From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S934136AbdKBQph (ORCPT ); Thu, 2 Nov 2017 12:45:37 -0400 Received: from bombadil.infradead.org ([65.50.211.133]:57201 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933812AbdKBQpg (ORCPT ); Thu, 2 Nov 2017 12:45:36 -0400 Date: Thu, 2 Nov 2017 17:45:27 +0100 From: Peter Zijlstra To: Alan Stern Cc: "Reshetova, Elena" , "linux-kernel@vger.kernel.org" , "gregkh@linuxfoundation.org" , "keescook@chromium.org" , "tglx@linutronix.de" , "mingo@redhat.com" , "ishkamiel@gmail.com" , Will Deacon , Paul McKenney , parri.andrea@gmail.com, boqun.feng@gmail.com, dhowells@redhat.com, david@fromorbit.com Subject: Re: [PATCH] refcount: provide same memory ordering guarantees as in atomic_t Message-ID: <20171102164527.mh3hgkrmpl5wup63@hirez.programming.kicks-ass.net> References: <20171102135742.7o4urtltgvhr6dku@hirez.programming.kicks-ass.net> <20171102160237.t2xkryg6joskf77y@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20171102160237.t2xkryg6joskf77y@hirez.programming.kicks-ass.net> User-Agent: NeoMutt/20170609 (1.8.3) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Nov 02, 2017 at 05:02:37PM +0100, Peter Zijlstra wrote: > On Thu, Nov 02, 2017 at 11:40:35AM -0400, Alan Stern wrote: > > On Thu, 2 Nov 2017, Peter Zijlstra wrote: > > > > > > Lock functions such as refcount_dec_and_lock() & > > > > refcount_dec_and_mutex_lock() Provide exactly the same guarantees as > > > > they atomic counterparts. > > > > > > Nope. The atomic_dec_and_lock() provides smp_mb() while > > > refcount_dec_and_lock() merely orders all prior load/store's against all > > > later load/store's. > > > > In fact there is no guaranteed ordering when refcount_dec_and_lock() > > returns false; > > It should provide a release: > > - if !=1, dec_not_one will provide release > - if ==1, dec_not_one will no-op, but then we'll acquire the lock and > dec_and_test will provide the release, even if the test fails and we > unlock again it should still dec. > > The one exception is when the counter is saturated, but in that case > we'll never free the object and the ordering is moot in any case. > > > it provides ordering only if the return value is true. > > In which case it provides acquire ordering (thanks to the spin_lock), > > and both release ordering and a control dependency (thanks to the > > refcount_dec_and_test). > > > > > The difference is subtle and involves at least 3 CPUs. I can't seem to > > > write up anything simple, keeps turning into monsters :/ Will, Paul, > > > have you got anything simple around? > > > > The combination of acquire + release is not the same as smp_mb, because > > acquire+release is nothing, its release+acquire that I meant which > should order things locally, but now that you've got me looking at it > again, we don't in fact do that. > > So refcount_dec_and_lock() will provide a release, irrespective of the > return value (assuming we're not saturated). If it returns true, it also > does an acquire for the lock. > > But combined they're acquire+release, which is unfortunate.. it means > the lock section and the refcount stuff overlaps, but I don't suppose > that's actually a problem. Need to consider more. Right, so in that case we have refcount==0 and are guaranteed no concurrency. So its fine.