[PATCH v3 10/14] locking/percpu-rwsem: Add percpu_is_write_locked() and percpu_is_read_locked()

[Peter Zijlstra]

On Mon, Jul 04, 2022 at 05:05:10PM +0200, Marco Elver wrote:

> +bool percpu_is_read_locked(struct percpu_rw_semaphore *sem)
> +{
> +	return per_cpu_sum(*sem->read_count) != 0;
> +}
> +EXPORT_SYMBOL_GPL(percpu_is_read_locked);

I don't think this is correct; read_count can have spurious increments.

If we look at __percpu_down_read_trylock(), it does roughly something
like this:

	this_cpu_inc(*sem->read_count);
	smp_mb();
	if (!sem->block)
		return true;
	this_cpu_dec(*sem->read_count);
	return false;

So percpu_is_read_locked() needs to ensure the read_count is non-zero
*and* that block is not set.

That said; I really dislike the whole _is_locked family with a passion.
Let me try and figure out what you need this for.