Re: [PATCH drm-misc-next v9 01/11] drm/gem: fix lockdep check for dma-resv lock

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On 8/8/23 09:21, Boris Brezillon wrote:
On Thu,  3 Aug 2023 18:52:20 +0200
Danilo Krummrich <dakr@xxxxxxxxxx> wrote:

When no custom lock is set to protect a GEMs GPUVA list, lockdep checks
should fall back to the GEM objects dma-resv lock. With the current
implementation we're setting the lock_dep_map of the GEM objects 'resv'
pointer (in case no custom lock_dep_map is set yet) on
drm_gem_private_object_init().

However, the GEM objects 'resv' pointer might still change after
drm_gem_private_object_init() is called, e.g. through
ttm_bo_init_reserved(). This can result in the wrong lock being tracked.

To fix this, call dma_resv_held() directly from
drm_gem_gpuva_assert_lock_held() and fall back to the GEMs lock_dep_map
pointer only if an actual custom lock is set.

Fixes: e6303f323b1a ("drm: manager to keep track of GPUs VA mappings")
Signed-off-by: Danilo Krummrich <dakr@xxxxxxxxxx>

Reviewed-by: Boris Brezillon <boris.brezillon@xxxxxxxxxxxxx>

but I'm wondering if it wouldn't be a good thing to add a
drm_gem_set_resv() helper, so the core can control drm_gem_object::resv
re-assignments (block them if it's happening after the GEM has been
exposed to the outside world or update auxiliary data if it's happening
before that).

I agree, this might be a good idea. There are quite a few places where drm_gem_object::resv is set from external code.


---
  include/drm/drm_gem.h | 15 +++++++++------
  1 file changed, 9 insertions(+), 6 deletions(-)

diff --git a/include/drm/drm_gem.h b/include/drm/drm_gem.h
index c0b13c43b459..bc9f6aa2f3fe 100644
--- a/include/drm/drm_gem.h
+++ b/include/drm/drm_gem.h
@@ -551,15 +551,17 @@ int drm_gem_evict(struct drm_gem_object *obj);
   * @lock: the lock used to protect the gpuva list. The locking primitive
   * must contain a dep_map field.
   *
- * Call this if you're not proctecting access to the gpuva list
- * with the dma-resv lock, otherwise, drm_gem_gpuva_init() takes care
- * of initializing lock_dep_map for you.
+ * Call this if you're not proctecting access to the gpuva list with the
+ * dma-resv lock, but with a custom lock.
   */
  #define drm_gem_gpuva_set_lock(obj, lock) \
-	if (!(obj)->gpuva.lock_dep_map) \
+	if (!WARN((obj)->gpuva.lock_dep_map, \
+		  "GEM GPUVA lock should be set only once.")) \
  		(obj)->gpuva.lock_dep_map = &(lock)->dep_map
  #define drm_gem_gpuva_assert_lock_held(obj) \
-	lockdep_assert(lock_is_held((obj)->gpuva.lock_dep_map))
+	lockdep_assert((obj)->gpuva.lock_dep_map ? \
+		       lock_is_held((obj)->gpuva.lock_dep_map) : \
+		       dma_resv_held((obj)->resv))
  #else
  #define drm_gem_gpuva_set_lock(obj, lock) do {} while (0)
  #define drm_gem_gpuva_assert_lock_held(obj) do {} while (0)
@@ -573,11 +575,12 @@ int drm_gem_evict(struct drm_gem_object *obj);
   *
   * Calling this function is only necessary for drivers intending to support the
   * &drm_driver_feature DRIVER_GEM_GPUVA.
+ *
+ * See also drm_gem_gpuva_set_lock().
   */
  static inline void drm_gem_gpuva_init(struct drm_gem_object *obj)
  {
  	INIT_LIST_HEAD(&obj->gpuva.list);
-	drm_gem_gpuva_set_lock(obj, &obj->resv->lock.base);
  }
/**





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