netdev - Re: [PATCH v3 10/14] RDMA/ionic: Register device ops for control path

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Message-ID: <1a7190d4-f3ef-744c-4e46-8cb255dee6cf@amd.com>
Date: Mon, 7 Jul 2025 20:26:20 +0530
From: Abhijit Gangurde <abhijit.gangurde@....com>
To: Leon Romanovsky <leon@...nel.org>
Cc: Jason Gunthorpe <jgg@...pe.ca>, shannon.nelson@....com,
 brett.creeley@....com, davem@...emloft.net, edumazet@...gle.com,
 kuba@...nel.org, pabeni@...hat.com, corbet@....net, andrew+netdev@...n.ch,
 allen.hubbe@....com, nikhil.agarwal@....com, linux-rdma@...r.kernel.org,
 netdev@...r.kernel.org, linux-doc@...r.kernel.org,
 linux-kernel@...r.kernel.org, Andrew Boyer <andrew.boyer@....com>
Subject: Re: [PATCH v3 10/14] RDMA/ionic: Register device ops for control path


On 7/7/25 12:51, Leon Romanovsky wrote:
> On Mon, Jul 07, 2025 at 10:57:13AM +0530, Abhijit Gangurde wrote:
>> On 7/4/25 22:38, Leon Romanovsky wrote:
>>> On Thu, Jul 03, 2025 at 12:49:30PM +0530, Abhijit Gangurde wrote:
>>>> On 7/2/25 23:30, Leon Romanovsky wrote:
>>>>> On Wed, Jul 02, 2025 at 10:18:03AM -0300, Jason Gunthorpe wrote:
>>>>>> On Tue, Jul 01, 2025 at 01:38:44PM +0300, Leon Romanovsky wrote:
>>>>>>>> +static void ionic_flush_qs(struct ionic_ibdev *dev)
>>>>>>>> +{
>>>>>>>> +	struct ionic_qp *qp, *qp_tmp;
>>>>>>>> +	struct ionic_cq *cq, *cq_tmp;
>>>>>>>> +	LIST_HEAD(flush_list);
>>>>>>>> +	unsigned long index;
>>>>>>>> +
>>>>>>>> +	/* Flush qp send and recv */
>>>>>>>> +	rcu_read_lock();
>>>>>>>> +	xa_for_each(&dev->qp_tbl, index, qp) {
>>>>>>>> +		kref_get(&qp->qp_kref);
>>>>>>>> +		list_add_tail(&qp->ibkill_flush_ent, &flush_list);
>>>>>>>> +	}
>>>>>>>> +	rcu_read_unlock();
>>>>>>> Same question as for CQ. What does RCU lock protect here?
>>>>>> It should protect the kref_get against free of qp. The qp memory must
>>>>>> be RCU freed.
>>>>> I'm not sure that this was intension here. Let's wait for an answer from the author.
>>>> As Jason mentioned, It was intended to protect the kref_get against free of
>>>> cq and qp
>>>> in the destroy path.
>>> How is it possible? IB/core is supposed to protect from accessing verbs
>>> resources post their release/destroy.
>>>
>>> After you answered what RCU is protecting, I don't see why you would
>>> have custom kref over QP/CQ/e.t.c objects.
>>>
>>> Thanks
>> The RCU protected kref here is making sure that all the hw events are
>> processed before destroy callback returns. Similarly, when driver is
>> going for ib_unregister_device, it is draining the pending WRs and events.
> I asked why do you have kref in first place? When ib_unregister_device
> is called all "pending MR" already supposed to be destroyed.
>
> Thansk

The custom kref on QP/CQ object is holding the completion for the 
destroy callback.
If any pending async hw events are being processed, destroy would wait 
on this completion
before it returns.

Thanks