Messages in this thread | | | Subject | Re: [PATCH v2] rwsem: Support optimistic spinning | From | Tim Chen <> | Date | Mon, 28 Apr 2014 17:50:49 -0700 |
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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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