On Wed, Aug 18, 2021 at 03:38:23PM +0800, Desmond Cheong Zhi Xi wrote:

The task_work_add function is needed to prevent userspace races with DRM modesetting rights.

Some DRM ioctls can change modesetting permissions while other concurrent users are performing modesetting. To prevent races with userspace, such functions should flush readers of old permissions before returning to user mode. As the function that changes permissions might itself be a reader of the old permissions, we intend to schedule this flush using task_work_add.

However, when DRM is compiled as a loadable kernel module without exporting task_work_add, we get the following compilation error:

ERROR: modpost: "task_work_add" [drivers/gpu/drm/drm.ko] undefined!

Reported-by: kernel test robot lkp@intel.com Signed-off-by: Desmond Cheong Zhi Xi desmondcheongzx@gmail.com

Just realized another benefit of pushing the dev->master_rwsem write locks down into ioctls that need them: We wouldn't need this function here exported for use in drm. But also I'm not sure that works any better than the design in your current patch set ... -Daniel

---

kernel/task_work.c | 1 + 1 file changed, 1 insertion(+)

diff --git a/kernel/task_work.c b/kernel/task_work.c index 1698fbe6f0e1..90000404af2b 100644 --- a/kernel/task_work.c +++ b/kernel/task_work.c @@ -60,6 +60,7 @@ int task_work_add(struct task_struct *task, struct callback_head *work, return 0; } +EXPORT_SYMBOL(task_work_add); /**

• task_work_cancel_match - cancel a pending work added by task_work_add()

-- 2.25.1