On Mon, Mar 02, 2026 at 08:14:05AM -0800, Shakeel Butt wrote:

Hi Chen, thanks for taking a look.

On Mon, Mar 02, 2026 at 09:50:53AM +0800, Chen Ridong wrote:

quoted

[...]

quoted

quoted

+	last = READ_ONCE(cfile->notified_at);
+	if (time_before_eq(jiffies, last + CGROUP_FILE_NOTIFY_MIN_INTV))
+		return;
+

Previously, if a notification arrived within the rate-limit window, we would
still call timer_reduce(&cfile->notify_timer, next) to schedule a deferred
notification.

With this change, returning early here bypasses that timer scheduling entirely.
Does this risk missing notifications that would have been delivered by the timer?

You are indeed right that this can cause missed notifications. After giving some
thought I think the lockless check-and-return can be pretty much simplified to
timer_pending() check. If timer is active, just do nothing and the notification
will be delivered eventually.

I will send the updated version soon. Any comments on the other two patches?

Something like the following:

From 598199723b50813b015393122796f6775eee02d7 Mon Sep 17 00:00:00 2001
From: Shakeel Butt <shakeel.butt@linux.dev>
Date: Sat, 28 Feb 2026 04:01:28 -0800
Subject: [PATCH] cgroup: add lockless fast-path checks to cgroup_file_notify()

Add two lockless checks before acquiring the lock:

1. READ_ONCE(cfile->kn) NULL check to skip torn-down files.
2. timer_pending() check to skip when a deferred notification
   timer is already armed.

Both checks have safe error directions -- a stale read can only
cause unnecessary lock acquisition, never a missed notification.

Annotate cfile->kn write sites with WRITE_ONCE() to pair with the
lockless reader.

Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
Reported-by: Jakub Kicinski <kuba@kernel.org>
---
 kernel/cgroup/cgroup.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 2b298a2cf206..6e816d27ee25 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c

@@ -1749,7 +1749,7 @@ static void cgroup_rm_file(struct cgroup *cgrp, const struct cftype *cft)
 		struct cgroup_file *cfile = (void *)css + cft->file_offset;
 
 		spin_lock_irq(&cgroup_file_kn_lock);
-		cfile->kn = NULL;
+		WRITE_ONCE(cfile->kn, NULL);
 		spin_unlock_irq(&cgroup_file_kn_lock);
 
 		timer_delete_sync(&cfile->notify_timer);

@@ -4430,7 +4430,7 @@ static int cgroup_add_file(struct cgroup_subsys_state *css, struct cgroup *cgrp,
 		timer_setup(&cfile->notify_timer, cgroup_file_notify_timer, 0);
 
 		spin_lock_irq(&cgroup_file_kn_lock);
-		cfile->kn = kn;
+		WRITE_ONCE(cfile->kn, kn);
 		spin_unlock_irq(&cgroup_file_kn_lock);
 	}
 

@@ -4689,6 +4689,12 @@ void cgroup_file_notify(struct cgroup_file *cfile)
 	unsigned long flags;
 	struct kernfs_node *kn = NULL;
 
+	if (!READ_ONCE(cfile->kn))
+		return;
+
+	if (timer_pending(&cfile->notify_timer))
+		return;
+
 	spin_lock_irqsave(&cgroup_file_kn_lock, flags);
 	if (cfile->kn) {
 		unsigned long last = cfile->notified_at;

-- 
2.47.3