locking/rwlocks: Add contention detection for rwlocks

rwlocks do not currently have any facility to detect contention
like spinlocks do. In order to allow users of rwlocks to better manage
latency, add contention detection for queued rwlocks.

CC: Ingo Molnar <mingo@redhat.com>
CC: Will Deacon <will@kernel.org>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Acked-by: Davidlohr Bueso <dbueso@suse.de>
Acked-by: Waiman Long <longman@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Ben Gardon <bgardon@google.com>
Message-Id: <20210202185734.1680553-7-bgardon@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

include/asm-generic/qrwlock.h

@@ -14,6 +14,7 @@
 #include <asm/processor.h>
 
 #include <asm-generic/qrwlock_types.h>
+#include <asm-generic/qspinlock.h>
 
 /*
  * Writer states & reader shift and bias.
@@ -116,6 +117,16 @@ static inline void queued_write_unlock(struct qrwlock *lock)
 	smp_store_release(&lock->wlocked, 0);
 }
 
+/**
+ * queued_rwlock_is_contended - check if the lock is contended
+ * @lock : Pointer to queue rwlock structure
+ * Return: 1 if lock contended, 0 otherwise
+ */
+static inline int queued_rwlock_is_contended(struct qrwlock *lock)
+{
+	return arch_spin_is_locked(&lock->wait_lock);
+}
+
 /*
  * Remapping rwlock architecture specific functions to the corresponding
  * queue rwlock functions.
@@ -126,5 +137,6 @@ static inline void queued_write_unlock(struct qrwlock *lock)
 #define arch_write_trylock(l)		queued_write_trylock(l)
 #define arch_read_unlock(l)		queued_read_unlock(l)
 #define arch_write_unlock(l)		queued_write_unlock(l)
+#define arch_rwlock_is_contended(l)	queued_rwlock_is_contended(l)
 
 #endif /* __ASM_GENERIC_QRWLOCK_H */

include/linux/rwlock.h

@@ -128,4 +128,11 @@ do {								\
 	1 : ({ local_irq_restore(flags); 0; }); \
 })
 
+#ifdef arch_rwlock_is_contended
+#define rwlock_is_contended(lock) \
+	 arch_rwlock_is_contended(&(lock)->raw_lock)
+#else
+#define rwlock_is_contended(lock)	((void)(lock), 0)
+#endif /* arch_rwlock_is_contended */
+
 #endif /* __LINUX_RWLOCK_H */