linux-kernel - Re: [PATCH RFC tip/core/rcu 15/18] rcu: give different levels of the rcu

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Date:	Wed, 16 Dec 2009 07:13:45 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, josh@...htriplett.org,
	dvhltc@...ibm.com, niv@...ibm.com, tglx@...utronix.de,
	rostedt@...dmis.org, Valdis.Kletnieks@...edu, dhowells@...hat.com
Subject: Re: [PATCH RFC tip/core/rcu 15/18] rcu: give different levels of
	the rcu_node hierarchy distinct lockdep names

On Wed, Dec 16, 2009 at 11:26:00AM +0100, Peter Zijlstra wrote:
> On Tue, 2009-12-15 at 15:02 -0800, Paul E. McKenney wrote:
> > From: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > 
> > Proposed for 2.6.34, not for inclusion.
> > 
> > Previously, each level of the rcu_node hierarchy had the same rather
> > unimaginative name: "&rcu_node_class[i]".  This makes lockdep diagnostics
> > involving these lockdep classes less helpful than would be nice.  This
> > patch fixes this by giving each level of the rcu_node hierarchy a distinct
> > name: "rcu_node_level_0", "rcu_node_level_1", and so on.
> > 
> > Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > ---
> >  kernel/rcutree.c |    9 ++++++++-
> >  1 files changed, 8 insertions(+), 1 deletions(-)
> > 
> > diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> > index 0a4c328..a6e45f6 100644
> > --- a/kernel/rcutree.c
> > +++ b/kernel/rcutree.c
> > @@ -1811,11 +1811,17 @@ static void __init rcu_init_levelspread(struct rcu_state *rsp)
> >   */
> >  static void __init rcu_init_one(struct rcu_state *rsp)
> >  {
> > +	static char *buf[] = { "rcu_node_level_0",
> > +			       "rcu_node_level_1",
> > +			       "rcu_node_level_2",
> > +			       "rcu_node_level_3" };  /* Match MAX_RCU_LVLS */
> >  	int cpustride = 1;
> >  	int i;
> >  	int j;
> >  	struct rcu_node *rnp;
> >  
> > +	WARN_ON_ONCE(MAX_RCU_LVLS > 4);  /* Fix buf[] initialization! */
> 
> So you're going to WARN here,
> 
> >  	/* Initialize the level-tracking arrays. */
> >  
> >  	for (i = 1; i < NUM_RCU_LVLS; i++)
> > @@ -1829,7 +1835,8 @@ static void __init rcu_init_one(struct rcu_state *rsp)
> >  		rnp = rsp->level[i];
> >  		for (j = 0; j < rsp->levelcnt[i]; j++, rnp++) {
> >  			spin_lock_init(&rnp->lock);
> > -			lockdep_set_class(&rnp->lock, &rcu_node_class[i]);
> > +			lockdep_set_class_and_name(&rnp->lock,
> > +						   &rcu_node_class[i], buf[i]);
> 
> and segfault here because i overruns its bounds?

Might or might not, depending on memory layout.

> Might as well BUG_ON then.

I will give BUILD_BUG_ON() a try, but with the array size computed at
compile time as you suggest elsewhere.

							Thanx, Paul
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