Re: [PATCH 0/4] Allow MMIO regions to be exported through dma-buf

[Jason Gunthorpe <jgg@nvidia.com>]

On Fri, Aug 19, 2022 at 03:33:04PM +0200, Christian König wrote:

> > > > So we could delete the try_buf and just rely on move being safe on
> > > > partially destroyed dma_buf's as part of the API design.
> > > I think that might be the more defensive approach. A comment on the
> > > dma_buf_move_notify() function should probably be a good idea.
> > IMHO, it is an anti-pattern. The caller should hold a strong reference
> > on an object before invoking any API surface. Upgrading a weak
> > reference to a strong reference requires the standard "try get" API.
> > 
> > But if you feel strongly I don't mind dropping the try_get around move.
> 
> Well I see it as well that both approaches are not ideal, but my gut feeling
> tells me that just documenting that dma_buf_move_notify() can still be
> called as long as the release callback wasn't called yet is probably the
> better approach.

The comment would say something like:

 "dma_resv_lock(), dma_buf_move_notify(), dma_resv_unlock() may be
  called with a 0 refcount so long as ops->release() hasn't returned"

Which is a really abnormal API design, IMHO.

Jason