From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-182.mta0.migadu.com (out-182.mta0.migadu.com [91.218.175.182]) (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 58EBB36E46E for ; Wed, 11 Mar 2026 01:01:17 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.182 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1773190878; cv=none; b=IDjsP7AoRB2OI5qrul+i7O53CaN3vV2kq3z2TyAJylUQiW86+vNCl2SUpL1PGUg20xRbdyypNOatbehd4k7CIEel86y8shGHJhwYDu7TwO8ONeubg9SYAHp6wWOKUicrqa+99KD9gyje90NpY8b2wp3dPCKJjbNncUzC19pLeLg= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1773190878; c=relaxed/simple; bh=Cj6aYpBIjzHQp9dQbgNhURSsAIaiBqiBxPXxNoGgK5Y=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=u6gl6Ib7KV07peNE/umqudvEApP7PdCJ2S4U2i278byD7IX4KQYvpVCGd6r3nyzw2A0JenoUqnVOYCm5xHLo/GFrQDVOWteTCbwGWin5DUdvN3sj7fnrJ8Ziucb5hFiXCgmDRwwwoVKuueOYLNnbLcqakkH7oDl4IjHsv8Z7koQ= 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=Ccsx1/kW; arc=none smtp.client-ip=91.218.175.182 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="Ccsx1/kW" 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=1773190875; 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=ql30l3PkbeNcbzr2KL87n3zGyPvc/RHjLKkGfbTuIH0=; b=Ccsx1/kWEQE637Iz9fq15VANKbX+Sc52j5gJqkyyS8WiTnvG1rt0Nq26WNuowFqsaeVz5e TVv7q7ZfgqfBPvuvsPAIWVAO0My4sYJy6kLqoP5o/4+GC0kpOqROj3vc/aHkuK1lY3OYYT wOsJX6uvUs1m9KF6WlfWhSG4VIhvuQI= From: Shakeel Butt To: Tejun Heo Cc: Johannes Weiner , =?UTF-8?q?Michal=20Koutn=C3=BD?= , Chen Ridong , Jakub Kicinski , Meta kernel team , linux-mm@kvack.org, netdev@vger.kernel.org, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org Subject: [PATCH v2 2/3] cgroup: add lockless fast-path checks to cgroup_file_notify() Date: Tue, 10 Mar 2026 18:01:00 -0700 Message-ID: <20260311010101.3306366-3-shakeel.butt@linux.dev> In-Reply-To: <20260311010101.3306366-1-shakeel.butt@linux.dev> References: <20260311010101.3306366-1-shakeel.butt@linux.dev> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT Add lockless checks before acquiring cgroup_file_kn_lock: 1. READ_ONCE(cfile->kn) NULL check to skip torn-down files. 2. READ_ONCE(cfile->notified_at) rate-limit check to skip when within the notification interval. If within the interval, arm the deferred timer via timer_reduce() and confirm it is pending before returning -- if the timer fired in between, fall through to the lock path so the notification is not lost. Both checks have safe error directions -- a stale read can only cause unnecessary lock acquisition, never a missed notification. The critical section is simplified to just taking a kernfs_get() reference and updating notified_at. Annotate cfile->kn and cfile->notified_at write sites with WRITE_ONCE() to pair with the lockless readers. Reported-by: Jakub Kicinski Signed-off-by: Shakeel Butt --- Changes since v1: - Moves the timer arming and rate limiting out of lock. kernel/cgroup/cgroup.c | 30 +++++++++++++++++------------- 1 file changed, 17 insertions(+), 13 deletions(-) diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c index b3fbeadb2b5a..b00f4c3242e0 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); @@ -4429,7 +4429,7 @@ 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); } @@ -4685,21 +4685,25 @@ 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); + next = last + CGROUP_FILE_NOTIFY_MIN_INTV; + if (time_in_range(jiffies, last, next)) { + timer_reduce(&cfile->notify_timer, next); + if (timer_pending(&cfile->notify_timer)) + 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; - - if (time_in_range(jiffies, last, next)) { - timer_reduce(&cfile->notify_timer, next); - } else { - kn = cfile->kn; - kernfs_get(kn); - cfile->notified_at = jiffies; - } + kn = cfile->kn; + kernfs_get(kn); + WRITE_ONCE(cfile->notified_at, jiffies); } spin_unlock_irqrestore(&cgroup_file_kn_lock, flags); -- 2.52.0