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From: Shakeel Butt To: Chen Ridong Cc: Tejun Heo , Johannes Weiner , Michal =?utf-8?Q?Koutn=C3=BD?= , Roman Gushchin , Kuniyuki Iwashima , Daniel Sedlak , Meta kernel team , linux-mm@kvack.org, netdev@vger.kernel.org, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org, Jakub Kicinski Subject: Re: [PATCH 2/3] cgroup: add lockless fast-path checks to cgroup_file_notify() Message-ID: References: <20260228142018.3178529-1-shakeel.butt@linux.dev> <20260228142018.3178529-3-shakeel.butt@linux.dev> <40c77bba-0862-4422-b23e-2a10cd01c728@huaweicloud.com> Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Migadu-Flow: FLOW_OUT 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 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 Reported-by: Jakub Kicinski --- 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