cgroups: don't put struct cgroupfs_root protected by RCU

We don't access struct cgroupfs_root in fast path, so we should not put
struct cgroupfs_root protected by RCU

But the comment in struct cgroup_subsys.root confuse us.

struct cgroup_subsys.root is used in these places:

1 find_css_set(): if (ss->root->subsys_list.next == &ss->sibling)
2 rebind_subsystems(): if (ss->root != &rootnode)
                       rcu_assign_pointer(ss->root, root);
                       rcu_assign_pointer(subsys[i]->root, &rootnode);
3 cgroup_has_css_refs(): if (ss->root != cgrp->root)
4 cgroup_init_subsys(): ss->root = &rootnode;
5 proc_cgroupstats_show(): ss->name, ss->root->subsys_bits,
                           ss->root->number_of_cgroups, !ss->disabled);
6 cgroup_clone(): root = subsys->root;
                  if ((root != subsys->root) ||

All these place we have held cgroup_lock() or we don't dereference to
struct cgroupfs_root.  It's means wo don't need RCU when use struct
cgroup_subsys.root, and we should not put struct cgroupfs_root protected
by RCU.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Reviewed-by: Paul Menage <menage@google.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Pavel Emelyanov <xemul@openvz.org>
Cc: Balbir Singh <balbir@in.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 2 insertions, 3 deletions

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 08b78c09b09a..f68dfd8dd53a 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -337,7 +337,6 @@ struct cgroup_subsys {
 #define MAX_CGROUP_TYPE_NAMELEN 32
         const char *name;
 
-        /* Protected by RCU */
         struct cgroupfs_root *root;
 
         struct list_head sibling;
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index a391ab3bdfc6..a288da176e46 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -713,7 +713,7 @@ static int rebind_subsystems(struct cgroupfs_root *root,
                         cgrp->subsys[i] = dummytop->subsys[i];
                         cgrp->subsys[i]->cgroup = cgrp;
                         list_add(&ss->sibling, &root->subsys_list);
-                        rcu_assign_pointer(ss->root, root);
+                        ss->root = root;
                         if (ss->bind)
                                 ss->bind(ss, cgrp);
 
@@ -725,7 +725,7 @@ static int rebind_subsystems(struct cgroupfs_root *root,
                                 ss->bind(ss, dummytop);
                         dummytop->subsys[i]->cgroup = dummytop;
                         cgrp->subsys[i] = NULL;
-                        rcu_assign_pointer(subsys[i]->root, &rootnode);
+                        subsys[i]->root = &rootnode;
                         list_del(&ss->sibling);
                 } else if (bit & final_bits) {
                         /* Subsystem state should already exist */