From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755291AbdDMR7W (ORCPT ); Thu, 13 Apr 2017 13:59:22 -0400 Received: from bombadil.infradead.org ([65.50.211.133]:45453 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754864AbdDMR7T (ORCPT ); Thu, 13 Apr 2017 13:59:19 -0400 Date: Thu, 13 Apr 2017 19:59:07 +0200 From: Peter Zijlstra To: "Paul E. McKenney" Cc: linux-kernel@vger.kernel.org, mingo@kernel.org, jiangshanlai@gmail.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, tglx@linutronix.de, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, fweisbec@gmail.com, oleg@redhat.com, bobby.prani@gmail.com, dvyukov@google.com, will.deacon@arm.com Subject: Re: [PATCH tip/core/rcu 07/13] rcu: Add smp_mb__after_atomic() to sync_exp_work_done() Message-ID: <20170413175907.hcrlh2z55zivs4v4@hirez.programming.kicks-ass.net> References: <20170412165441.GA17149@linux.vnet.ibm.com> <1492016149-18834-7-git-send-email-paulmck@linux.vnet.ibm.com> <20170413091832.phnfppqjjy6sislo@hirez.programming.kicks-ass.net> <20170413161042.GA3956@linux.vnet.ibm.com> <20170413162409.q5gsqfytjyirgfep@hirez.programming.kicks-ass.net> <20170413165755.GJ3956@linux.vnet.ibm.com> <20170413171027.snjqn4u54t2kdzgx@hirez.programming.kicks-ass.net> <20170413173951.GM3956@linux.vnet.ibm.com> <20170413175136.5qnzvqrmzyuvlqsj@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170413175136.5qnzvqrmzyuvlqsj@hirez.programming.kicks-ass.net> User-Agent: NeoMutt/20170113 (1.7.2) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Apr 13, 2017 at 07:51:36PM +0200, Peter Zijlstra wrote: > > I suppose that one alternative is the new variant of kerneldoc, though > > very few of these functions have comment headers, let alone kerneldoc > > headers. Which reminds me, the question of spin_unlock_wait() and > > spin_is_locked() semantics came up a bit ago. Here is what I believe > > to be the case. Does this match others' expectations? > > > > o spin_unlock_wait() semantics: > > > > 1. Any access in any critical section prior to the > > spin_unlock_wait() is visible to all code following > > (in program order) the spin_unlock_wait(). > > > > 2. Any access prior (in program order) to the > > spin_unlock_wait() is visible to any critical > > section following the spin_unlock_wait(). > > > > o spin_is_locked() semantics: Half of spin_unlock_wait(), > > but only if it returns false: > > > > 1. Any access in any critical section prior to the > > spin_unlock_wait() is visible to all code following > > (in program order) the spin_unlock_wait(). > > Urgh.. yes those are pain. The best advise is to not use them. > > 055ce0fd1b86 ("locking/qspinlock: Add comments") The big problem with spin_unlock_wait(), aside from the icky barrier semantics, is that it tends to end up prone to starvation. So where spin_lock()+spin_unlock() have guaranteed fwd progress if the lock is fair (ticket,queued,etc..) spin_unlock_wait() must often lack that guarantee. Equally, spin_unlock_wait() was intended to be 'cheap' and be a read-only loop, but in order to satisfy the barrier requirements, it ends up doing stores anyway (see for example the arm64 and ppc implementations).