cgroup: add __rcu modifier to cgroup->subsys[]

For the planned unified hierarchy, each css (cgroup_subsys_state) will
be RCU protected so that it can be created and destroyed individually
while allowing RCU accesses.  Previous changes ensured that all
cgroup->subsys[] accesses use the cgroup_css() accessor.  This patch
adds __rcu modifier to cgroup->subsys[], add matching RCU dereference
in cgroup_css() and convert all assignments to either
rcu_assign_pointer() or RCU_INIT_POINTER().

This change prepares for the actual RCUfication of css's and doesn't
introduce any visible behavior change.  The conversion is verified
with sparse and all accesses are properly RCU annotated.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>

2 files changed, 15 insertions, 6 deletions

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 8a5dc91fbaad..eb200b5794e7 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -204,7 +204,7 @@ struct cgroup {
         struct cgroup_name __rcu *name;
 
         /* Private pointers for each registered subsystem */
-        struct cgroup_subsys_state *subsys[CGROUP_SUBSYS_COUNT];
+        struct cgroup_subsys_state __rcu *subsys[CGROUP_SUBSYS_COUNT];
 
         struct cgroupfs_root *root;
 
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index d63beffd41e1..c27101622567 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -229,11 +229,16 @@ static int cgroup_addrm_files(struct cgroup *cgrp, struct cftype cfts[],
  * @subsys_id: the subsystem of interest
  *
  * Return @cgrp's css (cgroup_subsys_state) associated with @subsys_id.
+ * This function must be called either under cgroup_mutex or
+ * rcu_read_lock() and the caller is responsible for pinning the returned
+ * css if it wants to keep accessing it outside the said locks.  This
+ * function may return %NULL if @cgrp doesn't have @subsys_id enabled.
  */
 static struct cgroup_subsys_state *cgroup_css(struct cgroup *cgrp,
                                               int subsys_id)
 {
-        return cgrp->subsys[subsys_id];
+        return rcu_dereference_check(cgrp->subsys[subsys_id],
+                                     lockdep_is_held(&cgroup_mutex));
 }
 
 /* convenient tests for these bits */
@@ -1072,8 +1077,10 @@ static int rebind_subsystems(struct cgroupfs_root *root,
                         BUG_ON(!cgroup_css(cgroup_dummy_top, i));
                         BUG_ON(cgroup_css(cgroup_dummy_top, i)->cgroup != cgroup_dummy_top);
 
-                        cgrp->subsys[i] = cgroup_dummy_top->subsys[i];
+                        rcu_assign_pointer(cgrp->subsys[i],
+                                           cgroup_css(cgroup_dummy_top, i));
                         cgroup_css(cgrp, i)->cgroup = cgrp;
+
                         list_move(&ss->sibling, &root->subsys_list);
                         ss->root = root;
                         if (ss->bind)
@@ -1088,8 +1095,10 @@ static int rebind_subsystems(struct cgroupfs_root *root,
 
                         if (ss->bind)
                                 ss->bind(cgroup_css(cgroup_dummy_top, i));
+
                         cgroup_css(cgroup_dummy_top, i)->cgroup = cgroup_dummy_top;
-                        cgrp->subsys[i] = NULL;
+                        RCU_INIT_POINTER(cgrp->subsys[i], NULL);
+
                         cgroup_subsys[i]->root = &cgroup_dummy_root;
                         list_move(&ss->sibling, &cgroup_dummy_root.subsys_list);
 
@@ -4314,7 +4323,7 @@ static void init_cgroup_css(struct cgroup_subsys_state *css,
                 css->flags |= CSS_ROOT;
 
         BUG_ON(cgroup_css(cgrp, ss->subsys_id));
-        cgrp->subsys[ss->subsys_id] = css;
+        rcu_assign_pointer(cgrp->subsys[ss->subsys_id], css);
 }
 
 /* invoke ->css_online() on a new CSS and mark it online if successful */
@@ -4962,7 +4971,7 @@ void cgroup_unload_subsys(struct cgroup_subsys *ss)
          * also takes care of freeing the css_id.
          */
         ss->css_free(cgroup_css(cgroup_dummy_top, ss->subsys_id));
-        cgroup_dummy_top->subsys[ss->subsys_id] = NULL;
+        RCU_INIT_POINTER(cgroup_dummy_top->subsys[ss->subsys_id], NULL);
 
         mutex_unlock(&cgroup_mutex);
 }