Re: Are jump labels broken on 6.11-rc1?

Peter Zijlstra
2024-08-07T14:34:10Z

On Wed, Aug 07, 2024 at 04:03:12PM +0200, Thomas Gleixner wrote:

> > + if (static_key_dec(key, true)) // dec-not-one
>
> Eeew.

:-) I knew you'd hate on that

> Something like the below?
>
> Thanks,
>
> tglx
> ---
> @@ -250,49 +250,71 @@ void static_key_disable(struct static_ke
> }
> EXPORT_SYMBOL_GPL(static_key_disable);
>
> -static bool static_key_slow_try_dec(struct static_key *key)
> +static bool static_key_dec(struct static_key *key, bool dec_not_one)
> {
> + int v = atomic_read(&key->enabled);
>
> do {
> /*
> + * Warn about the '-1' case; since that means a decrement is
> + * concurrent with a first (0->1) increment. IOW people are
> + * trying to disable something that wasn't yet fully enabled.
> + * This suggests an ordering problem on the user side.
> + *
> + * Warn about the '0' case; simple underflow.
> */
> + if (WARN_ON_ONCE(v <= 0))
> + return v;
> +
> + if (dec_not_one && v == 1)
> + return v;
> +
> } while (!likely(atomic_try_cmpxchg(&key->enabled, &v, v - 1)));
>
> + return v;
> +}
> +
> +/*
> + * Fastpath: Decrement if the reference count is greater than one
> + *
> + * Returns false, if the reference count is 1 or -1 to force the caller
> + * into the slowpath.
> + *
> + * The -1 case is to handle a decrement during a concurrent first enable,
> + * which sets the count to -1 in static_key_slow_inc_cpuslocked(). As the
> + * slow path is serialized the caller will observe 1 once it acquired the
> + * jump_label_mutex, so the slow path can succeed.
> + */
> +static bool static_key_dec_not_one(struct static_key *key)
> +{
> + int v = static_key_dec(key, true);
> +
> + return v != 1 && v != -1;

if (v < 0)
return false;

/*
* Notably, 0 (underflow) returns true such that it bails out
* without doing anything.
*/
return v != 1;

Perhaps?

> +}
> +
> +/*
> + * Slowpath: Decrement and test whether the refcount hit 0.
> + *
> + * Returns true if the refcount hit zero, i.e. the previous value was one.
> + */
> +static bool static_key_dec_and_test(struct static_key *key)
> +{
> + int v = static_key_dec(key, false);
> +
> + lockdep_assert_held(&jump_label_mutex);
> + return v == 1;
> }

But yeah, this is nicer!