Re: rcu-refcount stacker performance

["Paul E. McKenney" <paulmck@us.ibm.com>]

On Thu, Jul 14, 2005 at 08:44:50AM -0500, serue@us.ibm.com wrote:
> Quoting Paul E. McKenney (paulmck@us.ibm.com):
> > My guess is that the reference count is indeed costing you quite a
> > bit.  I glance quickly at the patch, and most of the uses seem to
> > be of the form:
> > 
> > 	increment ref count
> > 	rcu_read_lock()
> > 	do something
> > 	rcu_read_unlock()
> > 	decrement ref count
> > 
> > Can't these cases rely solely on rcu_read_lock()?  Why do you also
> > need to increment the reference count in these cases?
> 
> The problem is on module unload: is it possible for CPU1 to be
> on "do something", and sleep, and, while it sleeps, CPU2 does
> rmmod(lsm), so that by the time CPU1 stops sleeping, the code it
> is executing has been freed?

OK, but in the above case, "do something" cannot be sleeping, since
it is under rcu_read_lock().

> Because stacker won't remove the lsm from the list of modules
> until mod->exit() is executed, and module_free(mod) happens
> immediately after that, the above scenario seems possible.

Right, if you have some other code path that sleeps (outside of
rcu_read_lock(), right?), then you need the reference count for that
code path.  But the code paths that do not sleep should be able to
dispense with the reference count, reducing the cache-line traffic.

						Thanx, Paul