From mboxrd@z Thu Jan 1 00:00:00 1970 From: Waiman Long Date: Wed, 15 Jun 2016 19:08:48 +0000 Subject: Re: [RFC PATCH-tip v2 1/6] locking/osq: Make lock/unlock proper acquire/release barrier Message-Id: <5761A7C0.6000709@hpe.com> List-Id: References: <1465944489-43440-1-git-send-email-Waiman.Long@hpe.com> <1465944489-43440-2-git-send-email-Waiman.Long@hpe.com> <20160615165659.GC2094@linux-80c1.suse> <20160615171250.GO30921@twins.programming.kicks-ass.net> In-Reply-To: <20160615171250.GO30921@twins.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Peter Zijlstra Cc: Davidlohr Bueso , Ingo Molnar , linux-kernel@vger.kernel.org, x86@kernel.org, linux-alpha@vger.kernel.org, linux-ia64@vger.kernel.org, linux-s390@vger.kernel.org, linux-arch@vger.kernel.org, Jason Low , Dave Chinner , Scott J Norton , Douglas Hatch On 06/15/2016 01:12 PM, Peter Zijlstra wrote: > On Wed, Jun 15, 2016 at 09:56:59AM -0700, Davidlohr Bueso wrote: >> On Tue, 14 Jun 2016, Waiman Long wrote: >>> +++ b/kernel/locking/osq_lock.c >>> @@ -115,7 +115,7 @@ bool osq_lock(struct optimistic_spin_queue *lock) >>> * cmpxchg in an attempt to undo our queueing. >>> */ >>> >>> - while (!READ_ONCE(node->locked)) { >>> + while (!smp_load_acquire(&node->locked)) { >> Hmm this being a polling path, that barrier can get pretty expensive and >> last I checked it was unnecessary: > I think he'll go rely on it later on. > > In any case, its fairly simple to cure, just add > smp_acquire__after_ctrl_dep() at the end. If we bail because > need_resched() we don't need the acquire I think. Yes, I only need the acquire barrier when the locking is successful. Thanks for the suggestion. I will make the change accordingly. Cheers, Longman