On Mon, 7 May 2007, 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.
So, by the same logic, every API that 1) returns something to userspace
by canceling its internal kernel state 2) is not based on shared
kernel/userspace memory, will break under your assumptions.
Scary, because there's a pretty long list.
- Davide
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