[RFC patch 3/5] futex: Split out unlock from queue_me()

[Thomas Gleixner]

Preparatory patch for a lockless empty check of the hash bucket plist
in futex_wake().

No functional change.

Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
---
 kernel/futex.c |   24 ++++++++++++++++--------
 1 file changed, 16 insertions(+), 8 deletions(-)

Index: linux-2.6/kernel/futex.c
===================================================================
--- linux-2.6.orig/kernel/futex.c
+++ linux-2.6/kernel/futex.c
@@ -1548,15 +1548,14 @@ static inline void queue_unlock(struct f
  * @q:	The futex_q to enqueue
  * @hb:	The destination hash bucket
  *
- * The hb->lock must be held by the caller, and is released here. A call to
- * queue_me() is typically paired with exactly one call to unqueue_me().  The
- * exceptions involve the PI related operations, which may use unqueue_me_pi()
- * or nothing if the unqueue is done as part of the wake process and the unqueue
- * state is implicit in the state of woken task (see futex_wait_requeue_pi() for
- * an example).
+ * The hb->lock must be held by the caller. A call to queue_me() is
+ * typically paired with exactly one call to unqueue_me(). The
+ * exceptions involve the PI related operations, which may use
+ * unqueue_me_pi() or nothing if the unqueue is done as part of the
+ * wake process and the unqueue state is implicit in the state of
+ * woken task (see futex_wait_requeue_pi() for an example).
  */
 static inline void queue_me(struct futex_q *q, struct futex_hash_bucket *hb)
-	__releases(&hb->lock)
 {
 	int prio;
 
@@ -1573,7 +1572,6 @@ static inline void queue_me(struct futex
 	plist_node_init(&q->list, prio);
 	plist_add(&q->list, &hb->chain);
 	q->task = current;
-	spin_unlock(&hb->lock);
 }
 
 /**
@@ -1834,6 +1832,11 @@ static void futex_wait_queue_me(struct f
 	 */
 	set_current_state(TASK_INTERRUPTIBLE);
 	queue_me(q, hb);
+	/*
+	 * Unlock _AFTER_ we queued ourself. See the comment describing
+	 * the futex ordering guarantees on top of this file.
+	 */
+	queue_unlock(hb);
 
 	/* Arm the timer */
 	if (timeout) {
@@ -2085,6 +2088,11 @@ retry_private:
 	 * Only actually queue now that the atomic ops are done:
 	 */
 	queue_me(&q, hb);
+	/*
+	 * Unlock _AFTER_ we queued ourself. See the comment describing
+	 * the futex ordering guarantees on top of this file.
+	 */
+	queue_unlock(hb);
 
 	WARN_ON(!q.pi_state);
 	/*


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