[PATCH 10/12] rwsem: down_read_unfair infrastructure support

[Michel Lespinasse <walken@google.com>]

Add rwsem_down_read_unfair_failed() function in the non-generic rwsem
library code, similar to  rwsem_down_read_failed() except that blocked
threads are placed at the head of the queue.

Signed-off-by: Michel Lespinasse <walken@google.com>
---
 lib/rwsem.c |   22 +++++++++++++++++++++-
 1 files changed, 21 insertions(+), 1 deletions(-)

diff --git a/lib/rwsem.c b/lib/rwsem.c
index 1702524..689d4a2 100644
--- a/lib/rwsem.c
+++ b/lib/rwsem.c
@@ -34,6 +34,7 @@ struct rwsem_waiter {
 	unsigned int flags;
 #define RWSEM_WAITING_FOR_READ	0x00000001
 #define RWSEM_WAITING_FOR_WRITE	0x00000002
+#define RWSEM_UNFAIR            0x00000004
 };
 
 #define RWSEM_WAKE_ANY        0 /* Wake whatever's at head of wait list */
@@ -174,7 +175,11 @@ rwsem_down_failed_common(struct rw_semaphore *sem,
 
 	if (list_empty(&sem->wait_list))
 		adjustment += RWSEM_WAITING_BIAS;
-	list_add_tail(&waiter->list, &sem->wait_list);
+
+	if (flags & RWSEM_UNFAIR)
+		list_add(&waiter.list, &sem->wait_list);
+	else
+		list_add_tail(&waiter.list, &sem->wait_list);
 
 	/* we're now waiting on the lock, but no longer actively read-locking */
 	count = rwsem_atomic_update(adjustment, sem);
@@ -215,6 +220,21 @@ rwsem_down_read_failed(struct rw_semaphore *sem)
 					-RWSEM_ACTIVE_READ_BIAS);
 }
 
+#ifdef __HAVE_DOWN_READ_UNFAIR
+
+/*
+ * wait for the read lock to be granted - skip waiting writers
+ */
+asmregparm struct rw_semaphore __sched *
+rwsem_down_read_unfair_failed(struct rw_semaphore *sem)
+{
+	return rwsem_down_failed_common(sem,
+					RWSEM_WAITING_FOR_READ | RWSEM_UNFAIR,
+					-RWSEM_ACTIVE_READ_BIAS);
+}
+
+#endif
+
 /*
  * wait for the write lock to be granted
  */
-- 
1.7.0.1