[PATCH 4/6] Changes css_set freeing mechanism to be under RCU

[Ben Blum <bblum@google.com>]

Changes css_set freeing mechanism to be under RCU

This is a prepatch for making the procs file writable. In order to free the
old css_sets for each task to be moved as they're being moved, the freeing
mechanism must be RCU-protected, or else we would have to have a call to
synchronize_rcu() for each task before freeing its old css_set.

Signed-off-by: Ben Blum <bblum@google.com>

---

 include/linux/cgroup.h |    3 +++
 kernel/cgroup.c        |    8 +++++++-
 2 files changed, 10 insertions(+), 1 deletions(-)

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index b934b72..24e3f1a 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -260,6 +260,9 @@ struct css_set {
 	 * during subsystem registration (at boot time).
 	 */
 	struct cgroup_subsys_state *subsys[CGROUP_SUBSYS_COUNT];
+
+	/* For RCU-protected deletion */
+	struct rcu_head rcu_head;
 };
 
 /*
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 7e6b183..637a54e 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -288,6 +288,12 @@ static void unlink_css_set(struct css_set *cg)
 	}
 }
 
+static void free_css_set_rcu(struct rcu_head *obj)
+{
+	struct css_set *cg = container_of(obj, struct css_set, rcu_head);
+	kfree(cg);
+}
+
 static void __put_css_set(struct css_set *cg, int taskexit)
 {
 	int i;
@@ -317,7 +323,7 @@ static void __put_css_set(struct css_set *cg, int taskexit)
 		}
 	}
 	rcu_read_unlock();
-	kfree(cg);
+	call_rcu(&cg->rcu_head, free_css_set_rcu);
 }
 
 /*