linux-kernel - [PATCH 1/3] locking/rwsem: Return void in __rwsem_mark

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Message-Id: <1470384285-32163-2-git-send-email-dave@stgolabs.net>
Date:	Fri,  5 Aug 2016 01:04:43 -0700
From:	Davidlohr Bueso <dave@...olabs.net>
To:	peterz@...radead.org, mingo@...nel.org
Cc:	Waiman.Long@...com, jason.low2@....com, wanpeng.li@...mail.com,
	dave@...olabs.net, linux-kernel@...r.kernel.org,
	Davidlohr Bueso <dbueso@...e.de>
Subject: [PATCH 1/3] locking/rwsem: Return void in __rwsem_mark_wake()

We currently return a rw_semaphore structure, which is the
same lock we passed to the function's argument in the first
place. While there are several functions that choose this
return value, the callers use it, for example, for things
like ERR_PTR. This is not the case for __rwsem_mark_wake(),
and in addition this function is really about the lock
waiters (which we know there are at this point), so its
somewhat odd to be returning the sem structure.

Signed-off-by: Davidlohr Bueso <dbueso@...e.de>
---
 kernel/locking/rwsem-xadd.c | 35 +++++++++++++++++------------------
 1 file changed, 17 insertions(+), 18 deletions(-)

diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index 447e08de1fab..b03623172277 100644
--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -121,16 +121,17 @@ enum rwsem_wake_type {
  * - woken process blocks are discarded from the list after having task zeroed
  * - writers are only marked woken if downgrading is false
  */
-static struct rw_semaphore *
-__rwsem_mark_wake(struct rw_semaphore *sem,
-		  enum rwsem_wake_type wake_type, struct wake_q_head *wake_q)
+static void __rwsem_mark_wake(struct rw_semaphore *sem,
+			      enum rwsem_wake_type wake_type,
+			      struct wake_q_head *wake_q)
 {
 	struct rwsem_waiter *waiter;
 	struct task_struct *tsk;
 	struct list_head *next;
-	long oldcount, woken, loop, adjustment;
+	long loop, oldcount, woken = 0, adjustment = 0;
 
 	waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
+
 	if (waiter->type == RWSEM_WAITING_FOR_WRITE) {
 		if (wake_type == RWSEM_WAKE_ANY) {
 			/*
@@ -142,19 +143,19 @@ __rwsem_mark_wake(struct rw_semaphore *sem,
 			 */
 			wake_q_add(wake_q, waiter->task);
 		}
-		goto out;
+
+		return;
 	}
 
-	/* Writers might steal the lock before we grant it to the next reader.
+	/*
+	 * Writers might steal the lock before we grant it to the next reader.
 	 * We prefer to do the first reader grant before counting readers
 	 * so we can bail out early if a writer stole the lock.
 	 */
-	adjustment = 0;
 	if (wake_type != RWSEM_WAKE_READ_OWNED) {
 		adjustment = RWSEM_ACTIVE_READ_BIAS;
  try_reader_grant:
 		oldcount = atomic_long_fetch_add(adjustment, &sem->count);
-
 		if (unlikely(oldcount < RWSEM_WAITING_BIAS)) {
 			/*
 			 * If the count is still less than RWSEM_WAITING_BIAS
@@ -164,7 +165,8 @@ __rwsem_mark_wake(struct rw_semaphore *sem,
 			 */
 			if (atomic_long_add_return(-adjustment, &sem->count) <
 			    RWSEM_WAITING_BIAS)
-				goto out;
+				return;
+
 			/* Last active locker left. Retry waking readers. */
 			goto try_reader_grant;
 		}
@@ -176,11 +178,11 @@ __rwsem_mark_wake(struct rw_semaphore *sem,
 		rwsem_set_reader_owned(sem);
 	}
 
-	/* Grant an infinite number of read locks to the readers at the front
+	/*
+	 * Grant an infinite number of read locks to the readers at the front
 	 * of the queue.  Note we increment the 'active part' of the count by
 	 * the number of readers before waking any processes up.
 	 */
-	woken = 0;
 	do {
 		woken++;
 
@@ -219,9 +221,6 @@ __rwsem_mark_wake(struct rw_semaphore *sem,
 
 	sem->wait_list.next = next;
 	next->prev = &sem->wait_list;
-
- out:
-	return sem;
 }
 
 /*
@@ -255,7 +254,7 @@ struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem)
 	if (count == RWSEM_WAITING_BIAS ||
 	    (count > RWSEM_WAITING_BIAS &&
 	     adjustment != -RWSEM_ACTIVE_READ_BIAS))
-		sem = __rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q);
+		__rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q);
 
 	raw_spin_unlock_irq(&sem->wait_lock);
 	wake_up_q(&wake_q);
@@ -505,7 +504,7 @@ __rwsem_down_write_failed_common(struct rw_semaphore *sem, int state)
 		if (count > RWSEM_WAITING_BIAS) {
 			WAKE_Q(wake_q);
 
-			sem = __rwsem_mark_wake(sem, RWSEM_WAKE_READERS, &wake_q);
+			__rwsem_mark_wake(sem, RWSEM_WAKE_READERS, &wake_q);
 			/*
 			 * The wakeup is normally called _after_ the wait_lock
 			 * is released, but given that we are proactively waking
@@ -616,7 +615,7 @@ locked:
 
 	/* do nothing if list empty */
 	if (!list_empty(&sem->wait_list))
-		sem = __rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q);
+		__rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q);
 
 	raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
 	wake_up_q(&wake_q);
@@ -640,7 +639,7 @@ struct rw_semaphore *rwsem_downgrade_wake(struct rw_semaphore *sem)
 
 	/* do nothing if list empty */
 	if (!list_empty(&sem->wait_list))
-		sem = __rwsem_mark_wake(sem, RWSEM_WAKE_READ_OWNED, &wake_q);
+		__rwsem_mark_wake(sem, RWSEM_WAKE_READ_OWNED, &wake_q);
 
 	raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
 	wake_up_q(&wake_q);
-- 
2.6.6