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SubjectRe: Futex queue_me/get_user ordering
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.


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