Re: [PATCH v4 11/16] locking/rwsem: Enable readers spinning on writer

[Peter Zijlstra]

On Sat, Apr 13, 2019 at 01:22:54PM -0400, Waiman Long wrote:
> @@ -549,7 +582,7 @@ static noinline enum owner_state rwsem_spin_on_owner(struct rw_semaphore *sem)
>  	return !owner ? OWNER_NULL : OWNER_READER;
>  }
>  
> -static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
> +static bool rwsem_optimistic_spin(struct rw_semaphore *sem, bool wlock)
>  {
>  	bool taken = false;
>  	bool is_rt_task = rt_task(current);
> @@ -558,9 +591,6 @@ static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
>  	preempt_disable();
>  
>  	/* sem->wait_lock should not be held when doing optimistic spinning */
> -	if (!rwsem_can_spin_on_owner(sem))
> -		goto done;
> -
>  	if (!osq_lock(&sem->osq))
>  		goto done;
>  
> @@ -580,10 +610,11 @@ static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
>  		/*
>  		 * Try to acquire the lock
>  		 */
> -		if (rwsem_try_write_lock_unqueued(sem)) {
> -			taken = true;
> +		taken = wlock ? rwsem_try_write_lock_unqueued(sem)
> +			      : rwsem_try_read_lock_unqueued(sem);
> +
> +		if (taken)
>  			break;
> -		}
>  
>  		/*
>  		 * An RT task cannot do optimistic spinning if it cannot

Alternatively you pass the trylock function as an argument:

static bool rwsem_optimistic_spin(struct rw_semaphore *sem,
				  bool (*trylock)(struct rw_semaphore *sem))
{
	...
		if (trylock(sem)) {
			taken = true;
			goto unlock;
		}
	...
}