Messages in this thread | | | Date | Tue, 19 Apr 2016 15:27:37 -0700 | From | Davidlohr Bueso <> | Subject | Re: [PATCH] kernel/futex: handle the case where we got a "late" waiter |
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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
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