From 1b164b876c36c3eb5561dd9b37702b04401b0166 Mon Sep 17 00:00:00 2001 From: Tejun Heo Date: Tue, 24 Mar 2026 10:21:25 -1000 Subject: cgroup: Wait for dying tasks to leave on rmdir a72f73c4dd9b ("cgroup: Don't expose dead tasks in cgroup") hid PF_EXITING tasks from cgroup.procs so that systemd doesn't see tasks that have already been reaped via waitpid(). However, the populated counter (nr_populated_csets) is only decremented when the task later passes through cgroup_task_dead() in finish_task_switch(). This means cgroup.procs can appear empty while the cgroup is still populated, causing rmdir to fail with -EBUSY. Fix this by making cgroup_rmdir() wait for dying tasks to fully leave. If the cgroup is populated but all remaining tasks have PF_EXITING set (the task iterator returns none due to the existing filter), wait for a kick from cgroup_task_dead() and retry. The wait is brief as tasks are removed from the cgroup's css_set between PF_EXITING assertion in do_exit() and cgroup_task_dead() in finish_task_switch(). v2: cgroup_is_populated() true to false transition happens under css_set_lock not cgroup_mutex, so retest under css_set_lock before sleeping to avoid missed wakeups (Sebastian). Fixes: a72f73c4dd9b ("cgroup: Don't expose dead tasks in cgroup") Reported-by: kernel test robot Closes: https://lore.kernel.org/oe-lkp/202603222104.2c81684e-lkp@intel.com Reported-by: Sebastian Andrzej Siewior Signed-off-by: Tejun Heo Reviewed-by: Sebastian Andrzej Siewior Cc: Bert Karwatzki Cc: Michal Koutny Cc: cgroups@vger.kernel.org --- include/linux/cgroup-defs.h | 3 +++ 1 file changed, 3 insertions(+) (limited to 'include') diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h index bb92f5c169ca..7f87399938fa 100644 --- a/include/linux/cgroup-defs.h +++ b/include/linux/cgroup-defs.h @@ -609,6 +609,9 @@ struct cgroup { /* used to wait for offlining of csses */ wait_queue_head_t offline_waitq; + /* used by cgroup_rmdir() to wait for dying tasks to leave */ + wait_queue_head_t dying_populated_waitq; + /* used to schedule release agent */ struct work_struct release_agent_work; -- cgit v1.2.3