dma-resv: Give the docs a do-over

Specifically document the new/clarified rules around how the shared
fences do not have any ordering requirements against the exclusive
fence.

But also document all the things a bit better, given how central
struct dma_resv to dynamic buffer management the docs have been very
inadequat.

- Lots more links to other pieces of the puzzle. Unfortunately
  ttm_buffer_object has no docs, so no links :-(

- Explain/complain a bit about dma_resv_locking_ctx(). I still don't
  like that one, but fixing the ttm call chains is going to be
  horrible. Plus we want to plug in real slowpath locking when we do
  that anyway.

- Main part of the patch is some actual docs for struct dma_resv.

Overall I think we still have a lot of bad naming in this area (e.g.
dma_resv.fence is singular, but contains the multiple shared fences),
but I think that's more indicative of how the semantics and rules are
just not great.

Another thing that's real awkard is how chaining exclusive fences
right now means direct dma_resv.exclusive_fence pointer access with an
rcu_assign_pointer. Not so great either.

v2:
- Fix a pile of typos (Matt, Jason)
- Hammer it in that breaking the rules leads to use-after-free issues
  around dma-buf sharing (Christian)

Reviewed-by: Christian König <christian.koenig@amd.com>
Cc: Jason Ekstrand <jason@jlekstrand.net>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: linux-media@vger.kernel.org
Cc: linaro-mm-sig@lists.linaro.org
Link: https://patchwork.freedesktop.org/patch/msgid/20210805104705.862416-21-daniel.vetter@ffwll.ch

1 files changed, 7 insertions, 0 deletions

diff --git a/include/linux/dma-buf.h b/include/linux/dma-buf.h
index 8b32b4bdd590..66470c37e471 100644
--- a/include/linux/dma-buf.h
+++ b/include/linux/dma-buf.h
@@ -420,6 +420,13 @@ struct dma_buf {
 	 * - Dynamic importers should set fences for any access that they can't
 	 *   disable immediately from their &dma_buf_attach_ops.move_notify
 	 *   callback.
+	 *
+	 * IMPORTANT:
+	 *
+	 * All drivers must obey the struct dma_resv rules, specifically the
+	 * rules for updating fences, see &dma_resv.fence_excl and
+	 * &dma_resv.fence. If these dependency rules are broken access tracking
+	 * can be lost resulting in use after free issues.
 	 */
 	struct dma_resv *resv;