Re: [PATCH v8 2/4] iommufd: Add iommufd_access_replace() API

[Jason Gunthorpe]

On Wed, Jul 26, 2023 at 01:50:28PM -0700, Nicolin Chen wrote:
> 
> > >  	rc = iopt_add_access(&new_ioas->iopt, access);
> > >  	if (rc) {
> > > -		mutex_unlock(&access->ioas_lock);
> > >  		iommufd_put_object(&new_ioas->obj);
> > > +		if (cur_ioas)
> > > +			WARN_ON(iommufd_access_change_pt(access,
> > > +							 cur_ioas->obj.id));
> > 
> > We've already dropped our ref to cur_ioas, so this is also racy with
> > destroy.
> 
> Would it be better by calling iommufd_access_detach() that holds
> the same mutex in the iommufd_access_destroy_object()? We could
> also unwrap the detach and delay the refcount_dec, as you did in
> your attaching patch.

It is better just to integrate it with this algorithm so we don't have
the refcounting issues, like I did


> 
> > This is what I came up with:
> > 
> > diff --git a/drivers/iommu/iommufd/device.c b/drivers/iommu/iommufd/device.c
> > index 57c0e81f5073b2..e55d6e902edb98 100644
> > --- a/drivers/iommu/iommufd/device.c
> > +++ b/drivers/iommu/iommufd/device.c
> > @@ -758,64 +758,101 @@ void iommufd_access_destroy(struct iommufd_access *access)
> >  }
> >  EXPORT_SYMBOL_NS_GPL(iommufd_access_destroy, IOMMUFD);
> >  
> > -void iommufd_access_detach(struct iommufd_access *access)
> > +static int iommufd_access_change_ioas(struct iommufd_access *access,
> > +				      struct iommufd_ioas *new_ioas)
> >  {
> >  	struct iommufd_ioas *cur_ioas = access->ioas;
> > +	int rc;
> > +
> > +	lockdep_assert_held(&access->ioas_lock);
> > +
> > +	/* We are racing with a concurrent detach, bail */
> > +	if (access->ioas_unpin)
> > +		return -EBUSY;
> 
> I think this should check access->ioas too? I mean:

> 
> +	/* We are racing with a concurrent detach, bail */
> +	if (!access->ioas && access->ioas_unpin)
> +		return -EBUSY;

Oh, yes, that should basically be 'cur_ioas != access->ioas_unpin' -
ie any difference means we are racing with the unmap call.

> > +	if (new_ioas) {
> > +		rc = iopt_add_access(&new_ioas->iopt, access);
> > +		if (rc) {
> > +			iommufd_put_object(&new_ioas->obj);
> > +			access->ioas = cur_ioas;
> > +			return rc;
> > +		}
> > +		iommufd_ref_to_users(&new_ioas->obj);
> > +	}
> > +
> > +	access->ioas = new_ioas;
> > +	access->ioas_unpin = new_ioas;
> >  	iopt_remove_access(&cur_ioas->iopt, access);
> 
> There was a bug in my earlier version, having the same flow by
> calling iopt_add_access() prior to iopt_remove_access(). But,
> doing that would override the access->iopt_access_list_id and
> it would then get unset by the following iopt_remove_access().

Ah, I was wondering about that order but didn't check it.

Maybe we just need to pass the ID into iopt_remove_access and keep the
right version on the stack.

> So, I came up with this version calling an iopt_remove_access()
> prior to iopt_add_access(), which requires an add-back the old
> ioas upon an failure at iopt_add_access(new_ioas).

That is also sort of reasonable if the refcounting is organized like
this does.

Jason