linux-kernel - Re: [PATCH tip/core/rcu 13/15] rcu: Introduce for_each_rcu

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Message-ID: <20120616053500.GC8252@leaf>
Date:	Fri, 15 Jun 2012 22:35:00 -0700
From:	Josh Triplett <josh@...htriplett.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, rostedt@...dmis.org, Valdis.Kletnieks@...edu,
	dhowells@...hat.com, eric.dumazet@...il.com, darren@...art.com,
	fweisbec@...il.com, patches@...aro.org
Subject: Re: [PATCH tip/core/rcu 13/15] rcu: Introduce for_each_rcu_flavor()
 and use it

On Fri, Jun 15, 2012 at 06:01:49PM -0700, Paul E. McKenney wrote:
> On Fri, Jun 15, 2012 at 04:52:40PM -0700, Josh Triplett wrote:
> > On Fri, Jun 15, 2012 at 02:06:08PM -0700, Paul E. McKenney wrote:
> > > diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> > > index bd4e41c..75ad92a 100644
> > > --- a/kernel/rcutree.c
> > > +++ b/kernel/rcutree.c
> > > @@ -84,6 +84,7 @@ struct rcu_state rcu_bh_state = RCU_STATE_INITIALIZER(rcu_bh, call_rcu_bh);
> > >  DEFINE_PER_CPU(struct rcu_data, rcu_bh_data);
> > >  
> > >  static struct rcu_state *rcu_state;
> > > +LIST_HEAD(rcu_struct_flavors);
> > 
> > Does any means exist to turn this into a constant array known at compile
> > time rather than a runtime linked list?  Having this as a compile-time
> > constant may allow the compiler to unroll for_each_rcu_flavor and
> > potentially inline the calls inside it.
> 
> I could do that, but none of the traversals is anywhere near performance
> critical, and all the ways I can think of to do this are uglier than
> the list.

All of the struct rcu_state instances exist at compile time, so you can
just create an array of pointers to them:

static struct rcu_state *const rcu_struct_flavors[] = {
    &rcu_data,
    &rcu_bh_data,
#ifdef CONFIG_TREE_PREEMPT_RCU
    &rcu_preempt_data,
#endif
};

Then just define for_each_rcu_flavor to iterate over that compile-time
constant array.  Any reason that wouldn't work?

- Josh Triplett
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