From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-188.mta0.migadu.com (out-188.mta0.migadu.com [91.218.175.188]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 32D3738736D for ; Sat, 28 Feb 2026 14:20:56 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.188 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1772288458; cv=none; b=iIR/M1dA4B0kIZwv4WYB28zJ4e8ehck9GTypbeHYoPcbUF1P6vvgeH0jIUpDchJer2a64ABygvx8xr+F2P7N/Tjzlq7sZ9tUBdU3gYm8hI2IkNWZK3maKVNHT2u9T5u7vLLp/1o8OXuMhnq0mTpF2iQiEV8IF6qse4FcHj2D71M= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1772288458; c=relaxed/simple; bh=nBmlPo5O/CjJtH+LnUDf9peXd0tMjIrIR3KIGzfZdA4=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=GRaQ18lvdeAar73SZ/jp/dFJrvJnrJyZAkgmEhpz2Z74HYETLvnWV82I+EVe4jkSTvzuT9j9ORbGLY+HZEIaT21Uk9s9teKU5eEOn3OuCfCtUTyWkeKyUqvbHqBXeSUft/Cjf4dV1vzBMhyjiOJSagFfJmJNPgYdpq2tBtjcqJ0= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=CoTmr6uO; arc=none smtp.client-ip=91.218.175.188 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="CoTmr6uO" X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1772288445; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=mBR7DSPv0SQTUni7LYAlRsXlj4uQvQDqYNhgjcl//5E=; b=CoTmr6uOZUtm/aSGvKdQHWKa+316V6kLKQ3lL9j/1AgwB07qO6ga1qSdBot1NMHwMCvzqI Nc8JlGKFJYpmR3QLBk0JvXCLav1konHLC2VKHKBodvXDu1asnQl0bmvbQawP4tOvijwRFz 5XerhCSv6PraS2o22S9wCKt/q2MOwtE= From: Shakeel Butt To: Tejun Heo Cc: Johannes Weiner , =?UTF-8?q?Michal=20Koutn=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: [PATCH 2/3] cgroup: add lockless fast-path checks to cgroup_file_notify() Date: Sat, 28 Feb 2026 06:20:17 -0800 Message-ID: <20260228142018.3178529-3-shakeel.butt@linux.dev> In-Reply-To: <20260228142018.3178529-1-shakeel.butt@linux.dev> References: <20260228142018.3178529-1-shakeel.butt@linux.dev> Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT Add two lockless checks before acquiring the lock: 1. READ_ONCE(cfile->kn) NULL check to skip torn-down files. 2. READ_ONCE(cfile->notified_at) check to skip when within the rate-limit window (~10ms). Both checks have safe error directions -- a stale read can only cause unnecessary lock acquisition, never a missed notification. Annotate all write sites with WRITE_ONCE() to pair with the lockless readers. The trade-off is that trailing timer_reduce() calls during bursts are skipped, so the deferred notification that delivers the final state may be lost. This is acceptable for the primary callers like __memcg_memory_event() where events keep arriving. Signed-off-by: Shakeel Butt Reported-by: Jakub Kicinski --- kernel/cgroup/cgroup.c | 21 ++++++++++++++------- 1 file changed, 14 insertions(+), 7 deletions(-) diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c index 33282c7d71e4..5473ebd0f6c1 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); } @@ -4686,20 +4686,27 @@ int cgroup_add_legacy_cftypes(struct cgroup_subsys *ss, struct cftype *cfts) */ void cgroup_file_notify(struct cgroup_file *cfile) { - unsigned long flags; + unsigned long flags, last, next; struct kernfs_node *kn = NULL; + if (!READ_ONCE(cfile->kn)) + return; + + last = READ_ONCE(cfile->notified_at); + if (time_before_eq(jiffies, last + CGROUP_FILE_NOTIFY_MIN_INTV)) + return; + spin_lock_irqsave(&cgroup_file_kn_lock, flags); if (cfile->kn) { - unsigned long last = cfile->notified_at; - unsigned long next = last + CGROUP_FILE_NOTIFY_MIN_INTV; + last = cfile->notified_at; + next = last + CGROUP_FILE_NOTIFY_MIN_INTV; - if (time_in_range(jiffies, last, next)) { + if (time_before_eq(jiffies, next)) { timer_reduce(&cfile->notify_timer, next); } else { kn = cfile->kn; kernfs_get(kn); - cfile->notified_at = jiffies; + WRITE_ONCE(cfile->notified_at, jiffies); } } spin_unlock_irqrestore(&cgroup_file_kn_lock, flags); -- 2.47.3