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[PATCH -v4 09/10] futex: Remove inconsistent hb/rt_mutex state magic

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  [PATCH -v4 09/10] futex: Remove inconsistent hb/rt_mutex state magic Peter Zijlstra <peterz@infradead.org> - 2016-12-13 10:10 +0100

#1540911 — [PATCH -v4 09/10] futex: Remove inconsistent hb/rt_mutex state magic

Now that we guarantee hb queue and rt_mutex waiter state match up, we
no longer need to deal with the fallout of when they don't.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 kernel/futex.c |   38 +-------------------------------------
 1 file changed, 1 insertion(+), 37 deletions(-)

--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -1396,13 +1396,7 @@ static int wake_futex_pi(u32 __user *uad
 	raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock);
 	new_owner = rt_mutex_next_owner(&pi_state->pi_mutex);
 
-	/*
-	 * It is possible that the next waiter (the one that brought
-	 * top_waiter owner to the kernel) timed out and is no longer
-	 * waiting on the lock.
-	 */
-	if (!new_owner)
-		new_owner = top_waiter->task;
+	BUG_ON(!new_owner);
 
 	/*
 	 * We pass it to the next owner. The WAITERS bit is always
@@ -2324,7 +2318,6 @@ static long futex_wait_restart(struct re
  */
 static int fixup_owner(u32 __user *uaddr, struct futex_q *q, int locked)
 {
-	struct task_struct *owner;
 	int ret = 0;
 
 	if (locked) {
@@ -2337,35 +2330,6 @@ static int fixup_owner(u32 __user *uaddr
 		goto out;
 	}
 
-	/*
-	 * Catch the rare case, where the lock was released when we were on the
-	 * way back before we locked the hash bucket.
-	 */
-	if (q->pi_state->owner == current) {
-		/*
-		 * Try to get the rt_mutex now. This might fail as some other
-		 * task acquired the rt_mutex after we removed ourself from the
-		 * rt_mutex waiters list.
-		 */
-		if (rt_mutex_futex_trylock(&q->pi_state->pi_mutex)) {
-			locked = 1;
-			goto out;
-		}
-
-		/*
-		 * pi_state is incorrect, some other task did a lock steal and
-		 * we returned due to timeout or signal without taking the
-		 * rt_mutex. Too late.
-		 */
-		raw_spin_lock_irq(&q->pi_state->pi_mutex.wait_lock);
-		owner = rt_mutex_owner(&q->pi_state->pi_mutex);
-		if (!owner)
-			owner = rt_mutex_next_owner(&q->pi_state->pi_mutex);
-		raw_spin_unlock_irq(&q->pi_state->pi_mutex.wait_lock);
-		ret = fixup_pi_state_owner(uaddr, q, owner);
-		goto out;
-	}
-
 	/*
 	 * Paranoia check. If we did not take the lock, then we should not be
 	 * the owner of the rt_mutex.

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