Re: [PATCH v2 5/5] rwsem: do optimistic spinning for writer lockacquisition

[Peter Zijlstra]

On Mon, Jun 24, 2013 at 04:20:51PM -0700, Tim Chen wrote:
> +#ifdef CONFIG_RWSEM_SPIN_ON_WRITE_OWNER
> +static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
> +{
> +	int retval;
> +	task_struct *owner;
> +
> +	rcu_read_lock();
> +	owner = sem->owner;

That wants to be: owner = ACCESS_ONCE(sem->owner);

> +
> +	/* Spin only if active writer running */
> +	if (owner)
> +		retval = owner->on_cpu;
> +	else
> +		retval = false;
> +
> +	rcu_read_unlock();
> +	/*
> +	 * if lock->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.
> +	 */
> +	return retval;
> +}
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