netdev - Re: [PATCH v3 for-next 05/13] IB/cm: Reference count ib_cm

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Date:	Wed, 13 May 2015 13:20:22 +0300
From:	Haggai Eran <haggaie@...lanox.com>
To:	Jason Gunthorpe <jgunthorpe@...idianresearch.com>
CC:	Doug Ledford <dledford@...hat.com>, <linux-rdma@...r.kernel.org>,
	<netdev@...r.kernel.org>, Liran Liss <liranl@...lanox.com>,
	Guy Shapiro <guysh@...lanox.com>,
	Shachar Raindel <raindel@...lanox.com>,
	Yotam Kenneth <yotamke@...lanox.com>
Subject: Re: [PATCH v3 for-next 05/13] IB/cm: Reference count ib_cm_ids

On 12/05/2015 21:54, Jason Gunthorpe wrote:
> On Tue, May 12, 2015 at 09:50:51AM +0300, Haggai Eran wrote:
>> Looking at the code though, I now notice that the other call site to
>> cm_destroy_id, from within the error path of cm_process_work could now
>> theoretically destroy an ID with existing references. Is that what you
>> meant?
> 
> No, but that is certainly a problem.
>  
>> Since only listening CM IDs are now shared in RDMA CM, this should not
>> happen in this patch-set, but perhaps the code can be changed to
>> make
> 
> I think you need to enforce those semantics..
> 
> Firstly, it looks to me like we, again, have two krefs, the one you
> added and the 'ref_count'  in the priv structure which is 99% of a
> kref. So, again, don't do that.
> 
> If you want to share listening CM IDs, then do exactly and only
> that. Use the existing ref count scheme for keeping track of the
> kfree/etc, 
The existing reference count scheme is for synchronization in
cm_destroy_id. The function waits for active handlers to complete before
destroying the id. Decrementing ref_count to zero doesn't cause the id
to be freed.

> and add some kind of sharable listen ref count.
That's basically what the patch does. I can change it from a kref to a
direct reference count if you prefer.

> Early exit
> from cm_destroy_id when the there are still people listening.
> 
> That sounds like it keeps the basic rule of cm_destroy_id being
> properly paired with the alloc, and allows listen sharing without the
> confusion of what does multiple destroy mean.

Won't you find it confusing if a call to ib_destroy_cm_id is successful
but the id actually continues to live? I prefer the API to explicitly
show that you are only decreasing the reference count and that the id
might not be destroyed immediately.

Regards,
Haggai
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