Re: [PATCH] iommufd: take dma_resv lock before dma_buf_unpin() in release path

[Ankit Soni]

On Tue, May 26, 2026 at 09:26:56AM -0300, Jason Gunthorpe wrote:
> On Tue, May 26, 2026 at 11:10:34AM +0000, Ankit Soni wrote:
> > dma_buf_unpin() requires the caller to hold the exporter's dma_resv
> > lock:
> > 
> >   void dma_buf_unpin(struct dma_buf_attachment *attach)
> >   {
> >           ...
> >           dma_resv_assert_held(dmabuf->resv);
> >           ...
> >   }
> > 
> > iopt_release_pages() calls dma_buf_unpin() without taking that lock,
> > so every iommufd_ioas_destroy()/iommufd_ioas_unmap() that releases
> > the last reference on a DMABUF-backed iopt_pages triggers a WARN.
> > This was hit while running tools/testing/selftests/iommu/iommufd:
> 
> Any idea why this is comming up now? Did I run the tests without some
> kind of debug option to turn on that assertion maybe?
> 
> Jason

The assertion is gated by CONFIG_LOCKDEP. My config has it on via
CONFIG_DEBUG_LOCK_ALLOC=y (LOCK_STAT, PROVE_LOCKING and
DEBUG_WW_MUTEX_SLOWPATH each select-chain to LOCKDEP as well).

-Ankit