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futex: rename field in futex_q to clarify single waiter semantics

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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>
This commit is contained in:
Darren Hart 2008-12-17 17:29:56 -08:00 committed by Ingo Molnar
parent 1acdac1046
commit 73500ac545
1 changed files with 6 additions and 5 deletions
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11
kernel/futex.c
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@ -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.