drm/panfrost: Make sure the shrinker does not reclaim referenced BOs

Userspace might tag a BO purgeable while it's still referenced by GPU
jobs. We need to make sure the shrinker does not purge such BOs until
all jobs referencing it are finished.

Fixes: 013b651013 ("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>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20191129135908.2439529-9-boris.brezillon@collabora.com

drivers/gpu/drm/panfrost/panfrost_drv.c

@@ -166,6 +166,7 @@ panfrost_lookup_bos(struct drm_device *dev,
 			break;
 		}
 
+		atomic_inc(&bo->gpu_usecount);
 		job->mappings[i] = mapping;
 	}
 

drivers/gpu/drm/panfrost/panfrost_gem.h

@@ -30,6 +30,12 @@ struct panfrost_gem_object {
 		struct mutex lock;
 	} mappings;
 
+	/*
+	 * Count the number of jobs referencing this BO so we don't let the
+	 * shrinker reclaim this object prematurely.
+	 */
+	atomic_t gpu_usecount;
+
 	bool noexec		:1;
 	bool is_heap		:1;
 };

drivers/gpu/drm/panfrost/panfrost_gem_shrinker.c

@@ -41,6 +41,9 @@ static bool panfrost_gem_purge(struct drm_gem_object *obj)
 	struct drm_gem_shmem_object *shmem = to_drm_gem_shmem_obj(obj);
 	struct panfrost_gem_object *bo = to_panfrost_bo(obj);
 
+	if (atomic_read(&bo->gpu_usecount))
+		return false;
+
 	if (!mutex_trylock(&shmem->pages_lock))
 		return false;
 

drivers/gpu/drm/panfrost/panfrost_job.c

@@ -269,8 +269,13 @@ static void panfrost_job_cleanup(struct kref *ref)
 	dma_fence_put(job->render_done_fence);
 
 	if (job->mappings) {
-		for (i = 0; i < job->bo_count; i++)
+		for (i = 0; i < job->bo_count; i++) {
+			if (!job->mappings[i])
+				break;
+
+			atomic_dec(&job->mappings[i]->obj->gpu_usecount);
 			panfrost_gem_mapping_put(job->mappings[i]);
+		}
 		kvfree(job->mappings);
 	}