linux-kernel - [PATCH 2/2] rtmutex: Wake up the waiters lockless while dropping the read lock.

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Date:   Tue, 28 Sep 2021 17:00:06 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     linux-kernel@...r.kernel.org
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Waiman Long <longman@...hat.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Davidlohr Bueso <dave@...olabs.net>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: [PATCH 2/2] rtmutex: Wake up the waiters lockless while dropping the read lock.

From: Thomas Gleixner <tglx@...utronix.de>

The rw_semaphore and rwlock_t implementation both wake the waiter while
holding the rt_mutex_base::wait_lock acquired.
This can be optimized by waking the waiter lockless outside of the
locked section to avoid a needless contention on the
rt_mutex_base::wait_lock lock.

Extend rt_mutex_wake_q_add() to also accept task and state and use it in
__rwbase_read_unlock().

Suggested-by: Davidlohr Bueso <dave@...olabs.net>
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
---
 kernel/locking/rtmutex.c   | 23 +++++++++++++++--------
 kernel/locking/rwbase_rt.c |  6 +++++-
 2 files changed, 20 insertions(+), 9 deletions(-)

diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
index cafc259ec59d3..0c6a48dfcecb3 100644
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -446,17 +446,24 @@ static __always_inline void rt_mutex_adjust_prio(struct task_struct *p)
 }
 
 /* RT mutex specific wake_q wrappers */
+static __always_inline void rt_mutex_wake_q_add_task(struct rt_wake_q_head *wqh,
+						     struct task_struct *task,
+						     unsigned int wake_state)
+{
+	if (IS_ENABLED(CONFIG_PREEMPT_RT) && wake_state == TASK_RTLOCK_WAIT) {
+		if (IS_ENABLED(CONFIG_PROVE_LOCKING))
+			WARN_ON_ONCE(wqh->rtlock_task);
+		get_task_struct(task);
+		wqh->rtlock_task = task;
+	} else {
+		wake_q_add(&wqh->head, task);
+	}
+}
+
 static __always_inline void rt_mutex_wake_q_add(struct rt_wake_q_head *wqh,
 						struct rt_mutex_waiter *w)
 {
-	if (IS_ENABLED(CONFIG_PREEMPT_RT) && w->wake_state == TASK_RTLOCK_WAIT) {
-		if (IS_ENABLED(CONFIG_PROVE_LOCKING))
-			WARN_ON_ONCE(wqh->rtlock_task);
-		get_task_struct(w->task);
-		wqh->rtlock_task = w->task;
-	} else {
-		wake_q_add(&wqh->head, w->task);
-	}
+	rt_mutex_wake_q_add_task(wqh, w->task, w->wake_state);
 }
 
 static __always_inline void rt_mutex_wake_up_q(struct rt_wake_q_head *wqh)
diff --git a/kernel/locking/rwbase_rt.c b/kernel/locking/rwbase_rt.c
index 88191f6e252c3..15c81100f0e26 100644
--- a/kernel/locking/rwbase_rt.c
+++ b/kernel/locking/rwbase_rt.c
@@ -148,6 +148,7 @@ static void __sched __rwbase_read_unlock(struct rwbase_rt *rwb,
 {
 	struct rt_mutex_base *rtm = &rwb->rtmutex;
 	struct task_struct *owner;
+	DEFINE_RT_WAKE_Q(wqh);
 
 	raw_spin_lock_irq(&rtm->wait_lock);
 	/*
@@ -158,9 +159,12 @@ static void __sched __rwbase_read_unlock(struct rwbase_rt *rwb,
 	 */
 	owner = rt_mutex_owner(rtm);
 	if (owner)
-		wake_up_state(owner, state);
+		rt_mutex_wake_q_add_task(&wqh, owner, state);
 
+	/* Pairs with the preempt_enable in rt_mutex_wake_up_q() */
+	preempt_disable();
 	raw_spin_unlock_irq(&rtm->wait_lock);
+	rt_mutex_wake_up_q(&wqh);
 }
 
 static __always_inline void rwbase_read_unlock(struct rwbase_rt *rwb,
-- 
2.33.0