Re: [PATCH 2/3] cgroup: add lockless fast-path checks to cgroup_file_notify()

[Shakeel Butt <shakeel.butt@linux.dev>]

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:
> > 
[...]
> > > +	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