Re: [PATCH -tip] locking/rtmutex: Reduce top-waiter blocking on a lock

[Waiman Long <waiman.long@hpe.com>]

On 09/23/2016 09:28 PM, Davidlohr Bueso wrote:
>
> +#ifdef CONFIG_RT_MUTEX_SPIN_ON_OWNER
> +static bool rt_mutex_spin_on_owner(struct rt_mutex *lock,
> +                   struct task_struct *owner)
> +{
> +    bool ret = true;
> +
> +    /*
> +     * The last owner could have just released the lock,
> +     * immediately try taking it again.
> +     */
> +    if (!owner)
> +        goto done;
> +
> +    rcu_read_lock();
> +    while (rt_mutex_owner(lock) == owner) {
> +        /*
> +         * Ensure we emit the owner->on_cpu, dereference _after_
> +         * checking lock->owner still matches owner. If that fails,
> +         * owner might point to freed memory. If it still matches,
> +         * the rcu_read_lock() ensures the memory stays valid.
> +         */
> +        barrier();
> +        if (!owner->on_cpu || need_resched()) {
> +            ret = false;
> +            break;
> +        }
> +
> +        cpu_relax_lowlatency();
> +    }
> +    rcu_read_unlock();
> +done:
> +    return ret;
> +}
> +

One issue that I saw is that the spinner may no longer be the top waiter 
while spinning. Should we also check this condition in the spin loop?

Cheers,
Longman