Messages in this thread | | | Date | Wed, 20 Apr 2016 09:36:42 +0200 (CEST) | From | Thomas Gleixner <> | Subject | Re: [PATCH] kernel/futex: handle the case where we got a "late" waiter |
| |
On Tue, 19 Apr 2016, Davidlohr Bueso wrote: > 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.
It can be called unconditionally w/o trying the TID->0 transition in user space first and we should handle that case.
Thanks,
tglx
| |