cgroup: Wait for dying tasks to leave on rmdir

a72f73c4dd ("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: a72f73c4dd ("cgroup: Don't expose dead tasks in cgroup")
Reported-by: kernel test robot <oliver.sang@intel.com>
Closes: https://lore.kernel.org/oe-lkp/202603222104.2c81684e-lkp@intel.com
Reported-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Bert Karwatzki <spasswolf@web.de>
Cc: Michal Koutny <mkoutny@suse.com>
Cc: cgroups@vger.kernel.org

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;
 

kernel/cgroup/cgroup.c

@@ -2126,6 +2126,7 @@ static void init_cgroup_housekeeping(struct cgroup *cgrp)
 #endif
 
 	init_waitqueue_head(&cgrp->offline_waitq);
+	init_waitqueue_head(&cgrp->dying_populated_waitq);
 	INIT_WORK(&cgrp->release_agent_work, cgroup1_release_agent);
 }
 
@@ -6224,6 +6225,76 @@ static int cgroup_destroy_locked(struct cgroup *cgrp)
 	return 0;
 };
 
+/**
+ * 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_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.
+ *
+ * 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.
+ *
+ * Each cgroup_task_dead() kicks the waitqueue via cset->cgrp_links, and we
+ * retry the full check from scratch.
+ *
+ * Must be called with cgroup_mutex held.
+ */
+static int cgroup_drain_dying(struct cgroup *cgrp)
+	__releases(&cgroup_mutex) __acquires(&cgroup_mutex)
+{
+	struct css_task_iter it;
+	struct task_struct *task;
+	DEFINE_WAIT(wait);
+
+	lockdep_assert_held(&cgroup_mutex);
+retry:
+	if (!cgroup_is_populated(cgrp))
+		return 0;
+
+	/* Same iterator as cgroup.threads - if any task is visible, it's busy */
+	css_task_iter_start(&cgrp->self, 0, &it);
+	task = css_task_iter_next(&it);
+	css_task_iter_end(&it);
+
+	if (task)
+		return -EBUSY;
+
+	/*
+	 * 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.
+	 */
+	spin_lock_irq(&css_set_lock);
+	if (!cgroup_is_populated(cgrp)) {
+		spin_unlock_irq(&css_set_lock);
+		return 0;
+	}
+	prepare_to_wait(&cgrp->dying_populated_waitq, &wait,
+			TASK_UNINTERRUPTIBLE);
+	spin_unlock_irq(&css_set_lock);
+	mutex_unlock(&cgroup_mutex);
+	schedule();
+	finish_wait(&cgrp->dying_populated_waitq, &wait);
+	mutex_lock(&cgroup_mutex);
+	goto retry;
+}
+
 int cgroup_rmdir(struct kernfs_node *kn)
 {
 	struct cgroup *cgrp;
@@ -6233,9 +6304,12 @@ int cgroup_rmdir(struct kernfs_node *kn)
 	if (!cgrp)
 		return 0;
 
-	ret = cgroup_destroy_locked(cgrp);
-	if (!ret)
-		TRACE_CGROUP_PATH(rmdir, cgrp);
+	ret = cgroup_drain_dying(cgrp);
+	if (!ret) {
+		ret = cgroup_destroy_locked(cgrp);
+		if (!ret)
+			TRACE_CGROUP_PATH(rmdir, cgrp);
+	}
 
 	cgroup_kn_unlock(kn);
 	return ret;
@@ -6995,6 +7069,7 @@ void cgroup_task_exit(struct task_struct *tsk)
 
 static void do_cgroup_task_dead(struct task_struct *tsk)
 {
+	struct cgrp_cset_link *link;
 	struct css_set *cset;
 	unsigned long flags;
 
@@ -7008,6 +7083,11 @@ static void do_cgroup_task_dead(struct task_struct *tsk)
 	if (thread_group_leader(tsk) && atomic_read(&tsk->signal->live))
 		list_add_tail(&tsk->cg_list, &cset->dying_tasks);
 
+	/* kick cgroup_drain_dying() waiters, see cgroup_rmdir() */
+	list_for_each_entry(link, &cset->cgrp_links, cgrp_link)
+		if (waitqueue_active(&link->cgrp->dying_populated_waitq))
+			wake_up(&link->cgrp->dying_populated_waitq);
+
 	if (dl_task(tsk))
 		dec_dl_tasks_cs(tsk);