[PATCH 5.4 45/80] drm/panfrost: Fix a race in panfrost_gem_free_object()

[Greg Kroah-Hartman]

From: Boris Brezillon <boris.brezillon@xxxxxxxxxxxxx>

commit aed44cbeae2b7674cd155ba5cc6506aafe46a94e upstream.

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@xxxxxxxxxxxxxxx>
Signed-off-by: Boris Brezillon <boris.brezillon@xxxxxxxxxxxxx>
Reviewed-by: Steven Price <steven.price@xxxxxxx>
Acked-by: Alyssa Rosenzweig <alyssa.rosenzweig@xxxxxxxxxxxxx>
Signed-off-by: Rob Herring <robh@xxxxxxxxxx>
Link: https://patchwork.freedesktop.org/patch/msgid/20191129135908.2439529-5-boris.brezillon@xxxxxxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 drivers/gpu/drm/panfrost/panfrost_gem.c |   15 ++++++++++-----
 1 file changed, 10 insertions(+), 5 deletions(-)

--- 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(str
 	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(str
 		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);
 }