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=-15.7 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED 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 01D84C433F5 for ; Thu, 9 Sep 2021 11:04:15 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id D174161100 for ; Thu, 9 Sep 2021 11:04:14 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235236AbhIILFT (ORCPT ); Thu, 9 Sep 2021 07:05:19 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:55548 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234473AbhIILFM (ORCPT ); Thu, 9 Sep 2021 07:05:12 -0400 Received: from desiato.infradead.org (desiato.infradead.org [IPv6:2001:8b0:10b:1:d65d:64ff:fe57:4e05]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 4BE24C061756 for ; Thu, 9 Sep 2021 04:04:03 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=desiato.20200630; h=Content-Type:MIME-Version:References: Subject:Cc:To:From:Date:Message-ID:Sender:Reply-To:Content-Transfer-Encoding: Content-ID:Content-Description:In-Reply-To; bh=jh+fs4xoGFyV+30/UJk/QCfrqRTmFnmoLT8WuvoXSvs=; b=S+2mckeR6qhrWkzQE2OniejtkY pzyQZ6s4HQ4ZYFKJPwWx3KWD7CCX00Oou3fy8xX8uwuEJw0ZUFV6Icr7t+5N92qjd36C9H2Lw7rl/ Tb6JSWc0nVPLIMJRMPbt32XF8qWkPEoi9Y5mpmri8CrYR80owHepUfwlnuKBr9iGxxwnZgJDf2zeF vppXj0C9hga3lCDB09ml/HbFXQ315s7yweZtLOq/KLAfaMU1Ms8iObWyFZUYf4joagg8gDuKtobq4 oLPKXLHbT0wHF/8zr/+C1FysohJ7kOtz1F4reOECOX5li7X3PhbJXI0dUwsozFzKwYZvPE8Bax8Yt ze5HhL9Q==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=noisy.programming.kicks-ass.net) by desiato.infradead.org with esmtpsa (Exim 4.94.2 #2 (Red Hat Linux)) id 1mOHqF-001rBA-KE; Thu, 09 Sep 2021 11:03:39 +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 D20D4300129; Thu, 9 Sep 2021 13:03:37 +0200 (CEST) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 0) id B6CBE2C3CBEF1; Thu, 9 Sep 2021 13:03:37 +0200 (CEST) Message-ID: <20210909110203.893845303@infradead.org> User-Agent: quilt/0.66 Date: Thu, 09 Sep 2021 12:59:18 +0200 From: Peter Zijlstra To: tglx@linutronix.de, boqun.feng@gmail.com Cc: linux-kernel@vger.kernel.org, peterz@infradead.org, Ingo Molnar , Juri Lelli , Steven Rostedt , Davidlohr Bueso , Will Deacon , Waiman Long , Sebastian Andrzej Siewior , Mike Galbraith , Daniel Bristot de Oliveira Subject: [PATCH 3/4] locking/rwbase: Fix rwbase_write_lock() vs __rwbase_read_lock() References: <20210909105915.757320973@infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Boqun noticed that the write-trylock sequence of load+set is broken in rwbase_write_lock()'s wait-loop since they're not both under the same wait_lock instance. Restructure the code to make this more obvious and correct. Reported-by: Boqun Feng Signed-off-by: Peter Zijlstra (Intel) --- kernel/locking/rwbase_rt.c | 44 ++++++++++++++++++++++++++------------------ 1 file changed, 26 insertions(+), 18 deletions(-) --- a/kernel/locking/rwbase_rt.c +++ b/kernel/locking/rwbase_rt.c @@ -196,6 +196,19 @@ static inline void rwbase_write_downgrad __rwbase_write_unlock(rwb, WRITER_BIAS - 1, flags); } +static inline bool __rwbase_write_trylock(struct rwbase_rt *rwb) +{ + /* Can do without CAS because we're serialized by wait_lock. */ + lockdep_assert_held(&rwb->rtmutex.wait_lock); + + if (!atomic_read(&rwb->readers)) { + atomic_set(&rwb->readers, WRITER_BIAS); + return 1; + } + + return 0; +} + static int __sched rwbase_write_lock(struct rwbase_rt *rwb, unsigned int state) { @@ -210,34 +223,30 @@ static int __sched rwbase_write_lock(str atomic_sub(READER_BIAS, &rwb->readers); raw_spin_lock_irqsave(&rtm->wait_lock, flags); - /* - * set_current_state() for rw_semaphore - * current_save_and_set_rtlock_wait_state() for rwlock - */ - rwbase_set_and_save_current_state(state); + if (__rwbase_write_trylock(rwb)) + goto out_unlock; - /* Block until all readers have left the critical section. */ - for (; atomic_read(&rwb->readers);) { + rwbase_set_and_save_current_state(state); + for (;;) { /* Optimized out for rwlocks */ if (rwbase_signal_pending_state(state, current)) { rwbase_restore_current_state(); __rwbase_write_unlock(rwb, 0, flags); return -EINTR; } + + if (__rwbase_write_trylock(rwb)) + break; + raw_spin_unlock_irqrestore(&rtm->wait_lock, flags); + rwbase_schedule(); + raw_spin_lock_irqsave(&rtm->wait_lock, flags); - /* - * Schedule and wait for the readers to leave the critical - * section. The last reader leaving it wakes the waiter. - */ - if (atomic_read(&rwb->readers) != 0) - rwbase_schedule(); set_current_state(state); - raw_spin_lock_irqsave(&rtm->wait_lock, flags); } - - atomic_set(&rwb->readers, WRITER_BIAS); rwbase_restore_current_state(); + +out_unlock: raw_spin_unlock_irqrestore(&rtm->wait_lock, flags); return 0; } @@ -253,8 +262,7 @@ static inline int rwbase_write_trylock(s atomic_sub(READER_BIAS, &rwb->readers); raw_spin_lock_irqsave(&rtm->wait_lock, flags); - if (!atomic_read(&rwb->readers)) { - atomic_set(&rwb->readers, WRITER_BIAS); + if (__rwbase_write_trylock(rwb)) { raw_spin_unlock_irqrestore(&rtm->wait_lock, flags); return 1; }