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Subject[PATCH 2.6.20-rc5 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.

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).

---
futex.c | 78 ++++++++++++++++++++++++++++++++++++++++------------------------
1 file changed, 49 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 2007-01-17 09:39:57.000000000 +0100
+++ linux-2.6/kernel/futex.c 2007-01-17 09:44:25.000000000 +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 +918,23 @@ 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);
}
@@ -969,8 +989,8 @@ 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 +1008,8 @@ 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 +1102,10 @@ 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 +1378,7 @@ 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 +1429,7 @@ 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 +1503,10 @@ 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 +1889,7 @@ 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

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