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

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Message-ID: <20250704170807.GO6278@unreal>
Date: Fri, 4 Jul 2025 20:08:07 +0300
From: Leon Romanovsky <leon@...nel.org>
To: Abhijit Gangurde <abhijit.gangurde@....com>
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 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