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

[Peter Zijlstra]

On Wed, Apr 17, 2019 at 01:34:01PM -0400, Waiman Long wrote:
> On 04/17/2019 09:56 AM, Peter Zijlstra wrote:
> > 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;
> > 		}
> > 	...
> > }
> >
> With retpoline, an indirect function call will be slower.

With compiler optimization we can avoid that. Just mark the function as
__always_inline, there's only two call-sites, each with a different
trylock.

It might have already done that anyway, and used constant propagation
on your bool, but the function pointer one is far easier to read.