Re: [Linux kernel bug] WARNING in static_key_slow_inc_cpuslocked

[Thomas Gleixner <tglx@linutronix.de>]

On Sun, Jun 09 2024 at 18:02, Thomas Gleixner wrote:
> On Sun, Jun 09 2024 at 10:25, Steven Rostedt wrote:
> Well the bug is there to detect inconsistency and that clearly works :)
>
> But I clearly can't read, because the jump label operations are
> serialized via jump_label_mutex. Hrm...

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.

I'll send a patch after testing it.

Thanks,

        tglx