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=-15.8 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 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 EF812C433EF for ; Tue, 14 Sep 2021 14:02:26 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id D586160EB6 for ; Tue, 14 Sep 2021 14:02:26 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233529AbhINODm (ORCPT ); Tue, 14 Sep 2021 10:03:42 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:60086 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233416AbhINODj (ORCPT ); Tue, 14 Sep 2021 10:03:39 -0400 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 14F3BC061574 for ; Tue, 14 Sep 2021 07:02:22 -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=ikIcpvvxpz3lHW8UImkJMnAPIGaYfFUZqIxswpaNdgQ=; b=aNJ7XWDgkm0L1CF1btO9FfvG1e UBsNhuSzqGYkh8x+pslQAHRVA19nkv5D7TqtT5jJV9Sws1mKcNe4tjKlIFPiyXwJ9Nszj+Fj4pgAN KLuA6NjFUz+QcX1J30hSNRRAkKfnttr8mDvCkq1xiWXn4VRYa9rL1P5q+Kc+s6md4qZMm88dzAgqw vq7vlDpr/DgJ0k6MetIP0LsegF3omAcY3LUd5hArq8c9Ieiyy/gQNNmIxsf8MsiXOFU84CJeYiXhd 9+x7uoCB5oMiPUENtGOMPjB/nrdz5U/MCmw1G1MvzGrDWWB6jkJdsj/MgdDks2WkW3F15IIEgLzKS yEiqXajw==; 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 #2 (Red Hat Linux)) id 1mQ8yc-00EiwU-Cr; Tue, 14 Sep 2021 14:00:23 +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 72FCC300129; Tue, 14 Sep 2021 15:59:56 +0200 (CEST) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 5BE62200C6C6A; Tue, 14 Sep 2021 15:59:56 +0200 (CEST) Date: Tue, 14 Sep 2021 15:59:56 +0200 From: Peter Zijlstra To: Thomas Gleixner Cc: boqun.feng@gmail.com, linux-kernel@vger.kernel.org, Ingo Molnar , Juri Lelli , Steven Rostedt , Davidlohr Bueso , Will Deacon , Waiman Long , Sebastian Andrzej Siewior , Mike Galbraith , Daniel Bristot de Oliveira Subject: Re: [PATCH 3/4] locking/rwbase: Fix rwbase_write_lock() vs __rwbase_read_lock() Message-ID: References: <20210909105915.757320973@infradead.org> <20210909110203.893845303@infradead.org> <87k0jjeh2v.ffs@tglx> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87k0jjeh2v.ffs@tglx> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Sep 14, 2021 at 09:45:12AM +0200, Thomas Gleixner wrote: > On Thu, Sep 09 2021 at 12:59, Peter Zijlstra wrote: > > > 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. > > Confused. > > lock(); A > > for (; atomic_read(readers);) { > ... > unlock(); > .. > lock(); B > } > > atomic_set(); > unlock(); A or B > > The read/set is always in the same lock instance. I really did make a mess of things didn't I :-/ It was some intermediate state that was broken. How's this then? --- Subject: locking/rwbase: Extract __rwbase_write_trylock() From: Peter Zijlstra Date: Thu, 09 Sep 2021 12:59:18 +0200 The code in rwbase_write_lock() is a little non-obvious vs the read+set 'trylock', extract the sequence into a helper function to clarify the code. This also provides a single site to fix fast-path ordering. 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; }