Re: [patch 2/4] jump_label: Fix concurrency issues in static_key_slow_dec()

[Thomas Gleixner]

On Mon, Jun 10 2024 at 19:57, Peter Zijlstra wrote:
> On Mon, Jun 10, 2024 at 02:46:36PM +0200, Thomas Gleixner wrote:
>
>> @@ -247,20 +247,25 @@ EXPORT_SYMBOL_GPL(static_key_disable);
>>  
>>  static bool static_key_slow_try_dec(struct static_key *key)
>>  {
>> +	int v;
>>  
>>  	/*
>> +	 * Go into the slow path if key::enabled is less than or equal than
>> +	 * one. One is valid to shut down the key, anything less than one
>> +	 * is an imbalance, which is handled at the call site.
>> +	 *
>> +	 * That includes the special case of '-1' which is set in
>> +	 * static_key_slow_inc_cpuslocked(), but that's harmless as it is
>> +	 * fully serialized in the slow path below. By the time this task
>> +	 * acquires the jump label lock the value is back to one and the
>> +	 * retry under the lock must succeed.
>
> Harmless yes, but it really should not happen to begin with. If this
> happens it means someone wants to disable a key that is in the middle of
> getting enabled for the first time.
>
> I'm tempted to want a WARN here instead. Hmm?

No strong opinion