[PATCH 1/2] cgroup: Fix incorrect warning from cgroup_apply_control_disable()

[Waiman Long <longman-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>]

It was found that the following warning was displayed when remounting
controllers from cgroup v2 to v1:

[ 8042.997778] WARNING: CPU: 88 PID: 80682 at kernel/cgroup/cgroup.c:3130 cgroup_apply_control_disable+0x158/0x190
   :
[ 8043.091109] RIP: 0010:cgroup_apply_control_disable+0x158/0x190
[ 8043.096946] Code: ff f6 45 54 01 74 39 48 8d 7d 10 48 c7 c6 e0 46 5a a4 e8 7b 67 33 00 e9 41 ff ff ff 49 8b 84 24 e8 01 00 00 0f b7 40 08 eb 95 <0f> 0b e9 5f ff ff ff 48 83 c4 08 5b 5d 41 5c 41 5d 41 5e 41 5f c3
[ 8043.115692] RSP: 0018:ffffba8a47c23d28 EFLAGS: 00010202
[ 8043.120916] RAX: 0000000000000036 RBX: ffffffffa624ce40 RCX: 000000000000181a
[ 8043.128047] RDX: ffffffffa63c43e0 RSI: ffffffffa63c43e0 RDI: ffff9d7284ee1000
[ 8043.135180] RBP: ffff9d72874c5800 R08: ffffffffa624b090 R09: 0000000000000004
[ 8043.142314] R10: ffffffffa624b080 R11: 0000000000002000 R12: ffff9d7284ee1000
[ 8043.149447] R13: ffff9d7284ee1000 R14: ffffffffa624ce70 R15: ffffffffa6269e20
[ 8043.156576] FS:  00007f7747cff740(0000) GS:ffff9d7a5fc00000(0000) knlGS:0000000000000000
[ 8043.164663] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 8043.170409] CR2: 00007f7747e96680 CR3: 0000000887d60001 CR4: 00000000007706e0
[ 8043.177539] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[ 8043.184673] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[ 8043.191804] PKRU: 55555554
[ 8043.194517] Call Trace:
[ 8043.196970]  rebind_subsystems+0x18c/0x470
[ 8043.201070]  cgroup_setup_root+0x16c/0x2f0
[ 8043.205177]  cgroup1_root_to_use+0x204/0x2a0
[ 8043.209456]  cgroup1_get_tree+0x3e/0x120
[ 8043.213384]  vfs_get_tree+0x22/0xb0
[ 8043.216883]  do_new_mount+0x176/0x2d0
[ 8043.220550]  __x64_sys_mount+0x103/0x140
[ 8043.224474]  do_syscall_64+0x38/0x90
[ 8043.228063]  entry_SYSCALL_64_after_hwframe+0x44/0xae

It was caused by the "WARN_ON_ONCE(percpu_ref_is_dying(&css->refcnt))"
check. It was because the css had not been completely removed from
a previous call to css_kill() by cgroup_apply_control_disable() and
so the warning was triggered when cgroup_apply_control_disable() was
called again soon afterward.

Eliminating this incorrect warning by using percpu_ref_is_dying() as
escape for further action instead.

Fixes: 945ba19968888 ("cgroup: combine cgroup_mutex locking and offline css draining")
Signed-off-by: Waiman Long <longman-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
---
 kernel/cgroup/cgroup.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 881ce1470beb..e31bca9fcd46 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -3140,7 +3140,16 @@ static void cgroup_apply_control_disable(struct cgroup *cgrp)
 			if (!css)
 				continue;
 
-			WARN_ON_ONCE(percpu_ref_is_dying(&css->refcnt));
+			/*
+			 * A kill_css() might have been called previously, but
+			 * the css may still linger for a while before being
+			 * removed. Skip it in this case.
+			 */
+			if (percpu_ref_is_dying(&css->refcnt)) {
+				WARN_ON_ONCE(css->parent &&
+					cgroup_ss_mask(dsct) & (1 << ss->id));
+				continue;
+			}
 
 			if (css->parent &&
 			    !(cgroup_ss_mask(dsct) & (1 << ss->id))) {
-- 
2.18.1