Re: [PATCH 2/4] ib_core: implement XRC RCV qp's

Roland Dreier
2010-05-05T22:56:46Z

> The uverbs layer DOES handle the counting/listing (see, for example,
> list_add_tail at the end of ib_uverbs_create_xrc_rcv_qp.

But that is the per-process list of rcv QPs... not the reference
counting for each rcv QP. Look at how trivial
ib_uverbs_destroy_xrc_rcv_qp is -- there is no reference counting of the
real object there.

> However, I had an additional problem -- to distribute async events received
> for the xrc_rcv QP to all registered processes (so that each could unregister
> and allow the QP to be destroyed -- the ref count going to zero).

> In my original implementation, the low-level driver was responsible for generating
> the events for all the processes. To move this mechanism to the core would require
> a fairly extensive re-write. I would need to introduce ib_core methods for create,
> reg, unreg, and destroy, since the uverbs layer operates per user process and does
> not track across multiple processes. I was concerned that modifications of this
> magnitude would introduce instability in XRC, and would require a significant QA cycle

It seems to me that it should, if anything, be easier to track all these
lists at the uverbs layer. We already have a global xrcd structure for
each xrcd that we could easily stick a list of rcv QPs in (and keep the
list of rcv QP registrations per rcv QP).

I do see your point that the changes might be fairly large (although the
total XRC code is not so big), but we shouldn't stick ourselves with a
wrong implementation just because that implementation was already tested.

> Finally, I do not believe that it is such a bad thing to have low-level driver
> procedures for reg/unreg here. If a given low-level driver has implementation
> details that it wishes to record, it should have the opportunity to do so.

Hmm. I'm having a hard time imagining what those details are -- I
really do feel that this tracking of per-process reference counting etc
is something that we should do once in the core, rather than hiding it
in low-level drivers.

- R.
--
Roland Dreier <rolandd-FYB4Gu1CFyUAvxtiuMwx3w@public.gmane.org> || For corporate legal information go to: