linux-kernel - [PATCH 3/5] locking/percpu-rwsem: Move __this_cpu

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Date:   Wed, 13 Nov 2019 11:21:18 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     peterz@...radead.org, mingo@...nel.org, will@...nel.org
Cc:     oleg@...hat.com, tglx@...utronix.de, linux-kernel@...r.kernel.org,
        bigeasy@...utronix.de, juri.lelli@...hat.com, williams@...hat.com,
        bristot@...hat.com, longman@...hat.com, dave@...olabs.net,
        jack@...e.com
Subject: [PATCH 3/5] locking/percpu-rwsem: Move __this_cpu_inc() into the slowpath

As preparation to rework __percpu_down_read() move the
__this_cpu_inc() into it.

Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 include/linux/percpu-rwsem.h  |   10 ++++++----
 kernel/locking/percpu-rwsem.c |    2 ++
 2 files changed, 8 insertions(+), 4 deletions(-)

--- a/include/linux/percpu-rwsem.h
+++ b/include/linux/percpu-rwsem.h
@@ -59,8 +59,9 @@ static inline void percpu_down_read(stru
 	 * and that once the synchronize_rcu() is done, the writer will see
 	 * anything we did within this RCU-sched read-size critical section.
 	 */
-	__this_cpu_inc(*sem->read_count);
-	if (unlikely(!rcu_sync_is_idle(&sem->rss)))
+	if (likely(rcu_sync_is_idle(&sem->rss)))
+		__this_cpu_inc(*sem->read_count);
+	else
 		__percpu_down_read(sem, false); /* Unconditional memory barrier */
 	/*
 	 * The preempt_enable() prevents the compiler from
@@ -77,8 +78,9 @@ static inline bool percpu_down_read_tryl
 	/*
 	 * Same as in percpu_down_read().
 	 */
-	__this_cpu_inc(*sem->read_count);
-	if (unlikely(!rcu_sync_is_idle(&sem->rss)))
+	if (likely(rcu_sync_is_idle(&sem->rss)))
+		__this_cpu_inc(*sem->read_count);
+	else
 		ret = __percpu_down_read(sem, true); /* Unconditional memory barrier */
 	preempt_enable();
 	/*
--- a/kernel/locking/percpu-rwsem.c
+++ b/kernel/locking/percpu-rwsem.c
@@ -47,6 +47,8 @@ EXPORT_SYMBOL_GPL(percpu_free_rwsem);
 
 bool __percpu_down_read(struct percpu_rw_semaphore *sem, bool try)
 {
+	__this_cpu_inc(*sem->read_count);
+
 	/*
 	 * Due to having preemption disabled the decrement happens on
 	 * the same CPU as the increment, avoiding the