On Fri, 15 Apr 2016, Sebastian Andrzej Siewior wrote:
futex_unlock_pi() gets uval before taking the hb lock. Now imagine someone in futex_lock_pi() took the lock. While futex_unlock_pi() waits for the hb lock, the LOCK_PI sets FUTEX_WAITERS and drops the lock. Now, futex_unlock_pi() figures out that there is waiter and invokes wake_futex_pi() with the old uval which does not yet have FUTEX_WAITERS set. This flaw lets cmpxchg_futex_value_locked() fail and return -EINVAL.
Hmm but if we're calling futex_unlock_pi() in the first place, doesn't that indicate that the uval already has FUTEX_WAITERS and therefore failed the TID->0 transition in userland? That or the thread is bogusly unlocking a lock that it doesn't own. This is of course different than the requeue_pi case which can specify set_waiters but also gets the value via get_futex_value_locked(). Is this a real issue or did you find it by code inspection? Thanks, Davidlohr
