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

[Hitoshi Mitake]

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.

I cannot agree with your solution, but if you are in a hurry, I'll agree with it
as a temporal solution. But I'll replace it with more safer way in the near
future.

Thanks,
Hitoshi