[RFC PATCH v2 4/5] futex: Add timeout support to TO futexes

[Waiman Long]

This patch adds the timeout support in other futex types to TO
futexes. However, the timeout isn't as precise as the other futex
types due to the fact timer expiration can't currently be detected
when the thread is waiting in the serialization mutex.

Signed-off-by: Waiman Long <Waiman.Long@xxxxxxx>
---
 kernel/futex.c |   40 +++++++++++++++++++++++++++++++---------
 1 files changed, 31 insertions(+), 9 deletions(-)

diff --git a/kernel/futex.c b/kernel/futex.c
index 7daba56..c73be79 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -3423,7 +3423,8 @@ static inline int futex_set_waiters_unlocked(u32 __user *uaddr, u32 *puval)
  * Return: 0 if futex acquired, < 0 if an error happens.
  */
 static int futex_spin_on_owner(u32 __user *uaddr, u32 vpid,
-			       struct futex_state *state)
+			       struct futex_state *state,
+			       struct hrtimer_sleeper *timeout)
 {
 	int ret, loop = TO_SPIN_THRESHOLD;
 	u32 uval, curval;
@@ -3483,8 +3484,11 @@ static int futex_spin_on_owner(u32 __user *uaddr, u32 vpid,
 			continue;
 		}
 
-		/* Check for signal */
-		if (signal_pending(current)) {
+		/* Check for timeout & signal */
+		if (timeout && !timeout->task) {
+			ret = -ETIMEDOUT;
+			break;
+		} else if (signal_pending(current)) {
 			ret = -EINTR;
 			break;
 		}
@@ -3594,6 +3598,12 @@ efault:
  * This function is not inlined so that it can show up in stack trace for
  * analysis purpose.
  *
+ * The timeout functionality isn't as precise as other other futex types
+ * as timer expiration will not be detected while waiting in the
+ * serialization mutex. One possible solution is to modify the expiration
+ * function to send out a signal as well so as to break the thread out of
+ * the mutex code if we really want more precise timeout.
+ *
  * Return:
  *  < 0 - an error happens
  *  0   - acquires the lock via futex_spin_on_owner()
@@ -3601,8 +3611,9 @@ efault:
  *  2   - lock handed off from unlocker
  */
 static noinline int
-futex_lock_to(u32 __user *uaddr, unsigned int flags)
+futex_lock_to(u32 __user *uaddr, unsigned int flags, ktime_t *time)
 {
+	struct hrtimer_sleeper timeout, *to = NULL;
 	struct futex_hash_bucket *hb;
 	union futex_key key = FUTEX_KEY_INIT;
 	struct futex_state *state;
@@ -3627,6 +3638,8 @@ futex_lock_to(u32 __user *uaddr, unsigned int flags)
 	if (refill_futex_state_cache())
 		return -ENOMEM;
 
+	futex_set_timer(time, &to, &timeout, flags, current->timer_slack_ns);
+
 	ret = get_futex_key(uaddr, flags & FLAGS_SHARED, &key, VERIFY_WRITE);
 	if (unlikely(ret))
 		goto out;
@@ -3651,10 +3664,15 @@ futex_lock_to(u32 __user *uaddr, unsigned int flags)
 	/*
 	 * Acquiring the serialization mutex.
 	 */
-	if (state->type != TYPE_TO)
+	if (state->type != TYPE_TO) {
 		ret = -EINVAL;
-	else
+	} else {
+		if (to)
+			hrtimer_start_expires(&to->timer, HRTIMER_MODE_ABS);
 		ret = mutex_lock_interruptible(&state->mutex);
+		if (ret && to && !timeout.task)
+			ret = -ETIMEDOUT;
+	}
 
 	if (unlikely(ret))
 		/*
@@ -3667,7 +3685,7 @@ futex_lock_to(u32 __user *uaddr, unsigned int flags)
 	 * As the mutex owner, we can now spin on the futex word as well as
 	 * the active-ness of the futex owner.
 	 */
-	ret = futex_spin_on_owner(uaddr, vpid, state);
+	ret = futex_spin_on_owner(uaddr, vpid, state, to ? &timeout : NULL);
 
 	mutex_unlock(&state->mutex);
 
@@ -3684,6 +3702,10 @@ out_put_state_key:
 	put_futex_key(&key);
 
 out:
+	if (to) {
+		hrtimer_cancel(&to->timer);
+		destroy_hrtimer_on_stack(&to->timer);
+	}
 	return ret;
 }
 
@@ -3831,7 +3853,7 @@ long do_futex(u32 __user *uaddr, int op, u32 val, ktime_t *timeout,
 		return futex_requeue(uaddr, flags, uaddr2, val, val2, &val3, 1);
 #ifdef CONFIG_SMP
 	case FUTEX_LOCK_TO:
-		return futex_lock_to(uaddr, flags);
+		return futex_lock_to(uaddr, flags, timeout);
 	case FUTEX_UNLOCK_TO:
 		return futex_unlock_to(uaddr, flags);
 #endif
@@ -3850,7 +3872,7 @@ SYSCALL_DEFINE6(futex, u32 __user *, uaddr, int, op, u32, val,
 	int cmd = op & FUTEX_CMD_MASK;
 
 	if (utime && (cmd == FUTEX_WAIT || cmd == FUTEX_LOCK_PI ||
-		      cmd == FUTEX_WAIT_BITSET ||
+		      cmd == FUTEX_WAIT_BITSET || cmd == FUTEX_LOCK_TO ||
 		      cmd == FUTEX_WAIT_REQUEUE_PI)) {
 		if (unlikely(should_fail_futex(!(op & FUTEX_PRIVATE_FLAG))))
 			return -EFAULT;
-- 
1.7.1