Subject: Add rwsem_is_contended()

Add a function to rw_semaphores to check if there are any processes
waiting for the lock.

Signed-off-by: Christoph Lameter <clameter@sgi.com>

---
 include/linux/rwsem.h |    2 ++
 include/linux/sched.h |    9 +++++++++
 lib/rwsem-spinlock.c  |   12 ++++++++++++
 lib/rwsem.c           |    6 ++++++
 4 files changed, 29 insertions(+)

Index: linux-2.6/include/linux/rwsem.h
===================================================================
--- linux-2.6.orig/include/linux/rwsem.h	2008-04-18 14:04:12.000000000 -0700
+++ linux-2.6/include/linux/rwsem.h	2008-04-18 14:05:12.000000000 -0700
@@ -59,6 +59,8 @@ extern void up_write(struct rw_semaphore
  */
 extern void downgrade_write(struct rw_semaphore *sem);
 
+extern int rwsem_is_contended(struct rw_semaphore *sem);
+
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
 /*
  * nested locking. NOTE: rwsems are not allowed to recurse
Index: linux-2.6/include/linux/sched.h
===================================================================
--- linux-2.6.orig/include/linux/sched.h	2008-04-18 14:04:16.000000000 -0700
+++ linux-2.6/include/linux/sched.h	2008-04-18 14:05:12.000000000 -0700
@@ -1984,6 +1984,15 @@ static inline int spin_needbreak(spinloc
 #endif
 }
 
+static inline int rwsem_needbreak(struct rw_semaphore *sem)
+{
+#ifdef CONFIG_PREEMPT
+	return rwsem_is_contended(sem);
+#else
+	return 0;
+#endif
+}
+
 /*
  * Reevaluate whether the task has signals pending delivery.
  * Wake the task if so.
Index: linux-2.6/lib/rwsem-spinlock.c
===================================================================
--- linux-2.6.orig/lib/rwsem-spinlock.c	2008-04-18 14:26:49.000000000 -0700
+++ linux-2.6/lib/rwsem-spinlock.c	2008-04-18 14:30:49.000000000 -0700
@@ -305,6 +305,18 @@ void __downgrade_write(struct rw_semapho
 	spin_unlock_irqrestore(&sem->wait_lock, flags);
 }
 
+int rwsem_is_contended(struct rw_semaphore *sem)
+{
+	/*
+	 * Racy check for an empty list. False positives or negatives
+	 * would be okay. False positive may cause a useless dropping of
+	 * locks. False negatives may cause locks to be held a bit
+	 * longer until the next check.
+	 */
+	return !list_empty(&sem->wait_list);
+}
+
+EXPORT_SYMBOL(rwsem_is_contended);
 EXPORT_SYMBOL(__init_rwsem);
 EXPORT_SYMBOL(__down_read);
 EXPORT_SYMBOL(__down_read_trylock);
Index: linux-2.6/lib/rwsem.c
===================================================================
--- linux-2.6.orig/lib/rwsem.c	2008-04-18 15:15:13.000000000 -0700
+++ linux-2.6/lib/rwsem.c	2008-04-18 15:15:23.000000000 -0700
@@ -251,6 +251,12 @@ asmregparm struct rw_semaphore *rwsem_do
 	return sem;
 }
 
+int rwsem_is_contended(struct rw_semaphore *sem)
+{
+	return !list_empty(&sem->wait_list);
+}
+
+EXPORT_SYMBOL(rwsem_is_contended);
 EXPORT_SYMBOL(rwsem_down_read_failed);
 EXPORT_SYMBOL(rwsem_down_write_failed);
 EXPORT_SYMBOL(rwsem_wake);