Subject: rwsem contended

From: Christoph Lameter <clameter@sgi.com>

Add a function to rw_semaphores to check if there are any processes
waiting for the semaphore. Add rwsem_needbreak to sched.h that works
in the same way as spinlock_needbreak().

Signed-off-by: Christoph Lameter <clameter@sgi.com>
Signed-off-by: Andrea Arcangeli <andrea@qumranet.com>
---

diff --git a/include/linux/rwsem.h b/include/linux/rwsem.h
--- a/include/linux/rwsem.h
+++ b/include/linux/rwsem.h
@@ -59,6 +59,8 @@
  */
 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
diff --git a/include/linux/sched.h b/include/linux/sched.h
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1984,6 +1984,15 @@
 #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.
diff --git a/lib/rwsem-spinlock.c b/lib/rwsem-spinlock.c
--- a/lib/rwsem-spinlock.c
+++ b/lib/rwsem-spinlock.c
@@ -305,6 +305,18 @@
 	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);
diff --git a/lib/rwsem.c b/lib/rwsem.c
--- a/lib/rwsem.c
+++ b/lib/rwsem.c
@@ -251,6 +251,18 @@
 	return sem;
 }
 
+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(rwsem_down_read_failed);
 EXPORT_SYMBOL(rwsem_down_write_failed);
 EXPORT_SYMBOL(rwsem_wake);