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cgroup: Fix cgroup_drain_dying() testing the wrong condition 

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: 1b164b876c ("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>
This commit is contained in:
Tejun Heo 2026-03-25 07:23:48 -10:00
parent 6680c162b4
commit 4c56a8ac68
1 changed files with 22 additions and 20 deletions
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42
kernel/cgroup/cgroup.c
View File

@ -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 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 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 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 @@ retry:
* 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;
}