[PATCH 10/12] rwsem-spinlock: wake all readers when first waiter is a reader

[Michel Lespinasse <walken@google.com>]

When the first queued waiter is a reader, wake all readers instead of
just those that are at the front of the queue. This should result in
increased parallelism for workloads that mix readers and writers.

Signed-off-by: Michel Lespinasse <walken@google.com>

---
 lib/rwsem-spinlock.c | 39 ++++++++++++++++-----------------------
 1 file changed, 16 insertions(+), 23 deletions(-)

diff --git a/lib/rwsem-spinlock.c b/lib/rwsem-spinlock.c
index 5f117f37ac0a..d22948eeaeae 100644
--- a/lib/rwsem-spinlock.c
+++ b/lib/rwsem-spinlock.c
@@ -70,36 +70,29 @@ __rwsem_do_wake(struct rw_semaphore *sem, int wakewrite)
 
 	waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
 
-	if (!wakewrite) {
-		if (waiter->type == RWSEM_WAITING_FOR_WRITE)
-			goto out;
-		goto dont_wake_writers;
-	}
-
-	/*
-	 * as we support write lock stealing, we can't set sem->activity
-	 * to -1 here to indicate we get the lock. Instead, we wake it up
-	 * to let it go get it again.
-	 */
 	if (waiter->type == RWSEM_WAITING_FOR_WRITE) {
-		wake_up_process(waiter->task);
+		if (wakewrite)
+			/* Wake up a writer. Note that we do not grant it the
+			 * lock - it will have to acquire it when it runs. */
+			wake_up_process(waiter->task);
 		goto out;
 	}
 
-	/* grant an infinite number of read locks to the front of the queue */
- dont_wake_writers:
+	/* grant read locks to all queued readers. */
 	woken = 0;
-	while (waiter->type == RWSEM_WAITING_FOR_READ) {
+	while (true) {
 		struct list_head *next = waiter->list.next;
 
-		list_del(&waiter->list);
-		tsk = waiter->task;
-		smp_mb();
-		waiter->task = NULL;
-		wake_up_process(tsk);
-		put_task_struct(tsk);
-		woken++;
-		if (list_empty(&sem->wait_list))
+		if (waiter->type != RWSEM_WAITING_FOR_WRITE) {
+			list_del(&waiter->list);
+			tsk = waiter->task;
+			smp_mb();
+			waiter->task = NULL;
+			wake_up_process(tsk);
+			put_task_struct(tsk);
+			woken++;
+		}
+		if (next == &sem->wait_list)
 			break;
 		waiter = list_entry(next, struct rwsem_waiter, list);
 	}
-- 
1.8.1.3