Re: [PATCH] rfc: threaded epoll_wait thundering herd

[Davi Arnaut]

Ulrich Drepper wrote:
> On 5/5/07, Davi Arnaut <davi@xxxxxxxxxxxxx> 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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