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    Subject[PATCH 2.6.20-rc4 1/4] futex priority based wakeup
    Hi,

    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.

    ---

    futex.c | 66 +++++++++++++++++++++++++++++++++++-----------------------------
    1 file changed, 37 insertions(+), 29 deletions(-)

    ---

    Signed-off-by: Sébastien Dugué <sebastien.dugue@bull.net>
    Signed-off-by: Pierre Peiffer <pierre.peiffer@bull.net>

    ---
    Index: linux-2.6/kernel/futex.c
    ===================================================================
    --- linux-2.6.orig/kernel/futex.c 2007-01-08 10:40:22.000000000 +0100
    +++ linux-2.6/kernel/futex.c 2007-01-08 10:42:07.000000000 +0100
    @@ -106,12 +106,12 @@ 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: */
    @@ -133,8 +133,8 @@ 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<<FUTEX_HASHBITS];
    @@ -465,13 +465,13 @@ lookup_pi_state(u32 uval, struct futex_h
    {
    struct futex_pi_state *pi_state = NULL;
    struct futex_q *this, *next;
    - struct list_head *head;
    + struct plist_head *head;
    struct task_struct *p;
    pid_t pid;

    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, &me->key)) {
    /*
    * Another waiter already exists - bump up
    @@ -535,12 +535,12 @@ 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);
    /*
    @@ -653,7 +653,7 @@ 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;

    @@ -667,7 +667,7 @@ 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;
    @@ -695,7 +695,7 @@ 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;

    @@ -768,7 +768,7 @@ 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)
    @@ -780,7 +780,7 @@ 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)
    @@ -807,7 +807,7 @@ 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;

    @@ -856,7 +856,7 @@ 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) {
    @@ -867,9 +867,13 @@ 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_key_refs(&key2);
    drop_count++;
    @@ -914,7 +918,11 @@ 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);
    + plist_node_init(&q->list, current->normal_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);
    }
    @@ -969,8 +977,8 @@ 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);

    @@ -988,8 +996,8 @@ 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);
    @@ -1082,10 +1090,10 @@ 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);

    @@ -1358,7 +1366,7 @@ 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;

    @@ -1409,7 +1417,7 @@ 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);
    @@ -1483,10 +1491,10 @@ 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;
    @@ -1869,7 +1877,7 @@ 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


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