From e9933557cb09ebbc1fa22f719c82fd64b1f3fbe5 Mon Sep 17 00:00:00 2001
From: Ben Gardon <bgardon@google.com>
Date: Tue, 2 Feb 2021 10:57:12 -0800
Subject: [PATCH] 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 | 24 ++++++++++++++++++------
 include/linux/rwlock.h        |  7 +++++++
 2 files changed, 25 insertions(+), 6 deletions(-)

diff --git a/include/asm-generic/qrwlock.h b/include/asm-generic/qrwlock.h
index 3aefde23d..352a9ebb9 100755
--- a/include/asm-generic/qrwlock.h
+++ b/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.
@@ -116,15 +117,26 @@ 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.
  */
-#define arch_read_lock(l)	queued_read_lock(l)
-#define arch_write_lock(l)	queued_write_lock(l)
-#define arch_read_trylock(l)	queued_read_trylock(l)
-#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_read_lock(l)		queued_read_lock(l)
+#define arch_write_lock(l)		queued_write_lock(l)
+#define arch_read_trylock(l)		queued_read_trylock(l)
+#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 */
diff --git a/include/linux/rwlock.h b/include/linux/rwlock.h
index 34c414ccb..93086de7b 100755
--- a/include/linux/rwlock.h
+++ b/include/linux/rwlock.h
@@ -134,4 +134,11 @@ 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 */