Re: [Linux kernel bug] WARNING in static_key_slow_inc_cpuslocked
From: Thomas Gleixner
Date: Sun Jun 09 2024 - 12:56:25 EST
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