Re: [PATCH v4 10/15] drm/shmem-helper: Take reservation lock instead of drm_gem_shmem locks

Daniel Vetter
2022-05-11T15:29:30Z

On Wed, May 11, 2022 at 06:14:00PM +0300, Dmitry Osipenko wrote:
> On 5/11/22 17:24, Christian König wrote:
> > Am 11.05.22 um 15:00 schrieb Daniel Vetter:
> >> On Tue, May 10, 2022 at 04:39:53PM +0300, Dmitry Osipenko wrote:
> >>> [SNIP]
> >>> Since vmapping implies implicit pinning, we can't use a separate lock in
> >>> drm_gem_shmem_vmap() because we need to protect the
> >>> drm_gem_shmem_get_pages(), which is invoked by drm_gem_shmem_vmap() to
> >>> pin the pages and requires the dma_resv_lock to be locked.
> >>>
> >>> Hence the problem is:
> >>>
> >>> 1. If dma-buf importer holds the dma_resv_lock and invokes
> >>> dma_buf_vmap() -> drm_gem_shmem_vmap(), then drm_gem_shmem_vmap() shall
> >>> not take the dma_resv_lock.
> >>>
> >>> 2. Since dma-buf locking convention isn't specified, we can't assume
> >>> that dma-buf importer holds the dma_resv_lock around dma_buf_vmap().
> >>>
> >>> The possible solutions are:
> >>>
> >>> 1. Specify the dma_resv_lock convention for dma-bufs and make all
> >>> drivers to follow it.
> >>>
> >>> 2. Make only DRM drivers to hold dma_resv_lock around dma_buf_vmap().
> >>> Other non-DRM drivers will get the lockdep warning.
> >>>
> >>> 3. Make drm_gem_shmem_vmap() to take the dma_resv_lock and get deadlock
> >>> if dma-buf importer holds the lock.
> >>>
> >>> ...
> >> Yeah this is all very annoying.
> >
> > Ah, yes that topic again :)
> >
> > I think we could relatively easily fix that by just defining and
> > enforcing that the dma_resv_lock must have be taken by the caller when
> > dma_buf_vmap() is called.
> >
> > A two step approach should work:
> > 1. Move the call to dma_resv_lock() into the dma_buf_vmap() function and
> > remove all lock taking from the vmap callback implementations.
> > 2. Move the call to dma_resv_lock() into the callers of dma_buf_vmap()
> > and enforce that the function is called with the lock held.
>
> I've doubts about the need to move out the dma_resv_lock() into the
> callers of dma_buf_vmap()..
>
> I looked through all the dma_buf_vmap() users and neither of them
> interacts with dma_resv_lock() at all, i.e. nobody takes the lock
> in/outside of dma_buf_vmap(). Hence it's easy and more practical to make
> dma_buf_mmap/vmap() to take the dma_resv_lock by themselves.

i915_gem_dmabuf_vmap -> i915_gem_object_pin_map_unlocked ->
i915_gem_object_lock -> dma_resv_lock

And all the ttm drivers should work similarly. So there's definitely
drivers which grab dma_resv_lock from their vmap callback.

> It's unclear to me which driver may ever want to do the mapping under
> the dma_resv_lock. But if we will ever have such a driver that will need
> to map imported buffer under dma_resv_lock, then we could always add the
> dma_buf_vmap_locked() variant of the function. In this case the locking
> rule will sound like this:
>
> "All dma-buf importers are responsible for holding the dma-reservation
> lock around the dmabuf->ops->mmap/vmap() calls."
>
> > It shouldn't be that hard to clean up. The last time I looked into it my
> > main problem was that we didn't had any easy unit test for it.
>
> Do we have any tests for dma-bufs at all? It's unclear to me what you
> are going to test in regards to the reservation locks, could you please
> clarify?

Unfortunately not really :-/ Only way really is to grab a driver which
needs vmap (those are mostly display drivers) on an imported buffer, and
see what happens.

2nd best is liberally sprinkling lockdep annotations all over the place
and throwing it at intel ci (not sure amd ci is accessible to the public)
and then hoping that's good enough. Stuff like might_lock and
dma_resv_assert_held.