Re: [take25 1/6] kevent: Description.

Evgeniy Polyakov
2006-11-24T16:14:53Z

On Fri, Nov 24, 2006 at 08:06:59AM -0800, Ulrich Drepper (drepper@redhat.com) wrote:
> Evgeniy Polyakov wrote:
> >>I know this is how it's done now. But it is not where it has to end.
> >>IMO we have to get to a solution where new events are posted to the ring
> >>buffer asynchronously, i.e., without a thread calling kevent_wait. And
> >>then you need the extra parameter and verification. Even if it's today
> >>not needed we have to future-proof the interface since it cannot be
> >>changed once in use.
> >
> >There is a special flag in kevent_user to wake it if there are no ready
> >events - kernel thread which has added new events will set it and thus
> >subsequent kevent_wait() will return with updated indexes - userspace
> >must check indexes after kevent_wait().
>
> You misunderstand. I don't want to return without waiting unconditionally.
>
> There is a race which has to be closed. It's exactly the same as in the
> futex syscall. I've shown the interaction between the kernel and the
> thread in the previous mail. There is inevitably a time difference
> between the thread checking whether the ring buffer is empty and the
> kernel putting the thread to sleep in the kevent_wait call.
>
> This is no problem with the current kevent_wait implementation since the
> ring buffer is not filled asynchronously. But if/when it will be the
> kernel might add something to the ring buffer _after_ the thread checks
> for an empty ring buffer and _before_ it enters the kernel in the
> kevent_wait syscall.
>
> The kevent_wait syscall will only wake the thread when a new event is
> posted. We do not in general want it to be woken when the ring buffer
> is non empty. This would create far too many unnecessary wakeups it
> there is more than one thread working on the queue.
>
> With the addition parameters for kevent_wait indicating when the calling
> thread last checked the ring buffer the kernel can find out whether the
> decision to call kevent_wait was made based on outdated information or
> not. Outdated in the case a new event has been posted. In this case
> the thread is not put to sleep but instead returns.

Read my mail again.

If kernel has put data asynchronously it will setup special flag, thus
kevent_wait() will not sleep and will return, so thread will check new
entries and process them.

> --
> ➧ Ulrich Drepper ➧ Red Hat, Inc. ➧ 444 Castro St ➧ Mountain View,
> CA ❖

--
Evgeniy Polyakov