Re: [PATCH v4 08/16] locking/rwsem: Make rwsem_spin_on_owner() return owner state

[Peter Zijlstra]

On Wed, Apr 17, 2019 at 02:41:01PM +0200, Peter Zijlstra wrote:
> On Sat, Apr 13, 2019 at 01:22:51PM -0400, Waiman Long wrote:
> > In the special case that there is no active lock and the handoff bit
> > is set, optimistic spinning has to be stopped.
> 
> > @@ -500,9 +521,19 @@ static noinline bool rwsem_spin_on_owner(struct rw_semaphore *sem)
> >  
> >  	/*
> >  	 * If there is a new owner or the owner is not set, we continue
> > -	 * spinning.
> > +	 * spinning except when here is no active locks and the handoff bit
> > +	 * is set. In this case, we have to stop spinning.
> >  	 */
> > -	return is_rwsem_owner_spinnable(READ_ONCE(sem->owner));
> > +	owner = READ_ONCE(sem->owner);
> > +	if (!is_rwsem_owner_spinnable(owner))
> > +		return OWNER_NONSPINNABLE;
> > +	if (owner && !is_rwsem_owner_reader(owner))
> > +		return OWNER_WRITER;
> > +
> > +	count = atomic_long_read(&sem->count);
> > +	if (RWSEM_COUNT_HANDOFF(count) && !RWSEM_COUNT_LOCKED(count))
> > +		return OWNER_NONSPINNABLE;
> > +	return !owner ? OWNER_NULL : OWNER_READER;
> >  }
> 
> So this fixes a straight up bug in the last patch (and thus should be
> done before so the bug never exists), and creates unreadable code while
> at it.
> 
> Also, I think only checking HANDOFF after the loop is wrong; the moment
> HANDOFF happens you have to terminate the loop, irrespective of what
> @owner does.
> 
> Does something like so work?
> 
> ---

enum owner_state {
	OWNER_NULL		= 1 << 0,
	OWNER_WRITER		= 1 << 1,
	OWNER_READER		= 1 << 2,
	OWNER_NONSPINNABLE	= 1 << 3,
};
#define OWNER_SPINNABLE		(OWNER_NULL | OWNER_WRITER)

static inline enum owner_state rwsem_owner_state(unsigned long owner)
{
	if (!owner)
		return OWNER_NULL;

	if (owner & RWSEM_ANONYMOUSLY_OWNED)
		return OWNER_NONSPINNABLE;

	if (owner & RWSEM_READER_OWNER)
		return OWNER_READER;

	return OWNER_WRITER;
}

static noinline enum owner_state rwsem_spin_on_owner(struct rw_semaphore *sem)
{
	struct task_struct *tmp, *owner = READ_ONCE(sem->owner);
	enum owner_state state = rwsem_owner_state((unsigned long)owner);

	if (state != OWNER_WRITER)
		return state;

	rcu_read_lock();
	for (;;) {
		if (atomic_long_read(&sem->count) & RWSEM_FLAG_HANDOFF) {
			state = OWNER_NONSPINNABLE;
			break;
		}

		tmp = READ_ONCE(sem->owner);
		if (tmp != owner) {
			state = rwsem_owner_state((unsigned long)tmp);
			break;
		}

		/*
		 * Ensure we emit the owner->on_cpu, dereference _after_
		 * checking sem->owner still matches owner, if that fails,
		 * owner might point to free()d memory, if it still matches,
		 * the rcu_read_lock() ensures the memory stays valid.
		 */
		barrier();

		if (need_resched() || !owner_on_cpu(owner)) {
			state = OWNER_NONSPINNABLE;
			break;
		}

		cpu_relax();
	}
	rcu_read_unlock();

	return state;
}