Re: [PATCH v2] rwsem: Support optimistic spinning

[Tim Chen]

On Mon, 2014-04-28 at 16:10 -0700, Paul E. McKenney wrote:

> > +#ifdef CONFIG_SMP
> > +static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
> > +{
> > +	int retval;
> > +	struct task_struct *owner;
> > +
> > +	rcu_read_lock();
> > +	owner = ACCESS_ONCE(sem->owner);
> 
> OK, I'll bite...
> 
> Why ACCESS_ONCE() instead of rcu_dereference()?

We're using it as a speculative check on the sem->owner to see
if the owner is running on the cpu.  The rcu_read_lock
is used for ensuring that the owner->on_cpu memory is
still valid.

> 
> (My first question was "where is the update side", but this is covered
> by task_struct allocation and deallocation.)

Tim

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