Ulrich Drepper wrote:
> On 5/5/07, Davi Arnaut <[email protected]> wrote:
>> A google search turns up a few users. It also addresses some complaints
>> from Drepper.
>
> There is a huge problem with this approach and we're back at the
> inadequate interface.
>
> select/poll/epoll are thread cancellation points. I.e., the thread
> can be canceled before returning to the user. If this cancellation
> happens between the kernel deciding to give this thread the event (and
> no other thread) and the thread testing for cancellation in the libc
> wrapper around the syscall, then the event is lost and the process(es)
> might hang.
>
> With kevent we in the end fixed the problem by requiring that part of
> the cancellation handling the thread tries to wake up another thread
> waiting for the event queue. This is easily possible since the event
> data is in the shared memory segment and it's just purely the thread
> wakeup that is needed.
>
> To make something like this work for poll you'd have to push back the
> revents fields of the result back to the kernel which might then cause
> another thread to be woken up. I find this too ugly to consider. You
> guys will not believe this but I really thought all these things
> through before writing the OLS paper. poll cannot be salvaged.
See Linus's message on this same thread.
> There is another thing about this selective wakeup: do I assume it
> correctly that if more than one file descriptor is reported ready more
> than one thread is woken? I think nothing else can be justified.
Correct.
> Will in this case both threads get the same set of descriptors
> reported or will they see disjunct sets?
Disjunct. In reality, only if the event is edge triggered.
--
Davi Arnaut
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