Re: [Linux kernel bug] WARNING in static_key_slow_inc_cpuslocked

[Peter Zijlstra <peterz@infradead.org>]

On Sun, Jun 09, 2024 at 06:56:14PM +0200, Thomas Gleixner wrote:

> Ok. Now I found if for real. It's in the jump label core:
> 
> CPU0                            CPU1
> 
> static_key_slow_dec()
>  static_key_slow_try_dec()
> 
>    key->enabled == 1
>    val = atomic_fetch_add_unless(&key->enabled, -1, 1);
>    if (val == 1)
>    	return false;
> 
>    jump_label_lock();
>    if (atomic_dec_and_test(&key->enabled)) {
>       --> key->enabled == 0
>       __jump_label_update()
> 
>                                 static_key_slow_dec()
>                                  static_key_slow_try_dec()
> 
>                                     key->enabled == 0
>                                     val = atomic_fetch_add_unless(&key->enabled, -1, 1);
> 
>                                     --> key->enabled == -1 <- FAIL
> 
> static_key_slow_try_dec() is buggy. It needs similar logic as
> static_key_slow_try_inc() to work correctly.
> 
> It's not only the 0, key->enabled can be -1 when the other CPU is in the
> slow path of enabling it.

Well, the -1 thing is in the 0->1 path, that is, the very first enabler.

That *should* not race with a disabler. If it does, there is external
confusion. (As I think the follow up email shows..)