Subject: Re: [PATCH net 3/3] netdevsim: psp: rcu protect psp_dev reference

From: Willem de Bruijn <[email-masked]>
Date: Wed, 06 May 2026 16:43:28 -0400

Daniel Zahka wrote:
> 
> On 5/6/26 3:34 PM, Willem de Bruijn wrote:
> > Daniel Zahka wrote
> >>   static ssize_t
> >> @@ -228,16 +237,23 @@ nsim_psp_rereg_write(struct file *file, const char __user *data, size_t count,
> >>   		     loff_t *ppos)
> >>   {
> >>   	struct netdevsim *ns = file->private_data;
> >> -	int err;
> >> +	struct psp_dev *psd;
> >> +	ssize_t ret;
> >>   
> >>   	mutex_lock(&ns->psp.rereg_lock);
> >> -	__nsim_psp_uninit(ns);
> >> +	__nsim_psp_uninit(ns, false);
> >> +
> >> +	psd = psp_dev_create(ns->netdev, &nsim_psp_ops, &nsim_psp_caps, ns);
> >> +	if (IS_ERR(psd)) {
> >> +		ret = PTR_ERR(psd);
> >> +		goto out;
> >> +	}
> > Do you want to create the new device first and only delete the old
> > state if that succeeds? To avoid a netdevsim in state without dev.
> >
> >>   
> >> -	ns->psp.dev = psp_dev_create(ns->netdev, &nsim_psp_ops,
> >> -				     &nsim_psp_caps, ns);
> >> -	err = PTR_ERR_OR_ZERO(ns->psp.dev);
> >> +	rcu_assign_pointer(ns->psp.dev, psd);
> >> +	ret = count;
> >> +out:
> >>   	mutex_unlock(&ns->psp.rereg_lock);
> >> -	return err ?: count;
> >> +	return ret;
> >>   }
> >>   
> 
> Unfortunately, the way we have psp_dev_unregister() written, it would 
> clear out the main_netdev->psp_dev field from the second 
> psp_dev_create() call. I don't believe there is use case to do this in a 
> real driver, so I'm not sure we need to change how the create/unregister 
> paths work.

Thanks for that context.

Reviewed-by: Willem de Bruijn <[email-masked]>