Re: [PATCH 2/4] task_work: don't run task_work if task_work_exited is queued

[Peter Zijlstra <peterz@infradead.org>]

You seem to have lost Oleg and Al from the Cc list..

On Mon, Apr 06, 2020 at 01:48:51PM -0600, Jens Axboe wrote:
> If task_work has already been run on task exit, we don't always know
> if it's safe to run again. Check for task_work_exited in the
> task_work_pending() helper. This makes it less fragile in calling
> from the exit files path, for example.
> 
> Signed-off-by: Jens Axboe <axboe@kernel.dk>
> ---
>  include/linux/task_work.h | 4 +++-
>  kernel/task_work.c        | 8 ++++----
>  2 files changed, 7 insertions(+), 5 deletions(-)
> 
> diff --git a/include/linux/task_work.h b/include/linux/task_work.h
> index 54c911bbf754..24f977a8fc35 100644
> --- a/include/linux/task_work.h
> +++ b/include/linux/task_work.h
> @@ -7,6 +7,8 @@
>  
>  typedef void (*task_work_func_t)(struct callback_head *);
>  
> +extern struct callback_head task_work_exited;
> +
>  static inline void
>  init_task_work(struct callback_head *twork, task_work_func_t func)
>  {
> @@ -19,7 +21,7 @@ void __task_work_run(void);
>  
>  static inline bool task_work_pending(void)
>  {
> -	return current->task_works;
> +	return current->task_works && current->task_works != &task_work_exited;
>  }

Hurmph..  not sure I like this. It inlines that second condition to
every caller of task_work_run() even though for pretty much all of them
this is impossible.