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

[Peter Zijlstra <peterz@infradead.org>]

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?

>  	 */
> +	v = atomic_read(&key->enabled);
> +	do {
> +		if (v <= 1)
> +			return false;
> +	} while (!likely(atomic_try_cmpxchg(&key->enabled, &v, v - 1)));
> +
>  	return true;
>  }