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

Hans Henrik Happe
2006-11-23T21:50:08Z

On Thursday 23 November 2006 21:00, Ulrich Drepper wrote:
> Evgeniy Polyakov wrote:
> > uidx is an index, starting from which there are unread entries. It is
> > updated by userspace when it commits entries, so it is 'consumer'
> > pointer, while kidx is an index where kernel will put new entries, i.e.
> > 'producer' index. We definitely need them both.
> > Userspace can only update (implicitly by calling kevent_commit()) uidx.
>
> Right, which is why exporting this entry is not needed. Keep the
> interface as small as possible.
>
> Userlevel has to maintain its own index. Just assume kevent_wait
> returns 10 new entries and you have multiple threads. In this case all
> threads take their turns and pick an entry from the ring buffer. This
> basically has to be done with something like this (I ignore wrap-arounds
> here to simplify the example):
>
> int getidx() {
> while (uidx < kidx)
> if (atomic_cmpxchg(uidx, uidx + 1, uidx) == 0)
> return uidx;
> return -1;
> }

I don't know if this falls under the simplification, but wouldn't there be a
race when reading/copying the event data? I guess this could be solved with
an extra user index.

--

Hans Henrik Happe