[PATCH -v6 07/13] futex: Rework inconsistent rt_mutex/futex_q state

From: Peter Zijlstra
Date: Wed Mar 22 2017 - 06:45:49 EST


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.

What the current code does in this case is use the futex_q waiter that
got us here; however when the rt_mutex_timed_futex_lock() has already
failed; this leaves things in a weird state, resulting in much
head-aches in fixup_owner().

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 state.

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)

CPU0

T1
lock_pi()
queue_me() <- Waiter is visible

preemption

T2
unlock_pi()
loops with -EAGAIN forever

Which is undesirable for PI primitives. Future patches will rectify
this. For now we want to get rid of the fixup magic.

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
kernel/futex.c | 50 ++++++++++++++------------------------------------
1 file changed, 14 insertions(+), 36 deletions(-)

--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -1404,12 +1404,19 @@ static int wake_futex_pi(u32 __user *uad
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) {
+ raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock);
+ return -EAGAIN;
+ }

/*
* We pass it to the next owner. The WAITERS bit is always
@@ -2332,7 +2339,6 @@ 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) {
@@ -2346,43 +2352,15 @@ static int fixup_owner(u32 __user *uaddr
}

/*
- * 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.
*/
- 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,
q->pi_state->pi_mutex.owner,
q->pi_state->owner);
+ }

out:
return ret ? ret : locked;