linux-kernel - [PATCH 1/2] locking/mutex: remove rcu_read_lock/unlock as we already disabled preemption

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Date:   Sun, 26 Sep 2021 18:16:23 +0800
From:   Yanfei Xu <yanfei.xu@...driver.com>
To:     peterz@...radead.org, mingo@...hat.com, will@...nel.org,
        longman@...hat.com, boqun.feng@...il.com
Cc:     linux-kernel@...r.kernel.org
Subject: [PATCH 1/2] locking/mutex: remove rcu_read_lock/unlock as we already disabled preemption

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@...driver.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