From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-5.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 3AC22C433ED for ; Mon, 26 Apr 2021 08:09:50 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 0A9C1610E6 for ; Mon, 26 Apr 2021 08:09:50 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232392AbhDZIKa (ORCPT ); Mon, 26 Apr 2021 04:10:30 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:46654 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231171AbhDZIK2 (ORCPT ); Mon, 26 Apr 2021 04:10:28 -0400 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 77E8DC061574 for ; Mon, 26 Apr 2021 01:09:47 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=mdNcf1AV82cZAqbKKVJR5a9TfZz/3J+tTladCWTiR98=; b=LRgz3EdAhWNWk9TSkxzT0qJNlm 6i3IPJ0Ehi54Vmv5Pb45pgVFW7oNjT2CC8t6CFZ46UCwekq+CuUnLIibEeQ5sTnikcdK75J46CqS5 vEM/V+dr8DPGUSlyLUbviCo8qe12c0tcT9PN9xLnIP2yyBWD+kU6L03M74+V/fXjqYfRty9A+5Ugx JtECoFuuJRFZl0+d7jvNq4TrcNxW3tVAA6Al3ihA3WOzqUkMcNXbEXB0gkfKw2to4nqyFylmGaOac BE/LLHDPM7TJUd9Bl0ZmbYoQgLms0GYV6/5a/Y2O+gpXI7PKmjS1pKXjLYPt2mVwZ9Z4UGbfay3zr 6GTgd0sw==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=noisy.programming.kicks-ass.net) by casper.infradead.org with esmtpsa (Exim 4.94 #2 (Red Hat Linux)) id 1lawI6-005LqG-Gh; Mon, 26 Apr 2021 08:08:48 +0000 Received: from hirez.programming.kicks-ass.net (hirez.programming.kicks-ass.net [192.168.1.225]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits)) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id 2D900300212; Mon, 26 Apr 2021 10:08:26 +0200 (CEST) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 1432E2C988F39; Mon, 26 Apr 2021 10:08:26 +0200 (CEST) Date: Mon, 26 Apr 2021 10:08:26 +0200 From: Peter Zijlstra To: Waiman Long Cc: Ingo Molnar , Will Deacon , Boqun Feng , linux-kernel@vger.kernel.org, Borislav Petkov , Ali Saidi , Steve Capper Subject: Re: [PATCH] locking/qrwlock: queued_write_lock_slowpath() cleanup Message-ID: References: <20210425200637.31298-1-longman@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210425200637.31298-1-longman@redhat.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, Apr 25, 2021 at 04:06:37PM -0400, Waiman Long wrote: > void queued_write_lock_slowpath(struct qrwlock *lock) > { > - int cnts; > + int cnts = 0; > > /* Put the writer into the wait queue */ > arch_spin_lock(&lock->wait_lock); > > /* Try to acquire the lock directly if no reader is present */ > if (!atomic_read(&lock->cnts) && > - (atomic_cmpxchg_acquire(&lock->cnts, 0, _QW_LOCKED) == 0)) > + atomic_try_cmpxchg_acquire(&lock->cnts, &cnts, _QW_LOCKED)) > goto unlock; Would not something like: if (!(cnts = atomic_read(&lock->cnts)) && atomic_try_cmpxchg_acquire(&lock->cnts, &cnts, _QW_LOCKED) goto unlock; Be clearer? > > - /* Set the waiting flag to notify readers that a writer is pending */ > - atomic_add(_QW_WAITING, &lock->cnts); > + /* > + * Set the waiting flag to notify readers that a writer is pending > + * > + * As only one writer who is the wait_lock owner can set the waiting > + * flag which will be cleared later on when acquiring the write lock, > + * we can easily replace atomic_or() by an atomic_add() if there is > + * an architecture where an atomic_add() performs better than an > + * atomic_or(). That might be a little overboard on the comment, but sure :-) I don't think there's any arch that doesn't have atomic_or(), like I wrote elsewhere, the one that's often an issue is atomic_fetch_or(). > + */ > + atomic_or(_QW_WAITING, &lock->cnts);