On 2018-07-04 10:31 AM, Christian König wrote: > Am 26.06.2018 um 16:31 schrieb Michel Dänzer: >> From: Michel Dänzer <michel.daenzer at amd.com> >> >> Fixes the BUG_ON spuriously triggering under the following >> circumstances: >> >> * ttm_eu_reserve_buffers processes a list containing multiple BOs using >>   the same reservation object, so it calls >>   reservation_object_reserve_shared with that reservation object once >>   for each such BO. >> * In reservation_object_reserve_shared, old->shared_count == >>   old->shared_max - 1, so obj->staged is freed in preparation of an >>   in-place update. >> * ttm_eu_fence_buffer_objects calls reservation_object_add_shared_fence >>   once for each of the BOs above, always with the same fence. >> * The first call adds the fence in the remaining free slot, after which >>   old->shared_count == old->shared_max. > > Well, the explanation here is not correct. For multiple BOs using the > same reservation object we won't call > reservation_object_add_shared_fence() multiple times because we move > those to the duplicates list in ttm_eu_reserve_buffers(). > > But this bug can still happen because we call > reservation_object_add_shared_fence() manually with fences for the same > context in a couple of places. > > One prominent case which comes to my mind are for the VM BOs during > updates. Another possibility are VRAM BOs which need to be cleared. Thanks. How about the following: * ttm_eu_reserve_buffers calls reservation_object_reserve_shared. * In reservation_object_reserve_shared, shared_count == shared_max - 1, so obj->staged is freed in preparation of an in-place update. * ttm_eu_fence_buffer_objects calls reservation_object_add_shared_fence, after which shared_count == shared_max. * The amdgpu driver also calls reservation_object_add_shared_fence for the same reservation object, and the BUG_ON triggers. However, nothing bad would happen in reservation_object_add_shared_inplace, since all fences use the same context, so they can only occupy a single slot. Prevent this by moving the BUG_ON to where an overflow would actually happen (e.g. if a buggy caller didn't call reservation_object_reserve_shared before). Also, I'll add a reference to https://bugs.freedesktop.org/106418 in v2, as I suspect this fix is necessary under the circumstances described there as well. -- Earthling Michel Dänzer | http://www.amd.com Libre software enthusiast | Mesa and X developer
