[PATCH V2 1/2] nvme-core: use xarray for ctrl ns tracking

[Keith Busch]

On Wed, Jul 01, 2020 at 03:12:35PM +0200, Christoph Hellwig wrote:
> [willy: a comment/request on the xa_load API below]
> 
> On Tue, Jun 30, 2020 at 07:25:16PM -0700, Chaitanya Kulkarni wrote:
> >  
> > +	rcu_read_lock();
> > +	do {
> > +		ns = xas_load(&xas);
> > +		if (xa_is_zero(ns))
> > +			ns = NULL;
> > +	} while (xas_retry(&xas, ns));
> > +	ns = ns && kref_get_unless_zero(&ns->kref) ? ns : NULL;
> > +	rcu_read_unlock();
> 
> This looks pretty weird, but I think the problem is one in the xarray
> API, as for the typical lookup pattern we'd want an xa_load with
> external RCU locking:
> 
> 	rcu_read_lock();
> 	ns = xa_load_rcu(&ctrl->namespaces, nsid);
> 	if (ns && !kref_get_unless_zero(&ns->kref))
> 		ns = NULL;
> 	rcu_read_unlock();
> 
> instead of duplicating this fairly arcane loop in all kinds of callers.

Is it not safe to use external RCU locking with the existing xa_load()?
The rcu_read_lock() api says nested read-side critical sections are
fine.