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=-17.5 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER,INCLUDES_PATCH,MAILING_LIST_MULTI,NICE_REPLY_A, SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 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 18533C433B4 for ; Mon, 26 Apr 2021 18:51:57 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id D273A61152 for ; Mon, 26 Apr 2021 18:51:56 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S239277AbhDZSwh (ORCPT ); Mon, 26 Apr 2021 14:52:37 -0400 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:22205 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S239257AbhDZSwe (ORCPT ); Mon, 26 Apr 2021 14:52:34 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1619463112; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=HfK9lfoYc+03KHKdRUvmbZAZpf80F/z+G4/xc8L1eD8=; b=F7B3bs2ZxniFFcrCPI6MSgxCrYKQh+CHOn4VsCNwzB30bMBL2ditGS2HfbniVBKbMBWT4y JG+7FAdqFqkJqjvm6f/xx/M2WyDrCDngX9r+9R4lMQOdNITK6FRVFsMgeeCz/LpfOhcxcv gmI011e3fAL+iYufF/B7MhOwm9zTRBk= Received: from mail-qk1-f198.google.com (mail-qk1-f198.google.com [209.85.222.198]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-437-p6Bc3WkVPaSfWewEvY2tTw-1; Mon, 26 Apr 2021 14:51:50 -0400 X-MC-Unique: p6Bc3WkVPaSfWewEvY2tTw-1 Received: by mail-qk1-f198.google.com with SMTP id e4-20020a37b5040000b02902df9a0070efso21344395qkf.18 for ; Mon, 26 Apr 2021 11:51:50 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:from:subject:to:cc:references:message-id:date :user-agent:mime-version:in-reply-to:content-transfer-encoding :content-language; bh=HfK9lfoYc+03KHKdRUvmbZAZpf80F/z+G4/xc8L1eD8=; b=f8usDYbN4cO+j2cEem/BUlTCD+jpAzF+8t2C1QZCWoHQH9ad7zi0cbyzWR8futBtZ2 Mg+IW9uvpmYCQc6OYQUei6tVb70qR2A+wgGac7VyMFXkjH1zaR0+AmpSnDiND48pn6H2 HRZhpgD6slPpLg0j9fb6GegdWQeCuIo2A/cyaWnFaLwPt6MeUooKJAzU1+FKmIizN1/6 Dqa373gCbxDGvds2dxrJBjd567e+/2pkN8UiXF8U3Zlr/5lxn2S4/PfZSnTEqaGXiQqf 5fk7EOuzp2Yw6NHxq2qpabPq9BE64mWGJ13zJSyq2PU/KuyfDkJNhT1V6RDFYCVipZAV Z0ug== X-Gm-Message-State: AOAM531dRvnhVf/gHS0vmOVnGNr7UmO3d3guf6nxD7fsMntEuD7YU1bp 7rImH4Mw/TuWUKbxpQx/aOsTwIuR0b+CXPXfj2DELlCGumZzUlETljINOBC/Kif/qwjjNT3gD6f cea6Kbjb2NPSTDWEwFOaB4kAN X-Received: by 2002:a37:8181:: with SMTP id c123mr19150369qkd.287.1619463110180; Mon, 26 Apr 2021 11:51:50 -0700 (PDT) X-Google-Smtp-Source: ABdhPJzS8X/VzurSrpDlNhl+L1bACm1B5oJnNe8qn+lgbdD5zkt3ZBvGh0XvLtQ/7lrv2aslazYIJg== X-Received: by 2002:a37:8181:: with SMTP id c123mr19150347qkd.287.1619463109942; Mon, 26 Apr 2021 11:51:49 -0700 (PDT) Received: from llong.remote.csb ([2601:191:8500:76c0::cdbc]) by smtp.gmail.com with ESMTPSA id e13sm13047982qtm.35.2021.04.26.11.51.49 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 26 Apr 2021 11:51:49 -0700 (PDT) From: Waiman Long X-Google-Original-From: Waiman Long Subject: Re: [PATCH] locking/qrwlock: queued_write_lock_slowpath() cleanup To: Peter Zijlstra , Ingo Molnar , Will Deacon , Boqun Feng Cc: linux-kernel@vger.kernel.org, Borislav Petkov , Ali Saidi , Steve Capper References: <20210426185017.19815-1-longman@redhat.com> Message-ID: Date: Mon, 26 Apr 2021 14:51:48 -0400 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.9.0 MIME-Version: 1.0 In-Reply-To: <20210426185017.19815-1-longman@redhat.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Content-Language: en-US Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 4/26/21 2:50 PM, Waiman Long wrote: > Make the code more readable by replacing the atomic_cmpxchg_acquire() > by an equivalent atomic_try_cmpxchg_acquire() and change atomic_add() > to atomic_or(). > > For architectures that use qrwlock, I do not find one that has an > atomic_add() defined but not an atomic_or(). I guess it should be fine > by changing atomic_add() to atomic_or(). > > Note that the previous use of atomic_add() isn't wrong as only one > writer that is the wait_lock owner can set the waiting flag and the > flag will be cleared later on when acquiring the write lock. > > Suggested-by: Linus Torvalds > Signed-off-by: Waiman Long > --- > kernel/locking/qrwlock.c | 6 +++--- > 1 file changed, 3 insertions(+), 3 deletions(-) > > diff --git a/kernel/locking/qrwlock.c b/kernel/locking/qrwlock.c > index b94f3831e963..ec36b73f4733 100644 > --- a/kernel/locking/qrwlock.c > +++ b/kernel/locking/qrwlock.c > @@ -66,12 +66,12 @@ void queued_write_lock_slowpath(struct qrwlock *lock) > 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)) > + if (!(cnts = atomic_read(&lock->cnts)) && > + atomic_try_cmpxchg_acquire(&lock->cnts, &cnts, _QW_LOCKED)) > goto unlock; > > /* Set the waiting flag to notify readers that a writer is pending */ > - atomic_add(_QW_WAITING, &lock->cnts); > + atomic_or(_QW_WAITING, &lock->cnts); > > /* When no more readers or writers, set the locked flag */ > do { Sorry, I missed the "v2" in the title. Cheers, Longman