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=-2.3 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_PASS,USER_AGENT_MUTT autolearn=ham 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 3618FC282DA for ; Wed, 17 Apr 2019 13:58:52 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id EEFDB206BA for ; Wed, 17 Apr 2019 13:58:51 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="kSu1jmql" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1732335AbfDQN6v (ORCPT ); Wed, 17 Apr 2019 09:58:51 -0400 Received: from merlin.infradead.org ([205.233.59.134]:57662 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729356AbfDQN6u (ORCPT ); Wed, 17 Apr 2019 09:58:50 -0400 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=merlin.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:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Id: List-Help:List-Unsubscribe:List-Subscribe:List-Post:List-Owner:List-Archive; bh=7jrL6lodRd2tckoibj5XcymQR7hqhog8uNUUZYt63g8=; b=kSu1jmqlOZvccWGxZIk1vCHId EtaWdONgoVTDNTreOK+aW6mSP99GHypSuWDdbfsP+5KoZuAdV3gpbAHaOCHvSoycKHg7lei9bDaz7 tHz0Vjql7l6PWIAcKAQGQHB+aiFN6ZNo1wKIq6D6Wme/0LFQ9G8t15+s4cBMip3G4WkA7svlaNyFQ Uhczn6M8GlqVLn/6Mr5ylEvY1IKHXEMMLh/+BWYPt9qMroEc8L2pk2BuZKgkm1Pecl/ROvWKww7cB wRieRqMM3GtVv/WJrcyVYacTydeneCnE2KMtYC3HtujD3pyjyktbNVtkWdywi9e9Jih9GTbukUNEF mnbXdqCgg==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=hirez.programming.kicks-ass.net) by merlin.infradead.org with esmtpsa (Exim 4.90_1 #2 (Red Hat Linux)) id 1hGl5H-0003SP-5l; Wed, 17 Apr 2019 13:58:43 +0000 Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id E058629B5292C; Wed, 17 Apr 2019 15:58:41 +0200 (CEST) Date: Wed, 17 Apr 2019 15:58:41 +0200 From: Peter Zijlstra To: Waiman Long Cc: Ingo Molnar , Will Deacon , Thomas Gleixner , linux-kernel@vger.kernel.org, x86@kernel.org, Davidlohr Bueso , Linus Torvalds , Tim Chen , huang ying Subject: Re: [PATCH v4 11/16] locking/rwsem: Enable readers spinning on writer Message-ID: <20190417135841.GI4038@hirez.programming.kicks-ass.net> References: <20190413172259.2740-1-longman@redhat.com> <20190413172259.2740-12-longman@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190413172259.2740-12-longman@redhat.com> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, Apr 13, 2019 at 01:22:54PM -0400, Waiman Long wrote: > +/* > + * Try to acquire read lock before the reader is put on wait queue. > + * Lock acquisition isn't allowed if the rwsem is locked or a writer handoff > + * is ongoing. > + */ > +static inline bool rwsem_try_read_lock_unqueued(struct rw_semaphore *sem) > +{ > + long count = atomic_long_read(&sem->count); > + > + if (RWSEM_COUNT_WLOCKED_OR_HANDOFF(count)) > + return false; > + > + count = atomic_long_fetch_add_acquire(RWSEM_READER_BIAS, &sem->count); > + if (!RWSEM_COUNT_WLOCKED_OR_HANDOFF(count)) { > + rwsem_set_reader_owned(sem); > + lockevent_inc(rwsem_opt_rlock); > + return true; > + } > + > + /* Back out the change */ > + atomic_long_add(-RWSEM_READER_BIAS, &sem->count); > + return false; > +} Doesn't a cmpxchg 'loop' make more sense here?