From nobody Tue Sep 9 22:08:37 2025 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 0A9E3C6FD1D for ; Mon, 27 Mar 2023 20:26:14 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230379AbjC0U0M (ORCPT ); Mon, 27 Mar 2023 16:26:12 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:54732 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229663AbjC0UZ4 (ORCPT ); Mon, 27 Mar 2023 16:25:56 -0400 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 4C0423C15 for ; Mon, 27 Mar 2023 13:24:29 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1679948668; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=c/k5fdQzdPEiui8WNN3fZp6fkCe06zAbO2ZueqkSVSw=; b=flPTfa4Ro0wVSBUhjzFpt+HpWiPu4glOt3D+WzGQ0H6TUKRF83mZxRMxlkC+0+9vzoqfQ7 Uz17Y4+VkXRX7UE4c6WD7aS9vt+Ubf9sG61ivht35rKHdTvCOPa94CRDyRMBzNdNNizrH6 I3VjA+Pt+S0OjiEMz52aTSPnxwE+Js0= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-595-XnRBMUz4NNubxjrYG_Xr8Q-1; Mon, 27 Mar 2023 16:24:25 -0400 X-MC-Unique: XnRBMUz4NNubxjrYG_Xr8Q-1 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.rdu2.redhat.com [10.11.54.4]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id BCFAE101A550; Mon, 27 Mar 2023 20:24:24 +0000 (UTC) Received: from llong.com (unknown [10.22.17.245]) by smtp.corp.redhat.com (Postfix) with ESMTP id 867632027040; Mon, 27 Mar 2023 20:24:24 +0000 (UTC) From: Waiman Long To: Peter Zijlstra , Ingo Molnar , Will Deacon , Boqun Feng Cc: linux-kernel@vger.kernel.org Subject: [PATCH v2 7/8] locking/rwsem: Use the force Date: Mon, 27 Mar 2023 16:24:12 -0400 Message-Id: <20230327202413.1955856-8-longman@redhat.com> In-Reply-To: <20230327202413.1955856-1-longman@redhat.com> References: <20230327202413.1955856-1-longman@redhat.com> MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable X-Scanned-By: MIMEDefang 3.1 on 10.11.54.4 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Type: text/plain; charset="utf-8" From: Peter Zijlstra Now that the writer adjustment is done from the wakeup side and HANDOFF guarantees spinning/stealing is disabled, use the combined guarantee it ignore spurious READER_BIAS and directly claim the lock. Signed-off-by: Peter Zijlstra (Intel) --- kernel/locking/rwsem.c | 33 +++++++++++++++++++++++++++++---- 1 file changed, 29 insertions(+), 4 deletions(-) diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c index ee8861effcc2..7bd26e64827a 100644 --- a/kernel/locking/rwsem.c +++ b/kernel/locking/rwsem.c @@ -377,8 +377,8 @@ rwsem_add_waiter(struct rw_semaphore *sem, struct rwsem= _waiter *waiter) /* * Remove a waiter from the wait_list and clear flags. * - * Both rwsem_reader_wake() and rwsem_try_write_lock() contain a full 'cop= y' of - * this function. Modify with care. + * Both rwsem_{reader,writer}_wake() and rwsem_try_write_lock() contain a = full + * 'copy' of this function. Modify with care. * * Return: true if wait_list isn't empty and false otherwise */ @@ -479,8 +479,33 @@ static void rwsem_writer_wake(struct rw_semaphore *sem, struct rwsem_waiter *waiter, struct wake_q_head *wake_q) { - if (rwsem_try_write_lock(sem, waiter)) - rwsem_waiter_wake(waiter, wake_q); + long count =3D atomic_long_read(&sem->count); + + /* + * Since rwsem_mark_wake() is only called (with WAKE_ANY) when + * the lock is unlocked, and the HANDOFF bit guarantees that + * all spinning / stealing is disabled, it is posssible to + * unconditionally claim the lock -- any READER_BIAS will be + * temporary. + */ + if ((count & (RWSEM_FLAG_HANDOFF|RWSEM_WRITER_LOCKED)) =3D=3D RWSEM_FLAG_= HANDOFF) { + unsigned long adjustment =3D RWSEM_WRITER_LOCKED - RWSEM_FLAG_HANDOFF; + + if (list_is_singular(&sem->wait_list)) + adjustment -=3D RWSEM_FLAG_WAITERS; + + atomic_long_add(adjustment, &sem->count); + /* + * Have rwsem_writer_wake() fully imply rwsem_del_waiter() on + * success. + */ + list_del(&waiter->list); + atomic_long_set(&sem->owner, (long)waiter->task); + + } else if (!rwsem_try_write_lock(sem, waiter)) + return; + + rwsem_waiter_wake(waiter, wake_q); } =20 static void rwsem_reader_wake(struct rw_semaphore *sem, --=20 2.31.1