[PATCH v2 10/13] rwsem: simplify __rwsem_do_wake

[Michel Lespinasse <walken@google.com>]

This is mostly for cleanup value:

- We don't need several gotos to handle the case where the first
  waiter is a writer. Two simple tests will do (and generate very
  similar code).

- In the remainder of the function, we know the first waiter is a reader,
  so we don't have to double check that. We can use do..while loops
  to iterate over the readers to wake (generates slightly better code).

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

---
 lib/rwsem-spinlock.c | 25 ++++++++-----------------
 lib/rwsem.c          | 26 ++++++++++++--------------
 2 files changed, 20 insertions(+), 31 deletions(-)

diff --git a/lib/rwsem-spinlock.c b/lib/rwsem-spinlock.c
index 5f117f37ac0a..20207a6e8ac3 100644
--- a/lib/rwsem-spinlock.c
+++ b/lib/rwsem-spinlock.c
@@ -70,26 +70,17 @@ __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) {
+	do {
 		struct list_head *next = waiter->list.next;
 
 		list_del(&waiter->list);
@@ -99,10 +90,10 @@ __rwsem_do_wake(struct rw_semaphore *sem, int wakewrite)
 		wake_up_process(tsk);
 		put_task_struct(tsk);
 		woken++;
-		if (list_empty(&sem->wait_list))
+		if (next == &sem->wait_list)
 			break;
 		waiter = list_entry(next, struct rwsem_waiter, list);
-	}
+	} while (waiter->type != RWSEM_WAITING_FOR_WRITE);
 
 	sem->activity += woken;
 
diff --git a/lib/rwsem.c b/lib/rwsem.c
index 0d50e46d5b0c..9a675fa9d78e 100644
--- a/lib/rwsem.c
+++ b/lib/rwsem.c
@@ -68,20 +68,17 @@ __rwsem_do_wake(struct rw_semaphore *sem, int wake_type)
 	signed long woken, loop, adjustment;
 
 	waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
-	if (waiter->type != RWSEM_WAITING_FOR_WRITE)
-		goto readers_only;
-
-	if (wake_type == RWSEM_WAKE_READ_OWNED)
-		/* Another active reader was observed, so wakeup is not
-		 * likely to succeed. Save the atomic op.
-		 */
+	if (waiter->type == RWSEM_WAITING_FOR_WRITE) {
+		if (wake_type != RWSEM_WAKE_READ_OWNED)
+			/* Wake writer at the front of the queue, but do not
+			 * grant it the lock yet as we want other writers
+			 * to be able to steal it.  Readers, on the other hand,
+			 * will block as they will notice the queued writer.
+			 */
+			wake_up_process(waiter->task);
 		goto out;
+	}
 
-	/* Wake up the writing waiter and let the task grab the sem: */
-	wake_up_process(waiter->task);
-	goto out;
-
- readers_only:
 	/* If we come here from up_xxxx(), another thread might have reached
 	 * rwsem_down_failed_common() before we acquired the spinlock and
 	 * woken up a waiter, making it now active.  We prefer to check for
@@ -125,7 +122,8 @@ __rwsem_do_wake(struct rw_semaphore *sem, int wake_type)
 	rwsem_atomic_add(adjustment, sem);
 
 	next = sem->wait_list.next;
-	for (loop = woken; loop > 0; loop--) {
+	loop = woken;
+	do {
 		waiter = list_entry(next, struct rwsem_waiter, list);
 		next = waiter->list.next;
 		tsk = waiter->task;
@@ -133,7 +131,7 @@ __rwsem_do_wake(struct rw_semaphore *sem, int wake_type)
 		waiter->task = NULL;
 		wake_up_process(tsk);
 		put_task_struct(tsk);
-	}
+	} while (--loop);
 
 	sem->wait_list.next = next;
 	next->prev = &sem->wait_list;
-- 
1.8.1.3