linux-kernel - [PATCH 2/2] locking: Provide spin_is_contended() on PREEMPT

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Date:   Mon,  6 Sep 2021 16:30:04 +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>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: [PATCH 2/2] locking: Provide spin_is_contended() on PREEMPT_RT

spin_is_contended() should return != 0 if there is a waiter pending on
the lock. This can be checked on PREEMPT_RT's spin lock implementation
by checking if the lock as any waiters pending.

Provide spin_is_contended() on PREEMPT_RT by checking if there are any
waiters.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
---
 include/linux/spinlock_rt.h  | 3 ++-
 kernel/locking/spinlock_rt.c | 6 ++++++
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/include/linux/spinlock_rt.h b/include/linux/spinlock_rt.h
index 835aedaf68acd..844bc8f9cf091 100644
--- a/include/linux/spinlock_rt.h
+++ b/include/linux/spinlock_rt.h
@@ -39,6 +39,7 @@ extern void rt_spin_unlock(spinlock_t *lock);
 extern void rt_spin_lock_unlock(spinlock_t *lock);
 extern int rt_spin_trylock_bh(spinlock_t *lock);
 extern int rt_spin_trylock(spinlock_t *lock);
+extern int rt_spin_is_contended(spinlock_t *lock);
 
 static __always_inline void spin_lock(spinlock_t *lock)
 {
@@ -145,7 +146,7 @@ static __always_inline void spin_unlock_irqrestore(spinlock_t *lock,
 #define spin_trylock_irqsave(lock, flags)		\
 	__cond_lock(lock, __spin_trylock_irqsave(lock, flags))
 
-#define spin_is_contended(lock)		(((void)(lock), 0))
+#define spin_is_contended(lock)		rt_spin_is_contended(lock)
 
 static inline int spin_is_locked(spinlock_t *lock)
 {
diff --git a/kernel/locking/spinlock_rt.c b/kernel/locking/spinlock_rt.c
index d2912e44d61fd..c9acb665277ff 100644
--- a/kernel/locking/spinlock_rt.c
+++ b/kernel/locking/spinlock_rt.c
@@ -118,6 +118,12 @@ int __sched rt_spin_trylock_bh(spinlock_t *lock)
 }
 EXPORT_SYMBOL(rt_spin_trylock_bh);
 
+int __sched rt_spin_is_contended(spinlock_t *lock)
+{
+	return rt_mutex_has_waiters(&lock->lock);
+}
+EXPORT_SYMBOL(rt_spin_is_contended);
+
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
 void __rt_spin_lock_init(spinlock_t *lock, const char *name,
 			 struct lock_class_key *key, bool percpu)
-- 
2.33.0