From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754506AbcETP2S (ORCPT ); Fri, 20 May 2016 11:28:18 -0400 Received: from merlin.infradead.org ([205.233.59.134]:40294 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752565AbcETP2R (ORCPT ); Fri, 20 May 2016 11:28:17 -0400 Date: Fri, 20 May 2016 17:28:00 +0200 From: Peter Zijlstra To: Davidlohr Bueso Cc: manfred@colorfullife.com, Waiman.Long@hpe.com, mingo@kernel.org, torvalds@linux-foundation.org, ggherdovich@suse.com, mgorman@techsingularity.net, linux-kernel@vger.kernel.org Subject: Re: sem_lock() vs qspinlocks Message-ID: <20160520152800.GP3205@twins.programming.kicks-ass.net> References: <20160520053926.GC31084@linux-uzut.site> <20160520074946.GA3193@twins.programming.kicks-ass.net> <20160520150049.GB7086@linux-uzut.site> <20160520150505.GG3193@twins.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20160520150505.GG3193@twins.programming.kicks-ass.net> User-Agent: Mutt/1.5.21 (2012-12-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, May 20, 2016 at 05:05:05PM +0200, Peter Zijlstra wrote: > On Fri, May 20, 2016 at 08:00:49AM -0700, Davidlohr Bueso wrote: > > On Fri, 20 May 2016, Peter Zijlstra wrote: > > > > >On Thu, May 19, 2016 at 10:39:26PM -0700, Davidlohr Bueso wrote: > > >> In addition, this makes me wonder if queued_spin_is_locked() should then be: > > >> > > >>- return atomic_read(&lock->val); > > >>+ return atomic_read(&lock->val) & _Q_LOCKED_MASK; > > >> > > >>And avoid considering pending waiters as locked. > > > > > >Probably > > > > Similarly, and I know you hate it, but afaict, then semantically > > queued_spin_is_contended() ought to be: > > > > - return atomic_read(&lock->val) & ~_Q_LOCKED_MASK; > > + return atomic_read(&lock->val); > > Nah, that would make it return true for (0,0,1), ie. uncontended locked. FWIW, the only usage of spin_is_contended() should be for lock breaking, see spin_needbreak(). This also means that #define spin_is_contended(l) (false) is a valid implementation, where the only down-side is worse latency. This is done (together with GENERIC_LOCKBREAK), to allow trivial test-and-set spinlock implementations; as these cannot tell if the lock is contended.