From: Darren Hart <[email protected]>
I've tripped over this 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 <[email protected]>
---
kernel/futex.c | 11 ++++++-----
1 files changed, 6 insertions(+), 5 deletions(-)
diff --git a/kernel/futex.c b/kernel/futex.c
index ba0d3b8..99f8acc 100644
--- 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.
--
Darren Hart
IBM Linux Technology Center
Real-Time Linux Team
* Darren Hart <[email protected]> wrote:
> From: Darren Hart <[email protected]>
>
> I've tripped over this 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 <[email protected]>
> ---
>
> kernel/futex.c | 11 ++++++-----
> 1 files changed, 6 insertions(+), 5 deletions(-)
applied to tip/core/futexes, thanks Darren!
> - wake_up_all(&q->waiters);
> + wake_up(&q->waiter);
yeah, the naming and the wake_up_all is a relic from the FUTEX_FD days -
but we removed that earlier this year:
| commit 82af7aca56c67061420d618cc5a30f0fd4106b80
| Author: Eric Sesterhenn <[email protected]>
| Date: Fri Jan 25 10:40:46 2008 +0100
|
| Removal of FUTEX_FD
Ingo