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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 v2] locking/mutex: remove rcu_read_lock/unlock as we already disabled preemption
Date:   Fri,  8 Oct 2021 11:25:18 +0800
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preempt_disable/enable() is equal to RCU read-side crital section,
and the mutex lock slowpath disabled the preemption for the optimistic
spinning code. Let's remove the rcu_read_lock/unlock for saving some 
cycles in hot codes.

Signed-off-by: Yanfei Xu <yanfei.xu@windriver.com>
---
v1->v2: fix the incorrect comment in code and commit message. 
        thanks for WaiMan's suggestion.

BTW, sorry for this late v2 due to a long vocation.

 kernel/locking/mutex.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
index 2fede72b6af5..2f654cfb10d9 100644
--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -351,13 +351,14 @@ 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 in optimistic spinning code. Thus the
+		 * task_strcut structure won't go away during the spinning
+		 * period
 		 */
 		barrier();
 
@@ -377,7 +378,6 @@ bool mutex_spin_on_owner(struct mutex *lock, struct task_struct *owner,
 
 		cpu_relax();
 	}
-	rcu_read_unlock();
 
 	return ret;
 }
-- 
2.27.0