Re: [PATCH] kthread: kthread_should_stop() checks if we're a kthread

[Petr Mladek <pmladek@suse.com>]

Adding Andrew into Cc. He usually takes changes in kernel/kthread.c.

On Mon 2023-11-20 17:15:03, Kent Overstreet wrote:
> bcachefs has a fair amount of code that may or may not be running from a
> kthread (it may instead be called by a userspace ioctl); having
> kthread_should_stop() check if we're a kthread enables a fair bit of
> cleanup and makes it safer to use.
> 
> Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>
> ---
>  kernel/kthread.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/kthread.c b/kernel/kthread.c
> index 1eea53050bab..fe6090ddf414 100644
> --- a/kernel/kthread.c
> +++ b/kernel/kthread.c
> @@ -155,7 +155,8 @@ void free_kthread_struct(struct task_struct *k)
>   */
>  bool kthread_should_stop(void)
>  {
> -	return test_bit(KTHREAD_SHOULD_STOP, &to_kthread(current)->flags);
> +	return (current->flags & PF_KTHREAD) &&
> +		test_bit(KTHREAD_SHOULD_STOP, &to_kthread(current)->flags);
>  }
>  EXPORT_SYMBOL(kthread_should_stop);

I agree that it makes the API more safe because &to_kthread(current)
is NULL when the process is not a kthread.

Well, I do not like the idea of quietly ignoring a misuse of
the kthread_*() API. I would personally prefer to do:

// define this in include/linux/kthread.h
static inline bool in_kthread(void)
{
	return current->flags & PF_KTHREAD
}

// add WARN() into kthread_should_stop()
bool kthread_should_stop(void)
{
	if (WARN_ON_ONCE(!in_kthread))
		return false;

	return test_bit(KTHREAD_SHOULD_STOP, &to_kthread(current)->flags);
}
EXPORT_SYMBOL(kthread_should_stop);


And use the following in bcachefs() code:

	if (in_kthread() && kthread_should_stop())
		goto exit;


Is see several advantages:

    + It will warn when the API is misused.

    + It will be more clear that the bcachefs code might be
      used in both kthread and userspace code.

    + in_kthread() might be used around other code which is
      needed only when the process is a kthread.

    + Similar check and WARN() might be used also in the other
      kthread() API.


Best Regards,
Petr