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

Peter Zijlstra
2009-10-08T09:29:33Z

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);
}

/*