Re: [PATCH RFC tip/core/rcu 15/18] rcu: give different levels of the rcu_node hierarchy distinct lockdep names

[Peter Zijlstra <peterz@infradead.org>]

On Tue, 2009-12-15 at 15:02 -0800, Paul E. McKenney wrote:
> From: Paul E. McKenney <paulmck@linux.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@linux.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 as well BUG_ON then.