cgroup/pids: Avoid spurious event notification

Currently when a process in a group forks and fails due to it's
parent's max restriction, all the cgroups from 'pids_forking' to root
will generate event notifications but only the cgroups from
'pids_over_limit' to root will increase the counter of PIDCG_MAX.

Consider this scenario: there are 4 groups A, B, C,and D, the
relationships are as follows, and user is watching on C.pids.events.

root->A->B->C->D

When a process in D forks and fails due to B.max restriction, the
user will get a spurious event notification because when he wakes up
and reads C.pids.events, he will find that the content has not changed.

To address this issue, only the cgroups from 'pids_over_limit' to root
will have their PIDCG_MAX counters increased and event notifications
generated.

Fixes: 385a635c ("cgroup/pids: Make event counters hierarchical")
Signed-off-by: Xiu Jianfeng <xiujianfeng@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/cgroup/pids.c

@@ -244,7 +244,6 @@ static void pids_event(struct pids_cgroup *pids_forking,
 		       struct pids_cgroup *pids_over_limit)
 {
 	struct pids_cgroup *p = pids_forking;
-	bool limit = false;
 
 	/* Only log the first time limit is hit. */
 	if (atomic64_inc_return(&p->events_local[PIDCG_FORKFAIL]) == 1) {
@@ -252,20 +251,17 @@ static void pids_event(struct pids_cgroup *pids_forking,
 		pr_cont_cgroup_path(p->css.cgroup);
 		pr_cont("\n");
 	}
-	cgroup_file_notify(&p->events_local_file);
 	if (!cgroup_subsys_on_dfl(pids_cgrp_subsys) ||
-	    cgrp_dfl_root.flags & CGRP_ROOT_PIDS_LOCAL_EVENTS)
+	    cgrp_dfl_root.flags & CGRP_ROOT_PIDS_LOCAL_EVENTS) {
+		cgroup_file_notify(&p->events_local_file);
 		return;
+	}
 
-	for (; parent_pids(p); p = parent_pids(p)) {
-		if (p == pids_over_limit) {
-			limit = true;
-			atomic64_inc(&p->events_local[PIDCG_MAX]);
-			cgroup_file_notify(&p->events_local_file);
-		}
-		if (limit)
-			atomic64_inc(&p->events[PIDCG_MAX]);
+	atomic64_inc(&pids_over_limit->events_local[PIDCG_MAX]);
+	cgroup_file_notify(&pids_over_limit->events_local_file);
 
+	for (p = pids_over_limit; parent_pids(p); p = parent_pids(p)) {
+		atomic64_inc(&p->events[PIDCG_MAX]);
 		cgroup_file_notify(&p->events_file);
 	}
 }