Re: [PATCH v2] rwsem: Support optimistic spinning

[Peter Zijlstra]

On Mon, Apr 28, 2014 at 03:09:01PM -0700, Davidlohr Bueso wrote:
> +static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
> +{
> +	int retval;

And yet the return value is bool.

> +	struct task_struct *owner;
> +
> +	rcu_read_lock();
> +	owner = ACCESS_ONCE(sem->owner);
> +
> +	/* Spin only if active writer running */
> +	if (owner)
> +		retval = owner->on_cpu;
> +	else {
> +		/*
> +		 * When the owner is not set, the sem owner may have just
> +		 * acquired it and not set the owner yet, or the sem has
> +		 * been released, or reader active.
> +		 */
> +		retval = false;
> +	}

And if you init the retval to false, you can leave this entire branch
out.

> +	rcu_read_unlock();
> +
> +	return retval;
> +}


Which yields the much shorter:

static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
{
	struct task_struct *owner;
	bool on_cpu = false;

	rcu_read_lock();
	owner = ACCESS_ONCE(sem->owner);
	if (owner)
		on_cpu = owner->on_cpu;
	rcu_read_unlock();

	return on_cpu;
}

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