Messages in this thread | | | Date | Thu, 18 Nov 2004 11:10:50 +0900 | From | Hidetoshi Seto <> | Subject | Re: Futex queue_me/get_user ordering |
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Jakub Jelinek wrote: > On Mon, Nov 15, 2004 at 01:22:18PM +0000, Jamie Lokier wrote: > >> 1. A lost wakeup. >> >> wait_A is woken, but wait_B is not, even though the second >> pthread_cond_signal is "observably" after wait_B. >> >> The operation order is observable in sense that wait_B could >> update the data structure which is protected by cond+mutex, and >> wake_Y could read that update prior to deciding to signal. >> >> _Logically_, what happens is that wait_A is woken by wake_X, but >> it does not immediately re-acquire the mutex. In this time >> window, wait_B and wake_Y both run, and then wait_A acquires the >> mutex. During this window, wait_A is able to absorb the second >> signal. >> >> It's not clear to me if POSIX requires wait_B to be signalled or >> not in this case. >> >> 2. Future lost wakeups. >> >> Future calls to pthread_cond_signal(cond) fail to wake wait_B, >> even much later, because cond's NPTL data structure is >> inconsistent. It's invariant is broken. >> >> This is a bug in NPTL and it's easy to fix. Never increment wake >> unconditionally. Instead, increment it conditionally when (a) >> FUTEX_WAKE returns 1, and also (b) when FUTEX_WAIT returns -EAGAIN. > > > If you think it is fixable in userland, please write at least the pseudo > code that you believe should work. We have spent quite a lot of time > on that code and don't believe this is solvable in userland. > E.g. the futex IMHO must be incremented before FUTEX_WAKE, as otherwise > the woken tasks wouldn't see the effect. > > I believe the only place this is solvable in is the kernel, by ensuring > atomicity (i.e. queuing task iff curval == expected_val operation atomic > wrt. futex_wake/futex_requeue in other tasks).
I agree. I think this is kernel problem.
Even if it is possible to avoid this problem by tricks in userland, I think it is ugly that it could happen that threads having randomness val could be waken. i.g.:
>>>> >> "returns 0 if the futex was not equal to the expected value, but >>>> >> the process was woken by a FUTEX_WAKE call."
Still now, update of manpage is unnecessary, I think.
Thanks, H.Seto
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