Hi.
On Tue, Mar 10, 2009 at 10:01:30PM +0100, Jesper Dangaard Brouer (hawk@xxxxxxx) wrote:
> >+static void __exit ipt_osf_fini(void)
> >+{
> >+ struct ipt_osf_finger *f;
> >+ int i;
> >+
> >+ cn_del_callback(&cn_osf_id);
> >+ xt_unregister_match(&ipt_osf_match);
> >+
> >+ rcu_read_lock();
> >+ for (i=0; i<ARRAY_SIZE(ipt_osf_fingers); ++i) {
> >+ struct ipt_osf_finger_storage *st = &ipt_osf_fingers[i];
> >+
> >+ list_for_each_entry_rcu(f, &st->finger_list, finger_entry) {
>
> spin_lock(&st->finger_lock); //???
> >+ list_del_rcu(&f->finger_entry);
> spin_unlock(&st->finger_lock);
>
> >+ call_rcu(&f->rcu_head, ipt_osf_finger_free_rcu);
> >+ }
> >+ }
> >+ rcu_read_unlock();
>
> Should the list_del_rcu() not be protected by a spinlock?
Not required at this place - all users are already unregistered and
no code can access this list except module exit path.
> >+ rcu_barrier();
>
> In some of my code I call synchronize_net(), is it enough to call
> rcu_barrier()?
It is enough here, rcu_barrier() will wait until all scheduled
call_rcu() are completed, that's what we need. But in some cases we
should only wait for the whole grace period to elapse, then one has to use
synchronize_rcu() and friends. rcu_barrier() will wait for the callbacks
to be executed, while they are executed after grace period has elapsed,
so it implicitly includes synchronize_rcu(), but effectively they are
the same: both functions register rcu callback and wait for the
completion, rcu_barrier() is a bit more enhanced, since it has several
types.
> What is the difference between:
>
> synchronize_rcu()
> synchronize_net()
Those are essentially the same - synchronize_net() has additional
might_sleep() call. Both will wait until grace period elapced - i.e.
all currently RCU protected sections completed.
> rcu_barrier()
It will wait until all scheduled rcu callbacks are executed.
So from the description they look different, but implementation
suggestes that effectively they are the same, except that there are a
bit different invocation types for the barrier.
--
Evgeniy Polyakov
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