[PATCH 2/5] cgroup: Annotate unlocked nr_populated_* accesses with READ_ONCE/WRITE_ONCE

[Tejun Heo <tj@kernel.org>]

cgroup_update_populated() updates nr_populated_csets,
nr_populated_domain_children, and nr_populated_threaded_children under
css_set_lock, but cgroup_has_tasks(), cgroup_is_populated(), and
cgroup_can_be_thread_root() read them without holding it. Use
READ_ONCE/WRITE_ONCE.

Signed-off-by: Tejun Heo <tj@kernel.org>
---
 include/linux/cgroup.h | 21 +++++++++++++++++----
 kernel/cgroup/cgroup.c | 11 +++++++----
 2 files changed, 24 insertions(+), 8 deletions(-)

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index ceb87507667e..9f8bef8f3a60 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -639,16 +639,29 @@ static inline bool task_under_cgroup_hierarchy(struct task_struct *task,
 	return cgroup_is_descendant(cset->dfl_cgrp, ancestor);
 }
 
+/*
+ * Populated counters: writes happen under css_set_lock. The accessors below
+ * may read unlocked. What an unpopulated result means depends on context:
+ *
+ * - No lock held. Just a snapshot. May race with concurrent updates and is
+ *   useful only as a hint.
+ *
+ * - cgroup_mutex held. Migration into the cgroup is blocked, so an observed
+ *   !populated stays !populated until cgroup_mutex is dropped.
+ *
+ * - CSS_DYING set. The css can no longer be repopulated, so !populated is
+ *   sticky once observed.
+ */
 static inline bool cgroup_has_tasks(struct cgroup *cgrp)
 {
-	return cgrp->nr_populated_csets;
+	return READ_ONCE(cgrp->nr_populated_csets);
 }
 
-/* no synchronization, the result can only be used as a hint */
 static inline bool cgroup_is_populated(struct cgroup *cgrp)
 {
-	return cgrp->nr_populated_csets + cgrp->nr_populated_domain_children +
-		cgrp->nr_populated_threaded_children;
+	return READ_ONCE(cgrp->nr_populated_csets) +
+		READ_ONCE(cgrp->nr_populated_domain_children) +
+		READ_ONCE(cgrp->nr_populated_threaded_children);
 }
 
 /* returns ino associated with a cgroup */
diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 7a94c2ea1036..d1395784871a 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -404,7 +404,7 @@ static bool cgroup_can_be_thread_root(struct cgroup *cgrp)
 		return false;
 
 	/* can only have either domain or threaded children */
-	if (cgrp->nr_populated_domain_children)
+	if (READ_ONCE(cgrp->nr_populated_domain_children))
 		return false;
 
 	/* and no domain controllers can be enabled */
@@ -783,12 +783,15 @@ static void cgroup_update_populated(struct cgroup *cgrp, bool populated)
 		bool was_populated = cgroup_is_populated(cgrp);
 
 		if (!child) {
-			cgrp->nr_populated_csets += adj;
+			WRITE_ONCE(cgrp->nr_populated_csets,
+				   cgrp->nr_populated_csets + adj);
 		} else {
 			if (cgroup_is_threaded(child))
-				cgrp->nr_populated_threaded_children += adj;
+				WRITE_ONCE(cgrp->nr_populated_threaded_children,
+					   cgrp->nr_populated_threaded_children + adj);
 			else
-				cgrp->nr_populated_domain_children += adj;
+				WRITE_ONCE(cgrp->nr_populated_domain_children,
+					   cgrp->nr_populated_domain_children + adj);
 		}
 
 		if (was_populated == cgroup_is_populated(cgrp))
-- 
2.54.0