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futex: remove the wait queue

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The waitqueue which is used in struct futex_q is a leftover from the
futexfd implementation. There is no need to use a waitqueue at all, as
the waiting task is the only user of it. The waitqueue just adds
additional locking and a loop in the wake up path which both can be
avoided.

We have already a task reference in struct futex_q which is used for
PI futexes. Use it for normal futexes as well and just wake up the
task directly.

The logic of signalling the futex wakeup via setting q->lock_ptr to
NULL is kept with the difference that we set it NULL before doing the
wakeup. This opens an exit race window vs. a non futex wake up of the
to be woken up task, which we prevent with get_task_struct /
put_task_struct on the waiter.

[ Impact: simplification ]

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
This commit is contained in:
Thomas Gleixner 2009-05-05 19:21:40 +02:00
parent b30505c81a
commit f1a11e0576
1 changed files with 26 additions and 34 deletions
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60
kernel/futex.c
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@ -100,8 +100,8 @@ struct futex_pi_state {
*/
struct futex_q {
struct plist_node list;
/* There can only be a single waiter */
wait_queue_head_t waiter;
/* Waiter reference */
struct task_struct *task;
/* Which hash list lock to use: */
spinlock_t *lock_ptr;
@ -111,7 +111,6 @@ struct futex_q {
/* Optional priority inheritance state: */
struct futex_pi_state *pi_state;
struct task_struct *task;
/* rt_waiter storage for requeue_pi: */
struct rt_mutex_waiter *rt_waiter;
@ -694,22 +693,29 @@ retry:
*/
static void wake_futex(struct futex_q *q)
{
struct task_struct *p = q->task;
/*
* We set q->lock_ptr = NULL _before_ we wake up the task. If
* a non futex wake up happens on another CPU then the task
* might exit and p would dereference a non existing task
* struct. Prevent this by holding a reference on p across the
* wake up.
*/
get_task_struct(p);
plist_del(&q->list, &q->list.plist);
/*
* 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(&q->waiter);
/*
* The waiting task can free the futex_q as soon as this is written,
* without taking any locks. This must come last.
*
* A memory barrier is required here to prevent the following store to
* lock_ptr from getting ahead of the wakeup. Clearing the lock at the
* end of wake_up() does not prevent this store from moving.
* The waiting task can free the futex_q as soon as
* q->lock_ptr = NULL is written, without taking any locks. A
* memory barrier is required here to prevent the following
* store to lock_ptr from getting ahead of the plist_del.
*/
smp_wmb();
q->lock_ptr = NULL;
wake_up_state(p, TASK_NORMAL);
put_task_struct(p);
}
static int wake_futex_pi(u32 __user *uaddr, u32 uval, struct futex_q *this)
@ -1003,7 +1009,7 @@ void requeue_pi_wake_futex(struct futex_q *q, union futex_key *key)
WARN_ON(!q->rt_waiter);
q->rt_waiter = NULL;
wake_up(&q->waiter);
wake_up_state(q->task, TASK_NORMAL);
}
/**
@ -1280,8 +1286,6 @@ static inline struct futex_hash_bucket *queue_lock(struct futex_q *q)
{
struct futex_hash_bucket *hb;
init_waitqueue_head(&q->waiter);
get_futex_key_refs(&q->key);
hb = hash_futex(&q->key);
q->lock_ptr = &hb->lock;
@ -1575,11 +1579,9 @@ out:
* @hb: the futex hash bucket, must be locked by the caller
* @q: the futex_q to queue up on
* @timeout: the prepared hrtimer_sleeper, or null for no timeout
* @wait: the wait_queue to add to the futex_q after queueing in the hb
*/
static void futex_wait_queue_me(struct futex_hash_bucket *hb, struct futex_q *q,
struct hrtimer_sleeper *timeout,
wait_queue_t *wait)
struct hrtimer_sleeper *timeout)
{
queue_me(q, hb);
@ -1587,19 +1589,11 @@ static void futex_wait_queue_me(struct futex_hash_bucket *hb, struct futex_q *q,
* There might have been scheduling since the queue_me(), as we
* cannot hold a spinlock across the get_user() in case it
* faults, and we cannot just set TASK_INTERRUPTIBLE state when
* queueing ourselves into the futex hash. This code thus has to
* queueing ourselves into the futex hash. This code thus has to
* rely on the futex_wake() code removing us from hash when it
* wakes us up.
*/
/* add_wait_queue is the barrier after __set_current_state. */
__set_current_state(TASK_INTERRUPTIBLE);
/*
* Add current as the futex_q waiter. We don't remove ourselves from
* the wait_queue because we are the only user of it.
*/
add_wait_queue(&q->waiter, wait);
set_current_state(TASK_INTERRUPTIBLE);
/* Arm the timer */
if (timeout) {
@ -1704,7 +1698,6 @@ static int futex_wait(u32 __user *uaddr, int fshared,
u32 val, ktime_t *abs_time, u32 bitset, int clockrt)
{
struct hrtimer_sleeper timeout, *to = NULL;
DECLARE_WAITQUEUE(wait, current);
struct restart_block *restart;
struct futex_hash_bucket *hb;
struct futex_q q;
@ -1733,7 +1726,7 @@ static int futex_wait(u32 __user *uaddr, int fshared,
goto out;
/* queue_me and wait for wakeup, timeout, or a signal. */
futex_wait_queue_me(hb, &q, to, &wait);
futex_wait_queue_me(hb, &q, to);
/* If we were woken (and unqueued), we succeeded, whatever. */
ret = 0;
@ -2147,7 +2140,6 @@ static int futex_wait_requeue_pi(u32 __user *uaddr, int fshared,
struct hrtimer_sleeper timeout, *to = NULL;
struct rt_mutex_waiter rt_waiter;
struct rt_mutex *pi_mutex = NULL;
DECLARE_WAITQUEUE(wait, current);
struct restart_block *restart;
struct futex_hash_bucket *hb;
union futex_key key2;
@ -2191,7 +2183,7 @@ static int futex_wait_requeue_pi(u32 __user *uaddr, int fshared,
}
/* Queue the futex_q, drop the hb lock, wait for wakeup. */
futex_wait_queue_me(hb, &q, to, &wait);
futex_wait_queue_me(hb, &q, to);
spin_lock(&hb->lock);
ret = handle_early_requeue_pi_wakeup(hb, &q, &key2, to);