Re: [PATCH tip/core/rcu 3/3] rcu: place root rcu_node structure in separate lockdep class

[Peter Zijlstra <peterz@infradead.org>]

On Tue, 2009-10-06 at 21:48 -0700, Paul E. McKenney wrote:
> From: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> 
> Before this patch, all of the rcu_node structures were in the same lockdep
> class, so that lockdep would complain when rcu_preempt_offline_tasks()
> acquired the root rcu_node structure's lock while holding one of the leaf
> rcu_nodes' locks.  This patch changes rcu_init_one() to use a separate
> spin_lock_init() for the root rcu_node structure's lock than is used for
> that of all of the rest of the rcu_node structures, which puts the root
> rcu_node structure's lock in its own lockdep class.
> 
> Suggested-by: Peter Zijlstra <peterz@infradead.org>
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> ---
>  kernel/rcutree.c |    4 +++-
>  1 files changed, 3 insertions(+), 1 deletions(-)
> 
> diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> index d8d9865..705f02a 100644
> --- a/kernel/rcutree.c
> +++ b/kernel/rcutree.c
> @@ -1647,7 +1647,8 @@ static void __init rcu_init_one(struct rcu_state *rsp)
>  		cpustride *= rsp->levelspread[i];
>  		rnp = rsp->level[i];
>  		for (j = 0; j < rsp->levelcnt[i]; j++, rnp++) {
> -			spin_lock_init(&rnp->lock);
> +			if (rnp != rcu_get_root(rsp))
> +				spin_lock_init(&rnp->lock);
>  			rnp->gpnum = 0;
>  			rnp->qsmask = 0;
>  			rnp->qsmaskinit = 0;
> @@ -1670,6 +1671,7 @@ static void __init rcu_init_one(struct rcu_state *rsp)
>  			INIT_LIST_HEAD(&rnp->blocked_tasks[1]);
>  		}
>  	}
> +	spin_lock_init(&rcu_get_root(rsp)->lock);
>  }
>  
>  /*


An alternative version that would not change any code in the !LOCKDEP
case would read as:

---
 kernel/rcutree.c |    4 ++++
 1 files changed, 4 insertions(+), 0 deletions(-)

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index d8d9865..113f274 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -1625,6 +1625,8 @@ static void __init rcu_init_levelspread(struct rcu_state *rsp)
 }
 #endif /* #else #ifdef CONFIG_RCU_FANOUT_EXACT */
 
+static struct lock_class_key rcu_root_class;
+
 /*
  * Helper function for rcu_init() that initializes one rcu_state structure.
  */
@@ -1670,6 +1672,7 @@ static void __init rcu_init_one(struct rcu_state *rsp)
 			INIT_LIST_HEAD(&rnp->blocked_tasks[1]);
 		}
 	}
+	lockdep_set_class(&rcu_get_root(rsp)->lock, &rcu_root_class);
 }
 
 /*