Re: [PATCH tip/core/rcu 45/86] net,act_police,rcu: remove rcu_barrier()

Paul E. McKenney
2011-05-02T22:21:58Z

On Mon, May 02, 2011 at 10:50:11AM -0700, Josh Triplett wrote:
> On Mon, May 02, 2011 at 01:36:18AM -0700, Paul E. McKenney wrote:
> > On Sun, May 01, 2011 at 08:59:35AM -0700, Josh Triplett wrote:
> > > On Sun, May 01, 2011 at 06:21:25AM -0700, Paul E. McKenney wrote:
> > > > From: Lai Jiangshan <laijs@cn.fujitsu.com>
> > > >
> > > > There is no callback of this module maybe queued
> > > > since we use kfree_rcu(), we can safely remove the rcu_barrier().
> > > >
> > > > Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
> > > > Acked-by: David S. Miller <davem@davemloft.net>
> > > > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > > > ---
> > > > net/sched/act_police.c | 1 -
> > > > 1 files changed, 0 insertions(+), 1 deletions(-)
> > > >
> > > > diff --git a/net/sched/act_police.c b/net/sched/act_police.c
> > > > index d6bcd64..b3b9b32 100644
> > > > --- a/net/sched/act_police.c
> > > > +++ b/net/sched/act_police.c
> > > > @@ -396,7 +396,6 @@ static void __exit
> > > > police_cleanup_module(void)
> > > > {
> > > > tcf_unregister_action(&act_police_ops);
> > > > - rcu_barrier(); /* Wait for completion of call_rcu()'s (tcf_police_free_rcu) */
> > > > }
> > >
> > > Very nice side-effect of having common callback code. Seems worth doing
> > > a review of other callers of rcu_barrier as well, to see if they still
> > > need to do so.
> >
> > Agreed, and good point on the review. /me wonders how this review could
> > be automated...
>
> Build everything as a module, and for each module check whether it uses
> the symbol rcu_barrier but not the symbol call_rcu. Same for the
> corresponding call_rcu_sched and call_rcu_bh.

Good point -- I will give this a shot when I find AC power.

> Also, Coccinelle could likely handle simple cases of
> call_rcu(function_that_calls_container_of_then_kfree), at least when
> the call and the function appear in the same source file.

Good point -- it wasn't too hard by hand this time, but it would be
a good addition to RCU checking.

Thanx, Paul