[PATCH] cgroup/pids: Restore pids.events notifications in local mode

[Guopeng Zhang <guopeng.zhang@linux.dev>]

From: Guopeng Zhang <zhangguopeng@kylinos.cn>

A fork rejected by the pids controller increments the counter reported by
pids.events. When local event accounting is selected, however, pids_event()
returns after notifying only events_local_file, leaving pids.events pollers
asleep.

On legacy hierarchies, pids.events.local does not exist. With
pids_localevents, pids.events reports the same local counter. In both
cases, pids.events changes without generating a notification.

This can be reproduced with a pids_localevents mount:

    mkdir /tmp/test
    mount -t cgroup2 -o pids_localevents none /tmp/test
    mkdir /tmp/test/t
    echo 1 > /tmp/test/t/pids.max
    cat /tmp/test/t/pids.events                 # max 0
    timeout 3 inotifywait -e modify /tmp/test/t/pids.events &
    sh -c 'echo $$ > /tmp/test/t/cgroup.procs; (true &)' 2>/dev/null
    wait
    cat /tmp/test/t/pids.events                 # max 1

Without this patch, inotifywait times out without reporting an event.
Notify pids.events before returning from the local event path.

Fixes: 3f26a885a068 ("cgroup/pids: Add pids.events.local")
Signed-off-by: Guopeng Zhang <zhangguopeng@kylinos.cn>
---
 kernel/cgroup/pids.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/kernel/cgroup/pids.c b/kernel/cgroup/pids.c
index ecbb839d2acb..8444e270554a 100644
--- a/kernel/cgroup/pids.c
+++ b/kernel/cgroup/pids.c
@@ -253,6 +253,11 @@ static void pids_event(struct pids_cgroup *pids_forking,
 	}
 	if (!cgroup_subsys_on_dfl(pids_cgrp_subsys) ||
 	    cgrp_dfl_root.flags & CGRP_ROOT_PIDS_LOCAL_EVENTS) {
+		/*
+		 * pids.events reports the local counter on legacy hierarchies and
+		 * when pids_localevents is enabled.
+		 */
+		cgroup_file_notify(&p->events_file);
 		cgroup_file_notify(&p->events_local_file);
 		return;
 	}
-- 
2.43.0