Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752288AbcLERTj (ORCPT ); Mon, 5 Dec 2016 12:19:39 -0500 Received: from mx1.redhat.com ([209.132.183.28]:26100 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751605AbcLERTi (ORCPT ); Mon, 5 Dec 2016 12:19:38 -0500 Date: Mon, 5 Dec 2016 18:19:17 +0100 From: Oleg Nesterov To: Peter Zijlstra Cc: Davidlohr Bueso , mingo@kernel.org, john.stultz@linaro.org, dimitrysh@google.com, linux-kernel@vger.kernel.org, Davidlohr Bueso Subject: Re: [PATCH v2 2/3] locking/percpu-rwsem: Rework writer block/wake to not use wait-queues Message-ID: <20161205171916.GC13035@redhat.com> References: <1479495277-9075-1-git-send-email-dave@stgolabs.net> <1479495277-9075-3-git-send-email-dave@stgolabs.net> <20161203021839.GB30078@linux-80c1.suse> <20161205083605.GQ3092@twins.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20161205083605.GQ3092@twins.programming.kicks-ass.net> User-Agent: Mutt/1.5.18 (2008-05-17) X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.29]); Mon, 05 Dec 2016 17:19:37 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 553 Lines: 21 On 12/05, Peter Zijlstra wrote: > > > + for (;;) { > > + set_current_state(TASK_UNINTERRUPTIBLE); > > + > > + if (readers_active_check(sem)) > > + break; > > + > > + schedule(); > > + } > > + > > + rcu_assign_pointer(sem->writer, NULL); > > And this one does not, and the value being NULL this actually reverts to > WRITE_ONCE(). Do we really care? We do not even need WRITE_ONCE() afaics, this is like __set_current_state(TASK_RUNNING) after the main loop. We can't avoid the spurious wakeups anyway after return from percpu_down_write(). Oleg.