[PATCH 6.19 035/311] cgroup: Fix cgroup_drain_dying() testing the wrong condition

Greg Kroah-Hartman
2026-04-08T18:52:41Z

From: Tejun Heo <tj@kernel.org>

[ Upstream commit 4c56a8ac6869855866de0bb368a4189739e1d24f ]

cgroup_drain_dying() was using cgroup_is_populated() to test whether there are
dying tasks to wait for. cgroup_is_populated() tests nr_populated_csets,
nr_populated_domain_children and nr_populated_threaded_children, but
cgroup_drain_dying() only needs to care about this cgroup's own tasks - whether
there are children is cgroup_destroy_locked()'s concern.

This caused hangs during shutdown. When systemd tried to rmdir a cgroup that had
no direct tasks but had a populated child, cgroup_drain_dying() would enter its
wait loop because cgroup_is_populated() was true from
nr_populated_domain_children. The task iterator found nothing to wait for, yet
the populated state never cleared because it was driven by live tasks in the
child cgroup.

Fix it by using cgroup_has_tasks() which only tests nr_populated_csets.

v3: Fix cgroup_is_populated() -> cgroup_has_tasks() (Sebastian).

v2: https://lore.kernel.org/r/20260323200205.1063629-1-tj@kernel.org

Reported-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Fixes: 1b164b876c36 ("cgroup: Wait for dying tasks to leave on rmdir")
Signed-off-by: Tejun Heo <tj@kernel.org>
Tested-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
kernel/cgroup/cgroup.c | 42 ++++++++++++++++++++++--------------------
1 file changed, 22 insertions(+), 20 deletions(-)

diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 257d1ddea1ada..9370100764904 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -6229,20 +6229,22 @@ static int cgroup_destroy_locked(struct cgroup *cgrp)
* cgroup_drain_dying - wait for dying tasks to leave before rmdir
* @cgrp: the cgroup being removed
*
- * The PF_EXITING filter in css_task_iter_advance() hides exiting tasks from
- * cgroup.procs so that userspace (e.g. 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.procs and cgroup.threads use css_task_iter which filters out
+ * PF_EXITING tasks so that userspace doesn't see tasks that have already been
+ * reaped via waitpid(). However, cgroup_has_tasks() - which tests whether the
+ * cgroup has non-empty css_sets - is only updated when dying tasks pass through
* cgroup_task_dead() in finish_task_switch(). This creates a window where
- * cgroup.procs appears empty but cgroup_is_populated() is still true, causing
- * rmdir to fail with -EBUSY.
+ * cgroup.procs reads empty but cgroup_has_tasks() is still true, making rmdir
+ * fail with -EBUSY from cgroup_destroy_locked() even though userspace sees no
+ * tasks.
+ *
+ * This function aligns cgroup_has_tasks() with what userspace can observe. If
+ * cgroup_has_tasks() but the task iterator sees nothing (all remaining tasks are
+ * PF_EXITING), we wait for cgroup_task_dead() to finish processing them. As the
+ * window between PF_EXITING and cgroup_task_dead() is short, the wait is brief.
*
- * This function bridges that gap. If the cgroup is populated but all remaining
- * tasks have PF_EXITING set, we wait for cgroup_task_dead() to process them.
- * Tasks are removed from the cgroup's css_set in cgroup_task_dead() called from
- * finish_task_switch(). As the window between PF_EXITING and cgroup_task_dead()
- * is short, the number of PF_EXITING tasks on the list is small and the wait
- * is brief.
+ * This function only concerns itself with this cgroup's own dying tasks.
+ * Whether the cgroup has children is cgroup_destroy_locked()'s problem.
*
* Each cgroup_task_dead() kicks the waitqueue via cset->cgrp_links, and we
* retry the full check from scratch.
@@ -6258,7 +6260,7 @@ static int cgroup_drain_dying(struct cgroup *cgrp)

lockdep_assert_held(&cgroup_mutex);
retry:
- if (!cgroup_is_populated(cgrp))
+ if (!cgroup_has_tasks(cgrp))
return 0;

/* Same iterator as cgroup.threads - if any task is visible, it's busy */
@@ -6273,15 +6275,15 @@ static int cgroup_drain_dying(struct cgroup *cgrp)
* All remaining tasks are PF_EXITING and will pass through
* cgroup_task_dead() shortly. Wait for a kick and retry.
*
- * cgroup_is_populated() can't transition from false to true while
- * we're holding cgroup_mutex, but the true to false transition
- * happens under css_set_lock (via cgroup_task_dead()). We must
- * retest and prepare_to_wait() under css_set_lock. Otherwise, the
- * transition can happen between our first test and
- * prepare_to_wait(), and we sleep with no one to wake us.
+ * cgroup_has_tasks() can't transition from false to true while we're
+ * holding cgroup_mutex, but the true to false transition happens
+ * under css_set_lock (via cgroup_task_dead()). We must retest and
+ * prepare_to_wait() under css_set_lock. Otherwise, the transition
+ * can happen between our first test and prepare_to_wait(), and we
+ * sleep with no one to wake us.
*/
spin_lock_irq(&css_set_lock);
- if (!cgroup_is_populated(cgrp)) {
+ if (!cgroup_has_tasks(cgrp)) {
spin_unlock_irq(&css_set_lock);
return 0;
}
--
2.53.0