PI futexes + lock stealing woes

Julia Cartwright
2017-11-29T17:57:01Z

Hey Thomas, Peter-

Gratian and I have been debugging into a nasty and difficult race w/
futexes seemingly the culprit. The original symptom we were seeing
was a seemingly spurious -EDEADLK from a futex(LOCK_PI) operation.

On further analysis, however, it appears the thread which gets the
spurious -EDEADLK has observed a weird futex state: a prior
futex(WAIT_REQUEUE_PI) operation has returned -ETIMEDOUT, but the uaddr2
futex word owner field indicates that it's the owner.

Here's an attempt to boil down this situation into a pseudo trace; I'm
happy to forward along the full traces as well, if that would be
helpful:

waiter waker stealer (prio > waiter)

futex(WAIT_REQUEUE_PI, uaddr, uaddr2,
timeout=[N ms])
futex_wait_requeue_pi()
futex_wait_queue_me()
freezable_schedule()
<scheduled out>
futex(LOCK_PI, uaddr2)
futex(CMP_REQUEUE_PI, uaddr,
uaddr2, 1, 0)
/* requeues waiter to uaddr2 */
futex(UNLOCK_PI, uaddr2)
wake_futex_pi()
cmp_futex_value_locked(uaddr, waiter)
wake_up_q()
<woken by waker>
<hrtimer_wakeup() fires,
clears sleeper->task>
futex(LOCK_PI, uaddr2)
__rt_mutex_start_proxy_lock()
try_to_take_rt_mutex() /* steals lock */
rt_mutex_set_owner(lock, stealer)
<preempted>
<scheduled in>
rt_mutex_wait_proxy_lock()
__rt_mutex_slowlock()
try_to_take_rt_mutex() /* fails, lock held by stealer */
if (timeout && !timeout->task)
return -ETIMEDOUT;
fixup_owner()
/* lock wasn't acquired, so,
fixup_pi_state_owner skipped */
return -ETIMEDOUT;

/* At this point, we've returned -ETIMEDOUT to userspace, but the
* futex word shows waiter to be the owner, and the pi_mutex has
* stealer as the owner */

futex_lock(LOCK_PI, uaddr2)
-> bails with EDEADLK, futex word says we're owner.

At some later point in execution, the stealer gets scheduled back in and
will do fixup_owner() which fixes up the futex word, but at that point
it's too late: the waiter has already observed the wonky state.

fixup_owner() used to have additional seemingly relevant checks in place
that were removed 73d786bd043eb ("futex: Rework inconsistent
rt_mutex/futex_q state").

The actual kernel we've been testing is 4.9.33-rt23, w/ 153fbd1226fb3
("futex: Fix more put_pi_state() vs. exit_pi_state_list() races")
cherry-picked w/ PREEMPT_RT_FULL. However, it appears that this issue
may affect v4.15-rc1?

Thoughts on how to move forward?

Nasty.

Julia