Today, all threads waiting for a given futex are woken in FIFO order (first waiter woken first) instead of priority order. This patch makes use of plist (pirotity ordered lists) instead of simple list in futex_hash_bucket. All non-RT threads are stored with priority MAX_RT_PRIO, causing them to be woken last, in FIFO order (RT-threads are woken first, in priority order). Signed-off-by: Sébastien Dugué Signed-off-by: Pierre Peiffer --- kernel/futex.c | 78 +++++++++++++++++++++++++++++++++++---------------------- 1 file changed, 49 insertions(+), 29 deletions(-) Index: b/kernel/futex.c =================================================================== --- a/kernel/futex.c +++ b/kernel/futex.c @@ -81,12 +81,12 @@ struct futex_pi_state { * we can wake only the relevant ones (hashed queues may be shared). * * A futex_q has a woken state, just like tasks have TASK_RUNNING. - * It is considered woken when list_empty(&q->list) || q->lock_ptr == 0. + * 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. */ struct futex_q { - struct list_head list; + struct plist_node list; wait_queue_head_t waiters; /* Which hash list lock to use: */ @@ -108,8 +108,8 @@ struct futex_q { * Split the global futex_lock into every hash list lock. */ struct futex_hash_bucket { - spinlock_t lock; - struct list_head chain; + spinlock_t lock; + struct plist_head chain; }; static struct futex_hash_bucket futex_queues[1<chain; - list_for_each_entry_safe(this, next, head, list) { + plist_for_each_entry_safe(this, next, head, list) { if (match_futex(&this->key, &me->key)) { /* * Another waiter already exists - bump up @@ -513,12 +513,12 @@ lookup_pi_state(u32 uval, struct futex_h */ static void wake_futex(struct futex_q *q) { - list_del_init(&q->list); + plist_del(&q->list, &q->list.plist); if (q->filp) send_sigio(&q->filp->f_owner, q->fd, POLL_IN); /* * The lock in wake_up_all() is a crucial memory barrier after the - * list_del_init() and also before assigning to q->lock_ptr. + * plist_del() and also before assigning to q->lock_ptr. */ wake_up_all(&q->waiters); /* @@ -631,7 +631,7 @@ static int futex_wake(u32 __user *uaddr, { struct futex_hash_bucket *hb; struct futex_q *this, *next; - struct list_head *head; + struct plist_head *head; union futex_key key; int ret; @@ -645,7 +645,7 @@ static int futex_wake(u32 __user *uaddr, spin_lock(&hb->lock); head = &hb->chain; - list_for_each_entry_safe(this, next, head, list) { + plist_for_each_entry_safe(this, next, head, list) { if (match_futex (&this->key, &key)) { if (this->pi_state) { ret = -EINVAL; @@ -673,7 +673,7 @@ futex_wake_op(u32 __user *uaddr1, u32 __ { union futex_key key1, key2; struct futex_hash_bucket *hb1, *hb2; - struct list_head *head; + struct plist_head *head; struct futex_q *this, *next; int ret, op_ret, attempt = 0; @@ -746,7 +746,7 @@ retry: head = &hb1->chain; - list_for_each_entry_safe(this, next, head, list) { + plist_for_each_entry_safe(this, next, head, list) { if (match_futex (&this->key, &key1)) { wake_futex(this); if (++ret >= nr_wake) @@ -758,7 +758,7 @@ retry: head = &hb2->chain; op_ret = 0; - list_for_each_entry_safe(this, next, head, list) { + plist_for_each_entry_safe(this, next, head, list) { if (match_futex (&this->key, &key2)) { wake_futex(this); if (++op_ret >= nr_wake2) @@ -785,7 +785,7 @@ static int futex_requeue(u32 __user *uad { union futex_key key1, key2; struct futex_hash_bucket *hb1, *hb2; - struct list_head *head1; + struct plist_head *head1; struct futex_q *this, *next; int ret, drop_count = 0; @@ -834,7 +834,7 @@ static int futex_requeue(u32 __user *uad } head1 = &hb1->chain; - list_for_each_entry_safe(this, next, head1, list) { + plist_for_each_entry_safe(this, next, head1, list) { if (!match_futex (&this->key, &key1)) continue; if (++ret <= nr_wake) { @@ -845,9 +845,13 @@ static int futex_requeue(u32 __user *uad * requeue. */ if (likely(head1 != &hb2->chain)) { - list_move_tail(&this->list, &hb2->chain); + plist_del(&this->list, &hb1->chain); + plist_add(&this->list, &hb2->chain); this->lock_ptr = &hb2->lock; - } +#ifdef CONFIG_DEBUG_PI_LIST + this->list.plist.lock = &hb2->lock; +#endif + } this->key = key2; get_futex_key_refs(&key2); drop_count++; @@ -892,7 +896,23 @@ queue_lock(struct futex_q *q, int fd, st static inline void __queue_me(struct futex_q *q, struct futex_hash_bucket *hb) { - list_add_tail(&q->list, &hb->chain); + int prio; + + /* + * The priority used to register this element is + * - either the real thread-priority for the real-time threads + * (i.e. threads with a priority lower than MAX_RT_PRIO) + * - or MAX_RT_PRIO for non-RT threads. + * Thus, all RT-threads are woken first in priority order, and + * the others are woken last, in FIFO order. + */ + prio = min(current->normal_prio, MAX_RT_PRIO); + + plist_node_init(&q->list, prio); +#ifdef CONFIG_DEBUG_PI_LIST + q->list.plist.lock = &hb->lock; +#endif + plist_add(&q->list, &hb->chain); q->task = current; spin_unlock(&hb->lock); } @@ -947,8 +967,8 @@ static int unqueue_me(struct futex_q *q) spin_unlock(lock_ptr); goto retry; } - WARN_ON(list_empty(&q->list)); - list_del(&q->list); + WARN_ON(plist_node_empty(&q->list)); + plist_del(&q->list, &q->list.plist); BUG_ON(q->pi_state); @@ -966,8 +986,8 @@ static int unqueue_me(struct futex_q *q) */ static void unqueue_me_pi(struct futex_q *q, struct futex_hash_bucket *hb) { - WARN_ON(list_empty(&q->list)); - list_del(&q->list); + WARN_ON(plist_node_empty(&q->list)); + plist_del(&q->list, &q->list.plist); BUG_ON(!q->pi_state); free_pi_state(q->pi_state); @@ -1060,10 +1080,10 @@ static int futex_wait(u32 __user *uaddr, __set_current_state(TASK_INTERRUPTIBLE); add_wait_queue(&q.waiters, &wait); /* - * !list_empty() is safe here without any lock. + * !plist_node_empty() is safe here without any lock. * q.lock_ptr != 0 is not safe, because of ordering against wakeup. */ - if (likely(!list_empty(&q.list))) + if (likely(!plist_node_empty(&q.list))) time = schedule_timeout(time); __set_current_state(TASK_RUNNING); @@ -1336,7 +1356,7 @@ static int futex_unlock_pi(u32 __user *u struct futex_hash_bucket *hb; struct futex_q *this, *next; u32 uval; - struct list_head *head; + struct plist_head *head; union futex_key key; int ret, attempt = 0; @@ -1387,7 +1407,7 @@ retry_locked: */ head = &hb->chain; - list_for_each_entry_safe(this, next, head, list) { + plist_for_each_entry_safe(this, next, head, list) { if (!match_futex (&this->key, &key)) continue; ret = wake_futex_pi(uaddr, uval, this); @@ -1461,10 +1481,10 @@ static unsigned int futex_poll(struct fi poll_wait(filp, &q->waiters, wait); /* - * list_empty() is safe here without any lock. + * plist_node_empty() is safe here without any lock. * q->lock_ptr != 0 is not safe, because of ordering against wakeup. */ - if (list_empty(&q->list)) + if (plist_node_empty(&q->list)) ret = POLLIN | POLLRDNORM; return ret; @@ -1847,7 +1867,7 @@ static int __init init(void) } for (i = 0; i < ARRAY_SIZE(futex_queues); i++) { - INIT_LIST_HEAD(&futex_queues[i].chain); + plist_head_init(&futex_queues[i].chain, &futex_queues[i].lock); spin_lock_init(&futex_queues[i].lock); } return 0; -- -- Pierre Peiffer - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/