drm/panfrost: Fix a race in panfrost_gem_free_object()

panfrost_gem_shrinker_scan() might purge a BO (release the sgt and
kill the GPU mapping) that's being freed by panfrost_gem_free_object()
if we don't remove the BO from the shrinker list at the beginning of
panfrost_gem_free_object().

Fixes: 013b65101315 ("drm/panfrost: Add madvise and shrinker support")
Cc: <stable@vger.kernel.org>
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Acked-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20191129135908.2439529-5-boris.brezillon@collabora.com

diff --git a/drivers/gpu/drm/panfrost/panfrost_gem.c b/drivers/gpu/drm/panfrost/panfrost_gem.c
index deca0c3..fa1b273 100644 (file)
--- a/drivers/gpu/drm/panfrost/panfrost_gem.c
+++ b/drivers/gpu/drm/panfrost/panfrost_gem.c
@@ -19,6 +19,16 @@ static void panfrost_gem_free_object(struct drm_gem_object *obj)
        struct panfrost_gem_object *bo = to_panfrost_bo(obj);
        struct panfrost_device *pfdev = obj->dev->dev_private;
 
+       /*
+        * Make sure the BO is no longer inserted in the shrinker list before
+        * taking care of the destruction itself. If we don't do that we have a
+        * race condition between this function and what's done in
+        * panfrost_gem_shrinker_scan().
+        */
+       mutex_lock(&pfdev->shrinker_lock);
+       list_del_init(&bo->base.madv_list);
+       mutex_unlock(&pfdev->shrinker_lock);
+
        if (bo->sgts) {
                int i;
                int n_sgt = bo->base.base.size / SZ_2M;
@@ -33,11 +43,6 @@ static void panfrost_gem_free_object(struct drm_gem_object *obj)
                kfree(bo->sgts);
        }
 
-       mutex_lock(&pfdev->shrinker_lock);
-       if (!list_empty(&bo->base.madv_list))
-               list_del(&bo->base.madv_list);
-       mutex_unlock(&pfdev->shrinker_lock);
-
        drm_gem_shmem_free_object(obj);
 }