From nobody Sun Apr 13 09:52:16 2025 Delivered-To: importer@patchew.org Received-SPF: pass (zoho.com: domain of gnu.org designates 208.118.235.17 as permitted sender) client-ip=208.118.235.17; envelope-from=qemu-devel-bounces+importer=patchew.org@nongnu.org; helo=lists.gnu.org; Authentication-Results: mx.zoho.com; spf=pass (zoho.com: domain of gnu.org designates 208.118.235.17 as permitted sender) smtp.mailfrom=qemu-devel-bounces+importer=patchew.org@nongnu.org; Return-Path: Received: from lists.gnu.org (lists.gnu.org [208.118.235.17]) by mx.zohomail.com with SMTPS id 1487584405770781.6717414161822; Mon, 20 Feb 2017 01:53:25 -0800 (PST) Received: from localhost ([::1]:37011 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1cfkeo-0006wg-IQ for importer@patchew.org; Mon, 20 Feb 2017 04:53:22 -0500 Received: from eggs.gnu.org ([2001:4830:134:3::10]:60209) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1cfkMN-0007Kz-M9 for qemu-devel@nongnu.org; Mon, 20 Feb 2017 04:34:20 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1cfkMI-0007TO-JA for qemu-devel@nongnu.org; Mon, 20 Feb 2017 04:34:19 -0500 Received: from mx1.redhat.com ([209.132.183.28]:33856) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1cfkMC-0007Rs-9Q for qemu-devel@nongnu.org; Mon, 20 Feb 2017 04:34:14 -0500 Received: from int-mx11.intmail.prod.int.phx2.redhat.com (int-mx11.intmail.prod.int.phx2.redhat.com [10.5.11.24]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 70BCAC05091E; Mon, 20 Feb 2017 09:34:08 +0000 (UTC) Received: from localhost (ovpn-116-137.ams2.redhat.com [10.36.116.137]) by int-mx11.intmail.prod.int.phx2.redhat.com (8.14.4/8.14.4) with ESMTP id v1K9Y7W3031709; Mon, 20 Feb 2017 04:34:07 -0500 From: Stefan Hajnoczi To: Date: Mon, 20 Feb 2017 09:33:04 +0000 Message-Id: <20170220093304.20515-25-stefanha@redhat.com> In-Reply-To: <20170220093304.20515-1-stefanha@redhat.com> References: <20170220093304.20515-1-stefanha@redhat.com> X-Scanned-By: MIMEDefang 2.68 on 10.5.11.24 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.31]); Mon, 20 Feb 2017 09:34:08 +0000 (UTC) X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 209.132.183.28 Subject: [Qemu-devel] [PULL 24/24] coroutine-lock: make CoRwlock thread-safe and fair X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.21 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Peter Maydell , Stefan Hajnoczi , Paolo Bonzini Errors-To: qemu-devel-bounces+importer=patchew.org@nongnu.org Sender: "Qemu-devel" X-ZohoMail: RSF_0 Z_629925259 SPT_0 Content-Transfer-Encoding: quoted-printable MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" From: Paolo Bonzini This adds a CoMutex around the existing CoQueue. Because the write-side can just take CoMutex, the old "writer" field is not necessary anymore. Instead of removing it altogether, count the number of pending writers during a read-side critical section and forbid further readers from entering. Signed-off-by: Paolo Bonzini Reviewed-by: Fam Zheng Message-id: 20170213181244.16297-7-pbonzini@redhat.com Signed-off-by: Stefan Hajnoczi --- include/qemu/coroutine.h | 3 ++- util/qemu-coroutine-lock.c | 35 ++++++++++++++++++++++++----------- 2 files changed, 26 insertions(+), 12 deletions(-) diff --git a/include/qemu/coroutine.h b/include/qemu/coroutine.h index d2de268..e60beaf 100644 --- a/include/qemu/coroutine.h +++ b/include/qemu/coroutine.h @@ -204,8 +204,9 @@ bool qemu_co_queue_empty(CoQueue *queue); =20 =20 typedef struct CoRwlock { - bool writer; + int pending_writer; int reader; + CoMutex mutex; CoQueue queue; } CoRwlock; =20 diff --git a/util/qemu-coroutine-lock.c b/util/qemu-coroutine-lock.c index b0a554f..6328eed 100644 --- a/util/qemu-coroutine-lock.c +++ b/util/qemu-coroutine-lock.c @@ -346,16 +346,22 @@ void qemu_co_rwlock_init(CoRwlock *lock) { memset(lock, 0, sizeof(*lock)); qemu_co_queue_init(&lock->queue); + qemu_co_mutex_init(&lock->mutex); } =20 void qemu_co_rwlock_rdlock(CoRwlock *lock) { Coroutine *self =3D qemu_coroutine_self(); =20 - while (lock->writer) { - qemu_co_queue_wait(&lock->queue, NULL); + qemu_co_mutex_lock(&lock->mutex); + /* For fairness, wait if a writer is in line. */ + while (lock->pending_writer) { + qemu_co_queue_wait(&lock->queue, &lock->mutex); } lock->reader++; + qemu_co_mutex_unlock(&lock->mutex); + + /* The rest of the read-side critical section is run without the mutex= . */ self->locks_held++; } =20 @@ -364,10 +370,13 @@ void qemu_co_rwlock_unlock(CoRwlock *lock) Coroutine *self =3D qemu_coroutine_self(); =20 assert(qemu_in_coroutine()); - if (lock->writer) { - lock->writer =3D false; + if (!lock->reader) { + /* The critical section started in qemu_co_rwlock_wrlock. */ qemu_co_queue_restart_all(&lock->queue); } else { + self->locks_held--; + + qemu_co_mutex_lock(&lock->mutex); lock->reader--; assert(lock->reader >=3D 0); /* Wakeup only one waiting writer */ @@ -375,16 +384,20 @@ void qemu_co_rwlock_unlock(CoRwlock *lock) qemu_co_queue_next(&lock->queue); } } - self->locks_held--; + qemu_co_mutex_unlock(&lock->mutex); } =20 void qemu_co_rwlock_wrlock(CoRwlock *lock) { - Coroutine *self =3D qemu_coroutine_self(); - - while (lock->writer || lock->reader) { - qemu_co_queue_wait(&lock->queue, NULL); + qemu_co_mutex_lock(&lock->mutex); + lock->pending_writer++; + while (lock->reader) { + qemu_co_queue_wait(&lock->queue, &lock->mutex); } - lock->writer =3D true; - self->locks_held++; + lock->pending_writer--; + + /* The rest of the write-side critical section is run with + * the mutex taken, so that lock->reader remains zero. + * There is no need to update self->locks_held. + */ } --=20 2.9.3