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=-5.1 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A, SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 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 AF8C7C433DB for ; Tue, 9 Mar 2021 11:12:10 +0000 (UTC) Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 28A7B65253 for ; Tue, 9 Mar 2021 11:12:10 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 28A7B65253 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:49160 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1lJaHZ-0000Py-6W for qemu-devel@archiver.kernel.org; Tue, 09 Mar 2021 06:12:09 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]:59288) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1lJaC5-00014b-EU for qemu-devel@nongnu.org; Tue, 09 Mar 2021 06:06:29 -0500 Received: from us-smtp-delivery-124.mimecast.com ([63.128.21.124]:60253) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_CBC_SHA1:256) (Exim 4.90_1) (envelope-from ) id 1lJaC3-0001BF-T6 for qemu-devel@nongnu.org; Tue, 09 Mar 2021 06:06:29 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1615287987; 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=0Pq8EJfDiYJYoqlSdSD0m0FRouDJg2ncmxP8m2FxexU=; b=iyrn5j9W/C8BztwqvaolVkTGe0WQlnZaxnY9QQeTBQnyOe/V7vF/QR2OAo/DYKgUw70PEC 1VjmAc8S7L+mhbDYjwwQFjoYqYgNisg8RMFnpj01YY1AVJp6bEzRvf3u5baEnniJWb0uKn Ck7KXhTk4WrVetADQaFxU075rHQAnG4= Received: from mail-ed1-f70.google.com (mail-ed1-f70.google.com [209.85.208.70]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-386-rKPryh8sNs2rwAL31R4uXw-1; Tue, 09 Mar 2021 06:06:25 -0500 X-MC-Unique: rKPryh8sNs2rwAL31R4uXw-1 Received: by mail-ed1-f70.google.com with SMTP id m8so844245edv.11 for ; Tue, 09 Mar 2021 03:06:25 -0800 (PST) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:subject:to:cc:references:from:message-id:date :user-agent:mime-version:in-reply-to:content-language :content-transfer-encoding; bh=0Pq8EJfDiYJYoqlSdSD0m0FRouDJg2ncmxP8m2FxexU=; b=NUBPQrN7XGWbl0U99BKd6rF+pa97CU0hxomuLRU+Uen9PoH2CRvgY2v187U6d4sONU +iuF9WiVHlfsHRo9wWQZRek5Y4xJcG8uuZEJgmeDk3Sujimaw3jUI93sYhb+DS83eFtu /5Zx1f90K9+2c8vqmM51QWyhuBUtx9MiNKWBdCc2nMTTayxxXMe+rFulTRZaC+pJjjC4 8G/MyMu5Bv1+s28mi6tmwTBIhajZPuxh9JJEIiIxGtFKkU77Y9oVe8+taf3bx30wQzNX +hvi6M66DjDZIBYlVF7oGIML5HRkpA0jV9yKnQJXnouuRdh+HLUVOqj6e6twcZ79sh9t pI5w== X-Gm-Message-State: AOAM530OZPDbgWDcEfuYHGX0eTHk1iQtpdrgTFUu2zaWTLLTGe8d/pk7 YWTm1E1nxTYuV5vEte6wr//um5SG/8hk+tXput1IY7SKOZXmEN5NCyPY8NuKLwby0dQjZ2YRudI wTQ0VnNphnXSY7wQ= X-Received: by 2002:a05:6402:10c6:: with SMTP id p6mr3369179edu.241.1615287984394; Tue, 09 Mar 2021 03:06:24 -0800 (PST) X-Google-Smtp-Source: ABdhPJxmVQmxIid/9+cJnyAJTiWybcARV8/wC0f4zztpxz9WF/+w9SX4U4B2b2BYcH/aGZ3ctmiCTQ== X-Received: by 2002:a05:6402:10c6:: with SMTP id p6mr3369160edu.241.1615287984182; Tue, 09 Mar 2021 03:06:24 -0800 (PST) Received: from ?IPv6:2001:b07:6468:f312:c8dd:75d4:99ab:290a? ([2001:b07:6468:f312:c8dd:75d4:99ab:290a]) by smtp.gmail.com with ESMTPSA id cy5sm8804048edb.46.2021.03.09.03.06.23 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 09 Mar 2021 03:06:23 -0800 (PST) Subject: Re: [RFC PATCH 4/4] coroutine/rwlock: Wake writers in preference to readers To: David Edmondson , qemu-devel@nongnu.org References: <20210309102157.365356-1-david.edmondson@oracle.com> <20210309102157.365356-5-david.edmondson@oracle.com> From: Paolo Bonzini Message-ID: <8de7f2e6-825f-a803-cf12-870559b5e787@redhat.com> Date: Tue, 9 Mar 2021 12:06:22 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.7.0 MIME-Version: 1.0 In-Reply-To: <20210309102157.365356-5-david.edmondson@oracle.com> Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=pbonzini@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Received-SPF: pass client-ip=63.128.21.124; envelope-from=pbonzini@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -29 X-Spam_score: -3.0 X-Spam_bar: --- X-Spam_report: (-3.0 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.251, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, NICE_REPLY_A=-0.001, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Kevin Wolf , Stefan Weil , Stefan Hajnoczi , qemu-block@nongnu.org, Max Reitz Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 09/03/21 11:21, David Edmondson wrote: > - /* The critical section started in qemu_co_rwlock_wrlock. */ > - qemu_co_queue_restart_all(&lock->queue); > + /* The critical section started in qemu_co_rwlock_wrlock or > + * qemu_co_rwlock_upgrade. > + */ > + qemu_co_queue_restart_all(&lock->wqueue); > + qemu_co_queue_restart_all(&lock->rqueue); Hmm, the devil is in the details---this is a thundering herd waiting to happen. But fortunately this can be fixed while making the unlock primitive even simpler: if (lock->reader) { self->locks_held--; /* Read-side critical sections do not keep lock->mutex. */ qemu_co_mutex_lock(&lock->mutex); lock->reader--; assert(lock->reader >= 0); } /* If there are no remaining readers wake one waiting writer * or all waiting readers. */ if (!lock->reader && !qemu_co_queue_next(&lock->wqueue)) { assert(!lock->pending_writer); qemu_co_queue_restart_all(&lock->rqueue); } Thanks, Paolo