:03PM +0100, Nicholas Mc Guire wrote: > > On Mon, 10 Feb 2014, Peter Zijlstra wrote: > > > > > On Mon, Feb 10, 2014 at 06:17:12PM +0100, Nicholas Mc Guire wrote: > > > > maybe I'm missing/missunderstanding something here but > > > > pi_unlock -> arch_spin_unlock is a full mb() > > > > > > Nope, arch_spin_unlock() on x86 is a single add[wb] without LOCK prefix. > > > > > > The lock and unlock primitives are in general specified to have ACQUIRE > > > resp. RELEASE semantics. > > > > > > See Documentation/memory-barriers.txt for far too much head-hurting > > > details. > > > > I did check that - but from the code check it seems to me to be using a > > lock prefix in the fast __add() path and an explicit smp_add() in the slow > > path (arch/x86/include/asm/spinlock.h:arch_spin_unlock) the __add from > > arch/x86/include/asm/cmpxchg.h does lock or am I missinterpreting this ? > > the other archs I believe were all doing explicit mb()/smp_mb() in the > > arch_spin_unlock - will go check this again. > > It uses UNLOCK_LOCK_PREFIX, which if you look carefully, is normally > always "". Only some 'broken' i386 chips require a LOCK there. thanks for the details - will go dig through this again - still missing seme bits. the first patch proposal is taken care of I guess :) thx! hofrat