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

Evgeniy Polyakov
2006-11-24T12:14:05Z

On Fri, Nov 24, 2006 at 03:05:31PM +0300, Evgeniy Polyakov (johnpol@2ka.mipt.ru) wrote:
> On Thu, Nov 23, 2006 at 02:33:16PM -0800, Ulrich Drepper (drepper@redhat.com) wrote:
> > Evgeniy Polyakov wrote:
> > >+ int kevent_commit(int ctl_fd, unsigned int start,
> > >+ unsigned int num, unsigned int over);
> >
> > I think we can simplify this interface:
> >
> > int kevent_commit(int ctl_fd, unsigned int new_tail,
> > unsigned int over);
> >
> > The kernel sets the ring_uidx value to the 'new_tail' value if the tail
> > pointer would be incremented (module wrap around) and is not higher then
> > the current front pointer. The test will be a bit complicated but not
> > more so than what the current code has to do to check for mistakes.
> >
> > This approach has the advantage that the commit calls don't have to be
> > synchronized. If one thread sets the tail pointer to, say, 10 and
> > another to 12, then it does not matter whether the first thread is
> > delayed. If it will eventually be executed the result is simply a no-op
> > and since second thread's action supersedes it.
> >
> > Maybe the current form is even impossible to use with explicit locking
> > at userlevel. What if one thread, which is about to call kevent_commit,
> > if indefinitely delayed. Then this commit request's value is never
> > taken into account and the tail pointer is always short of what it
> > should be.
>
> I like this interface, although current one does not allow special

...does not require...

> synchronization in userspace, since it calculates if new commit is in
> the area where previous commit was.
> Will change for the next release.

--
Evgeniy Polyakov