futex: rename field in futex_q to clarify single waiter semantics

Impact: simplify code

I've tripped over the naming of this field a couple times.

The futex_q uses a "waiters" list to represent a single blocked task and
then calles wake_up_all().

This can lead to confusion in trying to understand the intent of the code,
which is to have a single futex_q for every task waiting on a futex.

This patch corrects the problem, using a single pointer to the waiting
task, and an appropriate call to wake_up, rather than wake_up_all.

Compile and boot tested on an 8way x86_64 machine.

Signed-off-by: Darren Hart <dvhltc@us.ibm.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

diff --git a/kernel/futex.c b/kernel/futex.c
index ba0d3b8..99f8acc 100644 (file)
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -92,11 +92,12 @@ struct futex_pi_state {
  * A futex_q has a woken state, just like tasks have TASK_RUNNING.
  * It is considered woken when plist_node_empty(&q->list) || q->lock_ptr == 0.
  * The order of wakup is always to make the first condition true, then
- * wake up q->waiters, then make the second condition true.
+ * wake up q->waiter, then make the second condition true.
  */
 struct futex_q {
        struct plist_node list;
-       wait_queue_head_t waiters;
+       /* There can only be a single waiter */
+       wait_queue_head_t waiter;
 
        /* Which hash list lock to use: */
        spinlock_t *lock_ptr;
@@ -573,7 +574,7 @@ static void wake_futex(struct futex_q *q)
         * The lock in wake_up_all() is a crucial memory barrier after the
         * plist_del() and also before assigning to q->lock_ptr.
         */
-       wake_up_all(&q->waiters);
+       wake_up(&q->waiter);
        /*
         * The waiting task can free the futex_q as soon as this is written,
         * without taking any locks.  This must come last.
@@ -930,7 +931,7 @@ static inline struct futex_hash_bucket *queue_lock(struct futex_q *q)
 {
        struct futex_hash_bucket *hb;
 
-       init_waitqueue_head(&q->waiters);
+       init_waitqueue_head(&q->waiter);
 
        get_futex_key_refs(&q->key);
        hb = hash_futex(&q->key);
@@ -1221,7 +1222,7 @@ static int futex_wait(u32 __user *uaddr, int fshared,
 
        /* add_wait_queue is the barrier after __set_current_state. */
        __set_current_state(TASK_INTERRUPTIBLE);
-       add_wait_queue(&q.waiters, &wait);
+       add_wait_queue(&q.waiter, &wait);
        /*
         * !plist_node_empty() is safe here without any lock.
         * q.lock_ptr != 0 is not safe, because of ordering against wakeup.