Re: [PATCH v2] rwsem: Support optimistic spinning

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Apr 30, 2014 at 09:33:34AM -0700, Jason Low wrote:
> > static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
> > {
> >         struct task_struct *owner;
> >         bool on_cpu = false;
> 
> Wouldn't we want to initialize on_cpu = true. For the !owner case, I
> would expect that we want to spin for the lock.
> 
> >         rcu_read_lock();
> >         owner = ACCESS_ONCE(sem->owner);
> >         if (owner)
> >                 on_cpu = owner->on_cpu;
> >         rcu_read_unlock();
> >
> >         return on_cpu;
> > }

That would indeed be in line with that the mutex code does. Indeed!