From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 9B385D58E48 for ; Mon, 2 Mar 2026 17:00:31 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id DB7396B0005; Mon, 2 Mar 2026 12:00:30 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id D64EC6B0089; Mon, 2 Mar 2026 12:00:30 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id C70906B0092; Mon, 2 Mar 2026 12:00:30 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0013.hostedemail.com [216.40.44.13]) by kanga.kvack.org (Postfix) with ESMTP id B5BF16B0005 for ; Mon, 2 Mar 2026 12:00:30 -0500 (EST) Received: from smtpin07.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay03.hostedemail.com (Postfix) with ESMTP id 72F99B73C3 for ; Mon, 2 Mar 2026 17:00:30 +0000 (UTC) X-FDA: 84501736620.07.4E1C905 Received: from out-177.mta1.migadu.com (out-177.mta1.migadu.com [95.215.58.177]) by imf27.hostedemail.com (Postfix) with ESMTP id 843CD4001B for ; Mon, 2 Mar 2026 17:00:28 +0000 (UTC) Authentication-Results: imf27.hostedemail.com; dkim=pass header.d=linux.dev header.s=key1 header.b=QM+FemWK; spf=pass (imf27.hostedemail.com: domain of shakeel.butt@linux.dev designates 95.215.58.177 as permitted sender) smtp.mailfrom=shakeel.butt@linux.dev; dmarc=pass (policy=none) header.from=linux.dev ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1772470828; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=WbAs9ykKjGj1acVkdZLpnTflSlmI29KTifSwOcqBl6o=; b=42+qOf1PqswP6VLBSsqOmHRvj6IyuDuruJjB5CBi0+b7lgPofRanB7sEHom3Hx2Ol9u2Pt CzOzJVGu5tb1w/4KT3r4f/EiTR/hHGqtsMmQBlYmmE54iWNGl3hhM+28sPdKSPkgYolMhJ vPXZrvUsb3o1NL2rGTWOu9D6yn27aDQ= ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1772470828; a=rsa-sha256; cv=none; b=TcHcFSjEIpvCSRg4k9c7rV8mWeCyTo6GyXKWM+gnMBQ4Jyiyn1r/V7RJ6SKM4OqMFXcENK BObIUhP2mD3mTTJDFgSJWAfd6jCQcQMqCZWsyfeZ9+3/vF9mI52lEdURPX5XR3GLDei/BB TVPrDhpQJw+Yka7QuRQHCTZDQ6Dk1Kw= ARC-Authentication-Results: i=1; imf27.hostedemail.com; dkim=pass header.d=linux.dev header.s=key1 header.b=QM+FemWK; spf=pass (imf27.hostedemail.com: domain of shakeel.butt@linux.dev designates 95.215.58.177 as permitted sender) smtp.mailfrom=shakeel.butt@linux.dev; dmarc=pass (policy=none) header.from=linux.dev Date: Mon, 2 Mar 2026 09:00:10 -0800 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1772470826; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=WbAs9ykKjGj1acVkdZLpnTflSlmI29KTifSwOcqBl6o=; b=QM+FemWKuEjzR06V0+l7g/FTA9BP0C/iPlSfw9pkv9G5M1hdQfOly2TwTD4+rNwMhqH/bn T+8kvhEDBbuIhVFJ8GbNVA5bJCEsWUXglP1KG0rc5R/dx16wW2t2fj/+Z3PO0epJAgbm6B /qIfXUGfLqj4AceUt5U7OJ0Eh/hTSwA= X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. 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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Migadu-Flow: FLOW_OUT X-Rspam-User: X-Rspamd-Server: rspam11 X-Rspamd-Queue-Id: 843CD4001B X-Stat-Signature: 4xmfechwrjuut5gjq98e4qmukecb9ssg X-HE-Tag: 1772470828-389429 X-HE-Meta: U2FsdGVkX19y79m/aaxfmXvcg2qgAf0xapV668uH/lU2g7U/ram1iND1RykuVoyUqJmFJTSF1GGRF/hWsHq92NndfCOPQbLapxoZZKgQEPrTlrBJcfzOE3Kko4nnuuMlvSKjNWnheyETDkbiyP0kvYWKzSmZAf3DgJmnXrVTLX/vSQ9uDvdAVCqU2ovscoTMbpgWpRKzUCWfnMXcJgw/hjtVq6J/U1Ft5YhabJ2xk3zw4mK09QUTItzJqqkCDlhv87VFpk9gGZd5na1bunU2ZLC5qI+QZtYKJFjs4o6JNQZhM+cU/Fs1NHkUrl+pZuzIRVwXyfXbha3kuzr+XZ7lRmgARPYqyPam8/JHoTdobmLAP2PBC95TNwcXgdr5A+w0Xw3BQq+JrRT2grI5tpYkYQaRtgWhg1QFZ5AdOhAe6KjucPCYq/incSzuV6l0cw5Pw4iYlyEma0S0Ztfv/t+/Ycl6jfWyaRja0b4t/uwmI5umTeSKw1biL9qAvudTS2S4pqw32Jqcg2QVLrTmgYydsO3VuEQm7pUzlcsdPmiQ8H7IU9GWJWE/oAvLrRxyw0fljJ3/AB00m72bXGDNnZJuU/edvGiO8a7RN9D6PFoo1a/o4+DLYLmzKLLEjStVis5+sWS3qSV7a3lJ4HItQyaNnGFF/X4V8SYv5xzijaFLjVgv2txwUeitRZ2gT5+UG7Bd8G3EV5OiaPiYDcWpbe/+me6JRjyesxPCD8sCdLicoQy/mI3J25cJPW4ZGm6ILmXANN2uj0ZRFUUkFRRMkIMxoRUwfJDfNxCIQxHHYtfOa8djHM4JNxdpM7JF3B0LWuDuWeGUwdb/42z/L9/JfdZvMGrPgLS9v+KDI1wnDf5/G+ffnZs8SbUYW/lJvmgwJ2MOPRfDTnD8bWCfTLyn8TOlWen0ABYNJJmoBfVMIF2Sz9MclwfHGhB2dYyx5WRLSAM61ykQBwzGlsbVgOI6PwJ P9100tla JjO72lV+I7t0xwEh0+5KojoWifNqVQsC9OnCn+qCI9CwpNZ9qYKEB5WBLXiGuFAWgi6S2ZoL+8H+yYNU+oFfUgRv56zd/JiJ5FJU8X72x6qmk8CzHcCDDhUKzigtZIdq/It8vev0MiD1gMAeSC6iymLjn89bnN6i5R00FRSGnGlvngQ+ksNkZVQp56tieiRgKZHE2Bq9lCJQvHCSjLhddnIxQJumdsDK1wbgrhGmgluHFKt2c7HslfqpCrDTUB3EOhKqLoMztsaLquSoewCEfR65rvQ== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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