From 4c56a8ac6869855866de0bb368a4189739e1d24f Mon Sep 17 00:00:00 2001 From: Tejun Heo Date: Wed, 25 Mar 2026 07:23:48 -1000 Subject: 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 Fixes: 1b164b876c36 ("cgroup: Wait for dying tasks to leave on rmdir") Signed-off-by: Tejun Heo Tested-by: Sebastian Andrzej Siewior --- 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 2163054e1aa6..4ca3cb993da2 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 @@ 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; } -- cgit v1.2.3