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

[Hitoshi Mitake]

At Tue, 17 Dec 2013 15:45:53 +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.
> 
> Before anyone tries to violet the assumption, we can stop it by telling them not
> to. And reg/unreg is very low level API and wouldn't have many users of it. So
> every calling of them can be strictly checked and no burder for most of future
> patches.

I think we must not be overconfident in our attentiveness. We need a mechanical
method for doing this sort of checking.

If you add a FIXME comment to the patch, I'll agree it. I'll seek a safer way
with my internal improvements of event loop.

Thanks,
Hitoshi