Commit 73d786bd authored by Peter Zijlstra's avatar Peter Zijlstra Committed by Thomas Gleixner

futex: Rework inconsistent rt_mutex/futex_q state

There is a weird state in the futex_unlock_pi() path when it interleaves
with a concurrent futex_lock_pi() at the point where it drops hb->lock.

In this case, it can happen that the rt_mutex wait_list and the futex_q
disagree on pending waiters, in particular rt_mutex will find no pending
waiters where futex_q thinks there are. In this case the rt_mutex unlock
code cannot assign an owner.

The futex side fixup code has to cleanup the inconsistencies with quite a
bunch of interesting corner cases.

Simplify all this by changing wake_futex_pi() to return -EAGAIN when this
situation occurs. This then gives the futex_lock_pi() code the opportunity
to continue and the retried futex_unlock_pi() will now observe a coherent

The only problem is that this breaks RT timeliness guarantees. That
is, consider the following scenario:

  T1 and T2 are both pinned to CPU0. prio(T2) > prio(T1)


      queue_me()  <- Waiter is visible


	loops with -EAGAIN forever

Which is undesirable for PI primitives. Future patches will rectify
Signed-off-by: default avatarPeter Zijlstra (Intel) <>
Link: default avatarThomas Gleixner <>
parent bf92cf3a
......@@ -1404,12 +1404,19 @@ static int wake_futex_pi(u32 __user *uaddr, u32 uval, struct futex_q *top_waiter
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.
* When we interleave with futex_lock_pi() where it does
* rt_mutex_timed_futex_lock(), we might observe @this futex_q waiter,
* but the rt_mutex's wait_list can be empty (either still, or again,
* depending on which side we land).
* When this happens, give up our locks and try again, giving the
* futex_lock_pi() instance time to complete, either by waiting on the
* rtmutex or removing itself from the futex queue.
if (!new_owner)
new_owner = top_waiter->task;
if (!new_owner) {
return -EAGAIN;
* We pass it to the next owner. The WAITERS bit is always
......@@ -2332,7 +2339,6 @@ static long futex_wait_restart(struct restart_block *restart);
static int fixup_owner(u32 __user *uaddr, struct futex_q *q, int locked)
struct task_struct *owner;
int ret = 0;
if (locked) {
......@@ -2345,44 +2351,16 @@ static int fixup_owner(u32 __user *uaddr, struct futex_q *q, int locked)
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.
owner = rt_mutex_owner(&q->pi_state->pi_mutex);
if (!owner)
owner = rt_mutex_next_owner(&q->pi_state->pi_mutex);
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.
if (rt_mutex_owner(&q->pi_state->pi_mutex) == current)
if (rt_mutex_owner(&q->pi_state->pi_mutex) == current) {
printk(KERN_ERR "fixup_owner: ret = %d pi-mutex: %p "
"pi-state %p\n", ret,
return ret ? ret : locked;
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