linux-kernel - [PATCH] locking/rwsem: save unnecessary rwsem_set

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Date:   Thu, 13 Jun 2019 18:18:09 +0800
From:   Peng Wang <rocking@....edu.cn>
To:     peterz@...radead.org, mingo@...hat.com, will.deacon@....com
Cc:     linux-kernel@...r.kernel.org, Peng Wang <rocking@....edu.cn>
Subject: [PATCH] locking/rwsem: save unnecessary rwsem_set_owner() after slow path

rwsem_down_write_failed() and rwsem_down_write_failed_killable() return
with sem->owner set if acquire semaphore successfully, so we can skip
rwsem_set_owner() to keep pace with reading.

Signed-off-by: Peng Wang <rocking@....edu.cn>
---
 kernel/locking/rwsem.h | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/kernel/locking/rwsem.h b/kernel/locking/rwsem.h
index 64877f5294e3..4fa21f737151 100644
--- a/kernel/locking/rwsem.h
+++ b/kernel/locking/rwsem.h
@@ -225,7 +225,8 @@ static inline void __down_write(struct rw_semaphore *sem)
 					     &sem->count);
 	if (unlikely(tmp != RWSEM_ACTIVE_WRITE_BIAS))
 		rwsem_down_write_failed(sem);
-	rwsem_set_owner(sem);
+	else
+		rwsem_set_owner(sem);
 }
 
 static inline int __down_write_killable(struct rw_semaphore *sem)
@@ -234,10 +235,12 @@ static inline int __down_write_killable(struct rw_semaphore *sem)
 
 	tmp = atomic_long_add_return_acquire(RWSEM_ACTIVE_WRITE_BIAS,
 					     &sem->count);
-	if (unlikely(tmp != RWSEM_ACTIVE_WRITE_BIAS))
+	if (unlikely(tmp != RWSEM_ACTIVE_WRITE_BIAS)) {
 		if (IS_ERR(rwsem_down_write_failed_killable(sem)))
 			return -EINTR;
-	rwsem_set_owner(sem);
+	} else {
+		rwsem_set_owner(sem);
+	}
 	return 0;
 }
 
-- 
2.19.1