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SubjectRe: [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@redhat.com> 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@redhat.com>
>
> Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
>
> 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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