[PATCH] cgroup: Keep favordynmods enabled once per-threadgroup rwsem is active

[Guopeng Zhang]

cgroup_enable_per_threadgroup_rwsem is a one-way switch. Once it is
enabled, cgroup.procs writes use the per-threadgroup rwsem and
cgroup_threadgroup_change_begin()/end() use the same global state to
decide whether to take and release the per-threadgroup rwsem.

The disable path warned that the per-threadgroup rwsem mechanism could not
be disabled but still called rcu_sync_exit() and cleared
CGRP_ROOT_FAVOR_DYNMODS. That partially disabled favordynmods while the
global per-threadgroup rwsem mode remained enabled: cgroup.procs writes
would continue to use the per-threadgroup rwsem, while
cgroup_threadgroup_change_begin()/end() could observe the exited rcu_sync
state. The root would also no longer report favordynmods.

Make the transition match the documented one-way semantics. Call
rcu_sync_enter() only for the first favordynmods enable, and make later
disable attempts a no-op after warning once the per-threadgroup rwsem mode
has been enabled.

Fixes: 0568f89d4fb8 ("cgroup: replace global percpu_rwsem with per threadgroup resem when writing to cgroup.procs")
Signed-off-by: Guopeng Zhang <zhangguopeng@xxxxxxxxxx>
---
Manual AB test:

Before this patch:
  enable favordynmods:
    cgroup2 opts: rw,relatime,favordynmods
  disable attempt:
    cgroup2 opts: rw,relatime
  dmesg:
    cgroup: cgroup favordynmods: per threadgroup rwsem mechanism can't be disabled

After this patch:
  enable favordynmods:
    cgroup2 opts: rw,relatime,favordynmods
  disable attempt:
    cgroup2 opts: rw,relatime,favordynmods
  dmesg:
    cgroup: cgroup favordynmods: per threadgroup rwsem mechanism can't be disabled

 kernel/cgroup/cgroup.c | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 6152add0c5eb..fd10fb5b3598 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -1297,14 +1297,13 @@ void cgroup_favor_dynmods(struct cgroup_root *root, bool favor)
 	 */
 	percpu_down_write(&cgroup_threadgroup_rwsem);
 	if (favor && !favoring) {
-		cgroup_enable_per_threadgroup_rwsem = true;
-		rcu_sync_enter(&cgroup_threadgroup_rwsem.rss);
+		if (!cgroup_enable_per_threadgroup_rwsem) {
+			cgroup_enable_per_threadgroup_rwsem = true;
+			rcu_sync_enter(&cgroup_threadgroup_rwsem.rss);
+		}
 		root->flags |= CGRP_ROOT_FAVOR_DYNMODS;
 	} else if (!favor && favoring) {
-		if (cgroup_enable_per_threadgroup_rwsem)
-			pr_warn_once("cgroup favordynmods: per threadgroup rwsem mechanism can't be disabled\n");
-		rcu_sync_exit(&cgroup_threadgroup_rwsem.rss);
-		root->flags &= ~CGRP_ROOT_FAVOR_DYNMODS;
+		pr_warn_once("cgroup favordynmods: per threadgroup rwsem mechanism can't be disabled\n");
 	}
 	percpu_up_write(&cgroup_threadgroup_rwsem);
 }
-- 
2.43.0