[PATCH] cgroup/pid: fix the pid_cgrp attach bug in cgroup v2

[Hui Su]

pids_can_attach() should make sure the pids->counter not bigger
than pids->limit, so we should use pids_try_charge() here.

without the change:
root@test:/sys/fs/cgroup/test# cat pids.max
3
root@test:/sys/fs/cgroup/test# sleep 1000 &
[1] 3379
root@test:/sys/fs/cgroup/test# sleep 1000 &
[2] 3380
root@test:/sys/fs/cgroup/test# sleep 1000 &
[3] 3381
root@test:/sys/fs/cgroup/test# sleep 1000 &
[4] 3382
root@test:/sys/fs/cgroup/test# echo 3379 > cgroup.procs
root@test:/sys/fs/cgroup/test# echo 3380 > cgroup.procs
root@test:/sys/fs/cgroup/test# echo 3381 > cgroup.procs
root@test:/sys/fs/cgroup/test# echo 3382 > cgroup.procs
root@test:/sys/fs/cgroup/test# cat pids.max
3
root@test:/sys/fs/cgroup/test# cat pids.current
4
root@test:/sys/fs/cgroup/test# cat cgroup.procs
3379
3380
3381
3382
root@test:/sys/fs/cgroup/test#

with this change:
root@test:/sys/fs/cgroup/test# cat pids.current
3
root@test:/sys/fs/cgroup/test# cat pids.max
3
root@test:/sys/fs/cgroup/test# cat cgroup.procs
2614
3683
3684
root@test:/sys/fs/cgroup/test# sleep 10000 &
[5] 3733
root@test:/sys/fs/cgroup/test# echo 3733 > cgroup.procs
bash: echo: write error: Resource temporarily unavailable
root@test:/sys/fs/cgroup/test# cat cgroup.procs
2614
3683
3684
root@test:/sys/fs/cgroup/test# cat pids.current
3
root@test:/sys/fs/cgroup/test# dmesg |tail -n 1
[  863.612162] cgroup: attach rejected by pids controller in /test

Signed-off-by: Hui Su <suhui@xxxxxxxx>
---
 kernel/cgroup/pids.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/kernel/cgroup/pids.c b/kernel/cgroup/pids.c
index 511af87f685e..5e24990f28de 100644
--- a/kernel/cgroup/pids.c
+++ b/kernel/cgroup/pids.c
@@ -172,6 +172,7 @@ static int pids_can_attach(struct cgroup_taskset *tset)
 {
 	struct task_struct *task;
 	struct cgroup_subsys_state *dst_css;
+	int ret = 0;
 
 	cgroup_taskset_for_each(task, dst_css, tset) {
 		struct pids_cgroup *pids = css_pids(dst_css);
@@ -186,11 +187,17 @@ static int pids_can_attach(struct cgroup_taskset *tset)
 		old_css = task_css(task, pids_cgrp_id);
 		old_pids = css_pids(old_css);
 
-		pids_charge(pids, 1);
+		ret = pids_try_charge(pids, 1);
+		if (ret) {
+			pr_info("cgroup: attach rejected by pids controller in ");
+			pr_cont_cgroup_path(dst_css->cgroup);
+			pr_cont("\n");
+			break;
+		}
 		pids_uncharge(old_pids, 1);
 	}
 
-	return 0;
+	return ret;
 }
 
 static void pids_cancel_attach(struct cgroup_taskset *tset)
-- 
2.25.1