Re: [PATCH 09/14] iommufd: Add iommufd_device_replace()

[Jason Gunthorpe <jgg@nvidia.com>]

On Tue, Feb 28, 2023 at 09:50:52AM +0800, Baolu Lu wrote:
> On 2/27/23 9:58 PM, Jason Gunthorpe wrote:
> > On Sun, Feb 26, 2023 at 11:01:59AM +0800, Baolu Lu wrote:
> > > On 2/25/23 8:27 AM, Jason Gunthorpe wrote:
> > > > @@ -437,25 +517,77 @@ int iommufd_device_attach(struct iommufd_device *idev, u32 *pt_id)
> > > >    		struct iommufd_ioas *ioas =
> > > >    			container_of(pt_obj, struct iommufd_ioas, obj);
> > > > -		rc = iommufd_device_auto_get_domain(idev, ioas, pt_id);
> > > > -		if (rc)
> > > > +		destroy_hwpt = iommufd_device_auto_get_domain(idev, ioas, pt_id,
> > > > +							      do_attach);
> > > > +		if (IS_ERR(destroy_hwpt))
> > > >    			goto out_put_pt_obj;
> > > >    		break;
> > > >    	}
> > > >    	default:
> > > > -		rc = -EINVAL;
> > > > +		destroy_hwpt = ERR_PTR(-EINVAL);
> > > >    		goto out_put_pt_obj;
> > > >    	}
> > > > +	iommufd_put_object(pt_obj);
> > > > -	refcount_inc(&idev->obj.users);
> > > > -	rc = 0;
> > > > +	/* This destruction has to be after we unlock everything */
> > > > +	if (destroy_hwpt)
> > > Should this be
> > > 
> > > 	if (!IS_ERR_OR_NULL(destroy_hwpt))
> > > 
> > > ?
> > Never use IS_ERR_OR_NULL ..
> 
> Can you please elaborate a bit on this? I can still see a lot of use of
> it in the tree.

Yes, sadly. It is usually some signal of toxic confusion about what
things mean.

A function that returns an ERR_PTR should very rarely return NULL, and
if it does return NULL then NULL wasn't an error.

Further you should never store an ERR_PTR in some structure and then
later try to test it, that is madness.

So with properly structured code the need should not exist.

https://lore.kernel.org/all/20130109150427.GL3931@n2100.arm.linux.org.uk/

Jason