Messages in this thread | | | Date | Wed, 4 Jun 2014 13:57:34 -0400 | Subject | Re: [PATCH v2] rwsem: Support optimistic spinning | From | Andev <> |
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Hi,
On Tue, Apr 29, 2014 at 11:11 AM, Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote: > On Mon, Apr 28, 2014 at 05:50:49PM -0700, Tim Chen wrote: >> 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. > > OK, so if we read complete garbage, all that happens is that we > lose a bit of performance? If so, I am OK with it as long as there > is a comment (which Davidlohr suggested later in this thread). > > Thanx, Paul >
The latest code seems to be missing this comment. Could you please add this?
-- Pratapa Rudra
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