[PATCH v2] cgroup: Use data_race() for task->flags in task_css_set_check()

From: Guopeng Zhang <redacted>

task_css_set_check() uses rcu_dereference_check() to verify that
task->cgroups can be dereferenced. One accepted condition is that the
task is already exiting, tested by checking PF_EXITING in task->flags.

This check is only part of the CONFIG_PROVE_RCU lockdep predicate. This
was found by KCSAN during fuzz testing. KCSAN can report a data race
when another task flag bit is updated concurrently. One report shows
pids_release() reading task->flags through task_css_set_check() while
do_task_dead() sets PF_NOFREEZE:

  KCSAN: data-race in task_css() [inline]
  KCSAN: data-race in pids_release()

  task_css()
  pids_release()
  cgroup_release()
  release_task()
  wait_task_zombie()

  value changed: 0x0040004c -> 0x0040804c

The changed bit is PF_NOFREEZE, not PF_EXITING. PF_EXITING remains set
before and after the update, so the task_css_set_check() condition does
not change. This is not a race on task->cgroups and does not indicate
incorrect pids charging or uncharging.

tools/memory-model/Documentation/access-marking.txt recommends
data_race() for data-racy loads used only for diagnostic purposes. Use
data_race() here to mark the intended diagnostic-only access.

No functional change intended.

Suggested-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Guopeng Zhang <redacted>
---
Changes in v2:
- Use data_race() instead of READ_ONCE() for the diagnostic-only
  CONFIG_PROVE_RCU predicate, as suggested by Tejun.
- Update the changelog to match access-marking.txt guidance.

 include/linux/cgroup.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index f2aa46a4f871..b905208942bf 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h

@@ -480,7 +480,7 @@ static inline void cgroup_unlock(void)
 		rcu_read_lock_sched_held() ||				\
 		lockdep_is_held(&cgroup_mutex) ||			\
 		lockdep_is_held(&css_set_lock) ||			\
-		((task)->flags & PF_EXITING) || (__c))
+		(data_race((task)->flags) & PF_EXITING) || (__c))
 #else
 #define task_css_set_check(task, __c)					\
 	rcu_dereference((task)->cgroups)

-- 
2.25.1