[sheepdog] [PATCH v2 1/3] sheep: make {register, unregister}_event thread safe

[Hitoshi Mitake]

At Tue, 17 Dec 2013 16:43:08 +0900,
Hitoshi Mitake wrote:
> 
> At Tue, 17 Dec 2013 15:36:45 +0800,
> Liu Yuan wrote:
> > 
> > On Tue, Dec 17, 2013 at 04:26:09PM +0900, Hitoshi Mitake wrote:
> > > At Tue, 17 Dec 2013 15:11:30 +0800,
> > > Liu Yuan wrote:
> > > > 
> > > > On Tue, Dec 17, 2013 at 03:58:07PM +0900, Hitoshi Mitake wrote:
> > > > > At Tue, 17 Dec 2013 14:50:19 +0800,
> > > > > Liu Yuan wrote:
> > > > > > 
> > > > > > On Tue, Dec 17, 2013 at 03:42:29PM +0900, Hitoshi Mitake wrote:
> > > > > > > At Tue, 17 Dec 2013 14:31:56 +0800,
> > > > > > > Liu Yuan wrote:
> > > > > > > > 
> > > > > > > > This allow us to call even handling functions in worker thread
> > > > > > > > 
> > > > > > > > Signed-off-by: Liu Yuan <namei.unix at gmail.com>
> > > > > > > > ---
> > > > > > > 
> > > > > > > I think this change is dangerous. This permits worker threads to
> > > > > > > unregister events even if these events are processed in the main
> > > > > > > thread. Making a new work queue and delegate it to register/unregister
> > > > > > > events would be safer.
> > > > > > 
> > > > > > This scheme is not pratical for async request.
> > > > > > 
> > > > > > This is just internal API, which are supported to be called by programmers
> > > > > > and check it is correct.
> > > > > 
> > > > > The checking will cost us lots of time. The problem is a race
> > > > > condition caused by multiple threads.
> > > > 
> > > > Why we would face this kind of problem? we can make sure at any time, there will
> > > > be a single thread manipulate it, be it worker or main thread.
> > > > 
> > > > I think we are talking about different issues, you are supposed that events
> > > > handling will be generically thread-safe for multiple threads. But this wouldn't
> > > > happen I think. Instead, assumption 'one event will be only be manipulated by
> > > > single entity (thus no multiple threads case)' will hold true in the long run.
> > > 
> > > As you say, your patch doesn't violate the above condition (one event will be
> > > only be manipulated by single entity). But we cannot express that
> > > register_event() and unregister_event() are thread safe in the above special
> > > condition. So users (we programmers) would use in an invalid way in the future
> > > and debugging will be hard.
> > 
> > How about following commit log:
> > 
> >     sheep: allow {register,unregister}_event to be called in worker thread
> >     
> >     For now we can only call them in the main thread, which is designed for long
> >     running or infrequent events. This would be inefficient if we want to deal with
> >     short running and frequent events that register/unregister the events in the
> >     worker thread
> >     
> >     1. avoid to be trapped to main thread for performance
> >     2. make sure registeration is done before some other events.
> >     
> >     This doesn't mean we can manipulate the same event with multiple threads
> >     simultaneously, instead we still adhere to the assumpioin that
> >     
> >     - one event will only be manipluated by a single entity.
> 
> We will never read all commit messages before using some functions. The
> description should be comments in source tree.

Let's wrap it up. If you add a summary of the above message to the source tree
as a comment, I'll ack the patch set.

Thanks,
Hitoshi