[PATCH 1/2] rwsem: add rwsem_is_contended V3

[Josef Bacik <jbacik@fusionio.com>]

Btrfs needs a simple way to know if it needs to let go of it's read lock on a
rwsem.  Introduce rwsem_is_contended to check to see if there are any waiters on
this rwsem currently.  This is just a hueristic, it is meant to be light and not
100% accurate and called by somebody already holding on to the rwsem in either
read or write.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fusionio.com>
---
V2->V3: fixed the comment and simplified the function as per Peter's
suggestions.

V1->V2: took everybodys suggestions and simplified it to just one function in
rwsem.h so it works for both the spinlock case and non-spinlock case.

 include/linux/rwsem.h | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/include/linux/rwsem.h b/include/linux/rwsem.h
index 0616ffe..03f3b05 100644
--- a/include/linux/rwsem.h
+++ b/include/linux/rwsem.h
@@ -75,6 +75,17 @@ do {								\
 } while (0)
 
 /*
+ * This is the same regardless of which rwsem implementation that is being used.
+ * It is just a heuristic meant to be called by somebody alreadying holding the
+ * rwsem to see if somebody from an incompatible type is wanting access to the
+ * lock.
+ */
+static inline int rwsem_is_contended(struct rw_semaphore *sem)
+{
+	return !list_empty(&sem->wait_list);
+}
+
+/*
  * lock for reading
  */
 extern void down_read(struct rw_semaphore *sem);
-- 
1.8.3.1