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

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Message-ID: <4218f338-f3c8-460d-148b-20019d32b841@amd.com>
Date: Sun, 20 Jul 2025 14:09:00 +0530
From: Abhijit Gangurde <abhijit.gangurde@....com>
To: Jason Gunthorpe <jgg@...pe.ca>, Leon Romanovsky <leon@...nel.org>
Cc: 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 7/16/25 00:46, Jason Gunthorpe wrote:
> On Sun, Jul 13, 2025 at 09:27:53AM +0300, Leon Romanovsky wrote:
>> Let's do what all other drivers do, please. I prefer simplest solution
>> and objects that can potentially be around after verbs objects were
>> cleaned doesn't sound right.
> I think it is OK, at least QP makes sense and matches some other
> drivers.
>
> +static void ionic_qp_event(struct ionic_ibdev *dev, u32 qpid, u8 code)
> +{
> +       struct ib_event ibev;
> +       struct ionic_qp *qp;
> +
> +       rcu_read_lock();
> +       qp = xa_load(&dev->qp_tbl, qpid);
> +       if (qp)
> +               kref_get(&qp->qp_kref);
> +       rcu_read_unlock();
> +
>
> The above is an async event path, and the kref is effectively the open
> coded rwlock pattern we use often.
>
> The unlock triggers a completion:
>
> +       kref_put(&qp->qp_kref, ionic_qp_complete);
> +static inline void ionic_qp_complete(struct kref *kref)
> +{
> +       struct ionic_qp *qp = container_of(kref, struct ionic_qp, qp_kref);
> +
> +       complete(&qp->qp_rel_comp);
> +}
>
> Which acts as the unlock. And then qp destruction:
>
> +int ionic_destroy_qp(struct ib_qp *ibqp, struct ib_udata *udata)
> +{
> +       kref_put(&qp->qp_kref, ionic_qp_complete);
> +       wait_for_completion(&qp->qp_rel_comp);
>
> Which is the typical "write" side of the lock.
>
> So this is all normal, the qp doesn't outlive destroy, destroy waits
> for all the async event deliver to complete. It has to, we free the
> underlying memory in the core code.
>
> As long as the other case are like this it is fine
>
> +       xa_erase_irq(&dev->qp_tbl, qp->qpid);
> +       synchronize_rcu();
>
> This should go away though, don't like to see synchronize_rcu(). The
> idea is you kfree the QP with RCU. But the core code doesn't do that..
>
> So in the short term you should take the lock instead of using rcu:
>
>         xa_lock(&dev->qp_tbl);
>         qp = xa_load(&dev->qp_tbl, qpid);
>         if (qp)
>                 kref_get(&qp->qp_kref);
>
> Jason

Thank you, Jason, for reviewing the logic and explaining how the 
kref/RCU mechanism effectively ensures correct synchronization and clean 
tear down during async event handling and QP destruction. A similar 
mechanism is currently used for CQ event handling and destruction as well.

Your suggestion to avoid synchronize_rcu() and instead take the lock 
directly for xarray lookups makes sense. I will proceed to replace the 
RCU critical section with xa_lock()/xa_load(), as you outlined, to 
better align with current best practices—unless there are any objections.

Thanks again for the valuable feedback!

Abhijit