[tip PATCH v7 1/9] RFC: futex: futex_wait_queue_me()

Darren Hart
2009-04-03T20:40:05Z

Refactor futex_wait() in preparation for futex_wait_requeue_pi(). In order to
reuse a good chunk of the futex_wait() code for the upcoming
futex_wait_requeue_pi() function, this patch breaks out the queue-to-wakeup
section of futex_wait() into futex_wait_queue_me().

Changelog:
V7: -Updates from tglx
-Make futex_wait() to use current->timer_slack_ns directly
-Removed needless likely() optimization
-Minor comment updates and coding style fixes
-Move the declaration of the wait_queue back into the caller to avoid a
timeout/signal vs. wakeup race where the wait_queue_t may go out of
scope
V6: -Incremental build fixes
V4: -Nesting cleanups
-Delayed hrtimer start until after setting TASK_INTERRUPTIBLE
V1: -Initial version

Signed-off-by: Darren Hart <dvhltc@us.ibm.com>
Reviewed-by: Steven Rostedt <srostedt@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Sripathi Kodi <sripathik@in.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: John Stultz <johnstul@us.ibm.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Dinakar Guniguntala <dino@in.ibm.com>
Cc: Ulrich Drepper <drepper@redhat.com>
Cc: Eric Dumazet <dada1@cosmosbay.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Jakub Jelinek <jakub@redhat.com>
---

kernel/futex.c | 138 +++++++++++++++++++++++++++++++-------------------------
1 files changed, 76 insertions(+), 62 deletions(-)

diff --git a/kernel/futex.c b/kernel/futex.c
index 6b50a02..d1d3e67 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -1115,24 +1115,87 @@ handle_fault:

static long futex_wait_restart(struct restart_block *restart);

+/**
+ * futex_wait_queue_me() - queue_me() and wait for wakeup, timeout, or signal
+ * @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,
+ struct wait_queue_t *wait)
+{
+ queue_me(q, hb);
+
+ /*
+ * 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
+ * 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);
+
+ /* Arm the timer */
+ if (timeout) {
+ hrtimer_start_expires(&timeout->timer, HRTIMER_MODE_ABS);
+ if (!hrtimer_active(&timeout->timer))
+ timeout->task = NULL;
+ }
+
+ /*
+ * !plist_node_empty() is safe here without any lock.
+ * q.lock_ptr != 0 is not safe, because of ordering against wakeup.
+ */
+ if (likely(!plist_node_empty(&q->list))) {
+ /*
+ * If the timer has already expired, current will already be
+ * flagged for rescheduling. Only call schedule if there
+ * is no timeout, or if it has yet to expire.
+ */
+ if (!timeout || timeout->task)
+ schedule();
+ }
+ __set_current_state(TASK_RUNNING);
+}
+
static int futex_wait(u32 __user *uaddr, int fshared,
u32 val, ktime_t *abs_time, u32 bitset, int clockrt)
{
- struct task_struct *curr = current;
+ struct hrtimer_sleeper timeout, *to = NULL;
+ DECLARE_WAITQUEUE(wait, current);
struct restart_block *restart;
- DECLARE_WAITQUEUE(wait, curr);
struct futex_hash_bucket *hb;
struct futex_q q;
u32 uval;
int ret;
- struct hrtimer_sleeper t;
- int rem = 0;

if (!bitset)
return -EINVAL;

q.pi_state = NULL;
q.bitset = bitset;
+
+ if (abs_time) {
+ to = &timeout;
+
+ hrtimer_init_on_stack(&to->timer, clockrt ? CLOCK_REALTIME :
+ CLOCK_MONOTONIC, HRTIMER_MODE_ABS);
+ hrtimer_init_sleeper(to, current);
+ hrtimer_set_expires_range_ns(&to->timer, *abs_time,
+ current->timer_slack_ns);
+ }
+
retry:
q.key = FUTEX_KEY_INIT;
ret = get_futex_key(uaddr, fshared, &q.key);
@@ -1178,75 +1241,22 @@ retry_private:
goto retry;
}
ret = -EWOULDBLOCK;
+
+ /* Only actually queue if *uaddr contained val. */
if (unlikely(uval != val)) {
queue_unlock(&q, hb);
goto out_put_key;
}

- /* Only actually queue if *uaddr contained val. */
- queue_me(&q, hb);
-
- /*
- * 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
- * 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_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.
- */
- if (likely(!plist_node_empty(&q.list))) {
- if (!abs_time)
- schedule();
- else {
- hrtimer_init_on_stack(&t.timer,
- clockrt ? CLOCK_REALTIME :
- CLOCK_MONOTONIC,
- HRTIMER_MODE_ABS);
- hrtimer_init_sleeper(&t, current);
- hrtimer_set_expires_range_ns(&t.timer, *abs_time,
- current->timer_slack_ns);
-
- hrtimer_start_expires(&t.timer, HRTIMER_MODE_ABS);
- if (!hrtimer_active(&t.timer))
- t.task = NULL;
-
- /*
- * the timer could have already expired, in which
- * case current would be flagged for rescheduling.
- * Don't bother calling schedule.
- */
- if (likely(t.task))
- schedule();
-
- hrtimer_cancel(&t.timer);
-
- /* Flag if a timeout occured */
- rem = (t.task == NULL);
-
- destroy_hrtimer_on_stack(&t.timer);
- }
- }
- __set_current_state(TASK_RUNNING);
-
- /*
- * NOTE: we don't remove ourselves from the waitqueue because
- * we are the only user of it.
- */
+ /* queue_me and wait for wakeup, timeout, or a signal. */
+ futex_wait_queue_me(hb, &q, to, &wait);

/* If we were woken (and unqueued), we succeeded, whatever. */
ret = 0;
if (!unqueue_me(&q))
goto out_put_key;
ret = -ETIMEDOUT;
- if (rem)
+ if (to && !to->task)
goto out_put_key;

/*
@@ -1275,6 +1285,10 @@ retry_private:
out_put_key:
put_futex_key(fshared, &q.key);
out:
+ if (to) {
+ hrtimer_cancel(&to->timer);
+ destroy_hrtimer_on_stack(&to->timer);
+ }
return ret;
}