[openib-general] userspace event reporting

[Fab Tillier]

> From: Sean Hefty [mailto:sean.hefty at intel.com]
> Sent: Tuesday, July 12, 2005 11:53 AM
> 
> This fell out of the uCM connection ID discussion...
> 
> There's an issue reporting events to userspace clients for an object that a
> user
> may have destroyed.  The problem exists with user verbs, but is much more
> likely
> to be seen by a userspace CM client.  To avoid reporting events for a
> destroyed
> object, I think that something similar to the following could be used from
> userspace:

I had to put something like this in place in the Windows CM, except that I had
to use async destroy semantics to allow preserving the existing CM API.

> destroy() should set a state marking destruction and wait for a reference
> count
> to go to 0 before transitioning to the kernel.  The kernel code should destroy
> the associated kernel object and then discard any unclaimed events.

You don't need a state here - just preset the reference count to 1.  The
sequence should follow something like:

1. Tell kernel to destroy CM ID.
2. Kernel flushes all pending events, completes (with failure) any requests from
a previous call to get_event().
3. deref, and wait for zero.

> get_event() should check the object state and discard the event if the object
> is
> being destroyed.  It should increment a reference count before reporting any
> events.

No need to check for the state.  If the object was destroyed already, the user
has a bug and there's not much you can do about it.  Something like this:
1. inc ref_cnt
2. call kernel to get event.
3. if failure dec ref_cnt.

You need to bump the ref_cnt before making the kernel call to get the event to
prevent a call to destroy() from blowing the CM ID away while an event request
is outstanding (and the status isn't known yet).

> put_event() should decrement the reference count and unblock destroy if the
> reference count goes to 0.

Yep.

- Fab