From: Sebastian Andrzej Siewior Date: Fri, 6 Mar 2026 19:22:35 +0000 (+0100) Subject: cgroup: Don't expose dead tasks in cgroup X-Git-Tag: v7.0-rc4~19^2 X-Git-Url: http://git.ipfire.org/gitweb.cgi?a=commitdiff_plain;h=a72f73c4dd9b;p=thirdparty%2Fkernel%2Fstable.git cgroup: Don't expose dead tasks in cgroup Once a task exits it has its state set to TASK_DEAD and then it is removed from the cgroup it belonged to. The last step happens on the task gets out of its last schedule() invocation and is delayed on PREEMPT_RT due to locking constraints. As a result it is possible to receive a pid via waitpid() of a task which is still listed in cgroup.procs for the cgroup it belonged to. This is something that systemd does not expect and as a result it waits for its exit until a time out occurs. This can also be reproduced on !PREEMPT_RT kernel with a significant delay in do_exit() after exit_notify(). Hide the task from the output which have PF_EXITING set which is done before the parent is notified. Keeping zombies with live threads shouldn't break anything (suggested by Tejun). Reported-by: Bert Karwatzki Closes: https://lore.kernel.org/all/20260219164648.3014-1-spasswolf@web.de/ Tested-by: Bert Karwatzki Fixes: 9311e6c29b34 ("cgroup: Fix sleeping from invalid context warning on PREEMPT_RT") Cc: stable@vger.kernel.org # v6.19+ Signed-off-by: Sebastian Andrzej Siewior Signed-off-by: Tejun Heo --- diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c index be1d71dda3179..01fc2a93f3ef2 100644 --- a/kernel/cgroup/cgroup.c +++ b/kernel/cgroup/cgroup.c @@ -5109,6 +5109,12 @@ repeat: return; task = list_entry(it->task_pos, struct task_struct, cg_list); + /* + * Hide tasks that are exiting but not yet removed. Keep zombie + * leaders with live threads visible. + */ + if ((task->flags & PF_EXITING) && !atomic_read(&task->signal->live)) + goto repeat; if (it->flags & CSS_TASK_ITER_PROCS) { /* if PROCS, skip over tasks which aren't group leaders */