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

Also, the mutex case has a need_resched() test in there; any particular
reason you didn't mirror that?

See 46af29e479cc0 ("locking/mutexes: Return false if task need_resched() in mutex_can_spin_on_owner()")
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