linux-kernel - [tip: locking/core] locking/percpu-rwsem: Move __this_cpu

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Date:   Tue, 11 Feb 2020 12:48:23 -0000
From:   "tip-bot2 for Peter Zijlstra" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Davidlohr Bueso <dbueso@...e.de>,
        Will Deacon <will@...nel.org>,
        Waiman Long <longman@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>, x86 <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: [tip: locking/core] locking/percpu-rwsem: Move __this_cpu_inc() into
 the slowpath

The following commit has been merged into the locking/core branch of tip:

Commit-ID:     71365d40232110f7b029befc9033ea311d680611
Gitweb:        https://git.kernel.org/tip/71365d40232110f7b029befc9033ea311d680611
Author:        Peter Zijlstra <peterz@...radead.org>
AuthorDate:    Wed, 30 Oct 2019 20:17:51 +01:00
Committer:     Ingo Molnar <mingo@...nel.org>
CommitterDate: Tue, 11 Feb 2020 13:10:54 +01:00

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>
Signed-off-by: Ingo Molnar <mingo@...nel.org>
Reviewed-by: Davidlohr Bueso <dbueso@...e.de>
Acked-by: Will Deacon <will@...nel.org>
Acked-by: Waiman Long <longman@...hat.com>
Tested-by: Juri Lelli <juri.lelli@...hat.com>
Link: https://lkml.kernel.org/r/20200131151540.041600199@infradead.org
---
 include/linux/percpu-rwsem.h  | 10 ++++++----
 kernel/locking/percpu-rwsem.c |  2 ++
 2 files changed, 8 insertions(+), 4 deletions(-)

diff --git a/include/linux/percpu-rwsem.h b/include/linux/percpu-rwsem.h
index 4ceaa19..bb5b71c 100644
--- a/include/linux/percpu-rwsem.h
+++ b/include/linux/percpu-rwsem.h
@@ -59,8 +59,9 @@ static inline void percpu_down_read(struct percpu_rw_semaphore *sem)
 	 * 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_trylock(struct percpu_rw_semaphore *sem)
 	/*
 	 * 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();
 	/*
diff --git a/kernel/locking/percpu-rwsem.c b/kernel/locking/percpu-rwsem.c
index 969389d..becf925 100644
--- 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