i915_gem_busy_ioctl uses dma_resv_for_each_fence_unlocked() to iterate over the fences in an GEM object without holding a reference but only the RCU read side lock.
What can happen here is that the GEM object is destroyed concurrently while i915_gem_busy_ioctl is still running. This won't free the GEM objects memory, but still drops all the dma_fence references. Now when dma_resv_for_each_fence_unlocked() sees a destroyed dma_fence it assumes that a new fence list was installed and re-starts the loop. But in the case of a destroyed GEM object a new fence list is never installed, only the old one freed and therefore the iteration never finishes resulting in an endless loop. The solution is to drop the fence references only after the RCU grace period. The fixes tag is not necessary the patch introducing the problem, but the one making it so worse that we need to address it. This problem was pointed out by Sashiko-bot. Signed-off-by: Christian König <[email protected]> Fixes: 912ff2ebd695 ("drm/i915: use the new iterator in i915_gem_busy_ioctl v2") CC: [email protected] --- drivers/gpu/drm/i915/gem/i915_gem_object.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/drivers/gpu/drm/i915/gem/i915_gem_object.c b/drivers/gpu/drm/i915/gem/i915_gem_object.c index 5172d3982654..9e01f8b2079a 100644 --- a/drivers/gpu/drm/i915/gem/i915_gem_object.c +++ b/drivers/gpu/drm/i915/gem/i915_gem_object.c @@ -89,6 +89,7 @@ struct drm_i915_gem_object *i915_gem_object_alloc(void) void i915_gem_object_free(struct drm_i915_gem_object *obj) { + dma_resv_fini(&obj->base._resv); return kmem_cache_free(slab_objects, obj); } @@ -144,7 +145,6 @@ void __i915_gem_object_fini(struct drm_i915_gem_object *obj) { mutex_destroy(&obj->mm.get_page.lock); mutex_destroy(&obj->mm.get_dma_page.lock); - dma_resv_fini(&obj->base._resv); } /** -- 2.43.0
