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@xxxxxxxxx> > Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@xxxxxxxxx> 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 > -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch
