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Commit 73500ac5 authored by Darren Hart's avatar Darren Hart Committed by Ingo Molnar
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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: default avatarDarren Hart <dvhltc@us.ibm.com>
Acked-by: default avatarThomas Gleixner <tglx@linutronix.de>
Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
parent 1acdac10

kernel/futex.c

+6 −5
Original line number Diff line number Diff line
@@ -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.