Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
From:   Yanfei Xu <yanfei.xu@windriver.com>
To:     peterz@infradead.org, mingo@redhat.com, will@kernel.org,
        longman@redhat.com, boqun.feng@gmail.com
Cc:     linux-kernel@vger.kernel.org
Subject: [PATCH 1/2] locking/mutex: remove rcu_read_lock/unlock as we already disabled preemption
Date:   Sun, 26 Sep 2021 18:16:23 +0800
Message-Id: <20210926101624.2460704-1-yanfei.xu@windriver.com>
Content-Transfer-Encoding: 8bit
Content-Type: text/plain
MIME-Version: 1.0
X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1
X-MS-Exchange-AntiSpam-MessageData-0: =?us-ascii?Q?HaIszXx7FmjPJAJJqOXlunwywrhOiNgnGxvZvtF6GDIxWETAiPmt6/bDhRLw?=
 =?us-ascii?Q?f6HoNKfEDB/1oh2hcKuWhbVirvKoYylMdjfdnzKNGzrDqjlV00gg2fnzGYX/?=
 =?us-ascii?Q?o/qaEGm2PYdE3rxEH6+DGAeTxFy8DJ+Y1ozx2bmucPYSU16QaQbt3jcKyQY8?=
 =?us-ascii?Q?hDRTVx0mqj/4SE5rvzAGnz24NVLZYY3ZBv/WMxVB7mfwF7S4QmKckoH+Qk7R?=
 =?us-ascii?Q?T/2NcMWTfQA7cVlk/mmNKa2bkwSOIG/D++cwHqUFVQo4ZS5q66J/GMl/t37E?=
 =?us-ascii?Q?4XotcvxsVf31RwvCMtyDHyqZ6pTHtXxfVPCq993Pzcc/Rpo3zuloUXQvm774?=
 =?us-ascii?Q?JXkrGTXTOnfnSz5XdePKPZhLnEfoiS1Z7Ed45ta0idjnYZFlVboPHONxEhTW?=
 =?us-ascii?Q?LoLHlV8WInUkioQr5SoX/lwxLGYXU+T44BCsLsmX5rIwyfgWiVtTLYS46prZ?=
 =?us-ascii?Q?iHZFm5neSgE1bortVFI3dS0H6KnWd+9pJz4zA7wmpYS+3BuRaKXiCHk3pCYE?=
 =?us-ascii?Q?Ao8G//F4p4dqTFAcZELZVkUdwz9Tj1XrJvcAHX1dTjZmRUlZAce/hlZqdeWv?=
 =?us-ascii?Q?357tMlKFC8rTGd5SFmpufeqH1OJiZ+hhnNkZ64h2xF9EQeXXbBeD8QbM6NDe?=
 =?us-ascii?Q?AhYm4qc15tP18mZqzOJhraBBs0ZJAOgbIYVBXVk6CRShteRdX4zUvMy1urb3?=
 =?us-ascii?Q?jN5X6UZVZmBVLbY6bSl4bPNKotJml0mEliiBVdlJBoo6lxJYkrPLwoeaNBrQ?=
 =?us-ascii?Q?2Jr9Bg/jmbFvjl19eJs0HekuODPZvwO9nbaM77/XN4XFppMgu5vGxx1Rikz9?=
 =?us-ascii?Q?w5aagc9RngH/f2CpKM6r6i2ry1PCrtrfDQ07mrMWpbWk0uaQZFV3Sfx9gqSZ?=
 =?us-ascii?Q?6RZ2Kq5qOWGufb1IpZUuR0gfqzmtvm0t3WzwLnvvoLwvxO+SPx7iN9CtLwGl?=
 =?us-ascii?Q?W/tWoZ29dd8rmq0750+CGuENEmTpsBn8ArJZHgnbrQKfuwGGT+yXZkcVOFbn?=
 =?us-ascii?Q?eA+uAqyl0Q31zF0LmAu22a6Lj/YNNlxUro2ewC/NhVyI3ESC1FVnaptEk0Bn?=
 =?us-ascii?Q?ZWkc2D1o1xGPY0xvc6DyEcvOaC5GcpRYROy8y920ZRtY1Rmtv+og82VXNWgi?=
 =?us-ascii?Q?v+qp2ne+PQeh3kfUMK9c7Dve5xXP8grbXnAaSGT2lrNGgSJpgf55tq4/I5/6?=
 =?us-ascii?Q?kmfGDDjtuc/cLIv0Fhh2Gdt9Ju5j66FuptDY6DtzVcWpMYuAvcACa4vcuRPq?=
 =?us-ascii?Q?dAbr/jgTZ2em67DzIvirozkfWLmFtMeW8tWuK882BH1HdDY26d8cArNqvshk?=
 =?us-ascii?Q?Vgnlev4dy9Me7bxMcl2cUDoW?=
X-OriginatorOrg: windriver.com
X-MS-Exchange-CrossTenant-Network-Message-Id: 95ef300b-6926-4037-d62e-08d980d6bfe0
X-MS-Exchange-CrossTenant-AuthSource: DM8PR11MB5734.namprd11.prod.outlook.com
X-MS-Exchange-CrossTenant-AuthAs: Internal
X-MS-Exchange-CrossTenant-OriginalArrivalTime: 26 Sep 2021 10:16:48.1818
 (UTC)
X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted
X-MS-Exchange-CrossTenant-Id: 8ddb2873-a1ad-4a18-ae4e-4644631433be
X-MS-Exchange-CrossTenant-MailboxType: HOSTED
X-MS-Exchange-CrossTenant-UserPrincipalName: C9SSiZK7Tza+3b4ntjbVCcb8hpgLA5moFvcq8dNljAPkyVgDupMPvQREetSeYtf9HD1X++8oXPbCgKlE1BL91g==
X-MS-Exchange-Transport-CrossTenantHeadersStamped: DM6PR11MB3148
X-Proofpoint-GUID: _d5LK_YWZHCA67jGFr5cA0coRFIew_J-
X-Proofpoint-ORIG-GUID: _d5LK_YWZHCA67jGFr5cA0coRFIew_J-
X-Proofpoint-Virus-Version: vendor=baseguard
 engine=ICAP:2.0.182.1,Aquarius:18.0.790,Hydra:6.0.391,FMLib:17.0.607.475
 definitions=2021-09-26_03,2021-09-24_02,2020-04-07_01
X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 malwarescore=0
 mlxlogscore=875 spamscore=0 mlxscore=0 lowpriorityscore=0 adultscore=0
 bulkscore=0 phishscore=0 impostorscore=0 suspectscore=0 priorityscore=1501
 clxscore=1011 classifier=spam adjust=0 reason=mlx scancount=1
 engine=8.12.0-2109230001 definitions=main-2109260071
Precedence: bulk
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

preempt_disable/enable() is equal to RCU read-side crital section,
and the mutex lock slowpath disable the preemption throughout the
entire slowpath. Let's remove the rcu_read_lock/unlock for saving
some cycles in hot codes.

Signed-off-by: Yanfei Xu <yanfei.xu@windriver.com>
---
 kernel/locking/mutex.c | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
index d456579d0952..54658c5db7d0 100644
--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -348,13 +348,13 @@ bool mutex_spin_on_owner(struct mutex *lock, struct task_struct *owner,
 {
 	bool ret = true;
 
-	rcu_read_lock();
 	while (__mutex_owner(lock) == owner) {
 		/*
 		 * Ensure we emit the owner->on_cpu, dereference _after_
-		 * checking lock->owner still matches owner. If that fails,
-		 * owner might point to freed memory. If it still matches,
-		 * the rcu_read_lock() ensures the memory stays valid.
+		 * checking lock->owner still matches owner. And we already
+		 * disabled preemption which is equal to the RCU read-side
+		 * crital section throughout the entire progress of the mutex
+		 * lock slowpath, thus the memory stays valid.
 		 */
 		barrier();
 
@@ -374,7 +374,6 @@ bool mutex_spin_on_owner(struct mutex *lock, struct task_struct *owner,
 
 		cpu_relax();
 	}
-	rcu_read_unlock();
 
 	return ret;
 }
-- 
2.27.0