From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mta1.migadu.com (out-63.mta1.migadu.com [95.215.58.63]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 84B4F372ECD for ; Thu, 3 Sep 2026 06:15:23 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.63 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788416125; cv=none; b=FoEQJ3369Mes9cj7Q5eLTaeotVyEgpLfml7brP0e27n4mxuwy2JHZaZuZ7TDDSSav6mOS1IJeQLcFTgHhnSb16uLdoiB8UUpFLnxdtqxcGHbqp16aSFKuPKHvORgyW7mGlg5EWlv58J7X+SWuRbUNoRPQvfn3y/wgg/sHuuMbII= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788416125; c=relaxed/simple; bh=VW2G9+S+grYb3iReEIYAciUQOjbf6ENRNXOUTjyONew=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=sGqo41XXTsv7xOhpQLx5LbLt6rDetdfzSJtmHhakvjHn6ePiZMzARyYzvU039PN6bHqtu1OXpVOrctkHf9I5Tms5pZVwP+HYOkcKtp2r7jTucCYLqf/JmufwAyo5kd53ZsWhJg7KwDVyrp7D5PRmP8wTc2OJsQJZQjZ/dSN23Ts= 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=tBm7ZQTX; arc=none smtp.client-ip=95.215.58.63 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="tBm7ZQTX" X-Envelope-To: linux-kselftest@vger.kernel.org DKIM-Signature: a=rsa-sha256; bh=VW2G9+S+grYb3iReEIYAciUQOjbf6ENRNXOUTjyONew=; c=simple/simple; d=linux.dev; h=from:to:subject:date:message-id:mime-version:content-type; s=key1; t=1788416121; v=1; x=1789020921; b=tBm7ZQTX2Dm6E5zwJFfDBpbZS3DL2Rs/wANv0RSwBMOXI4lXSHIXmYMy8p8cm2m4+Lh/kbE5 xqJD8CWATxUJs4PCjcjjwxhxGwGSwfA7U014kcvM/Oi+qCSWyyRZScj8eneqDCWkUOhtHdUrqWs 3feaIJinHAYiEFgzb4CJClZU= X-Envelope-To: linux-kselftest@vger.kernel.org Received: by smtp.migadu.com with ESMTPS id 4d603cdf1e5a2158; Thu, 03 Sep 2026 06:15:21 +0000 X-Mizu-Trace-ID: 4d603cdf1e5a2158 X-Migadu-Flow: FLOW_OUT Date: Wed, 2 Sep 2026 23:15:16 -0700 From: Shakeel Butt To: Greg Kroah-Hartman Cc: Tejun Heo , Christian Brauner , Meta kernel team , linux-kselftest@vger.kernel.org, driver-core@lists.linux.dev, linux-kernel@vger.kernel.org Subject: Re: [PATCH 1/3] kernfs: take kernfs_rename_lock for same-parent renames too Message-ID: <20260903060812.GA18952@shakeel.butt@linux.dev> References: <20260903040253.670020-1-shakeel.butt@linux.dev> <2026090330-cytoplasm-resilient-900b@gregkh> <6a990797.3e7a366d.3bd849.72d1SMTPIN_ADDED_BROKEN@mx.google.com> <2026090351-turbofan-tux-d75e@gregkh> Precedence: bulk X-Mailing-List: linux-kselftest@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: <2026090351-turbofan-tux-d75e@gregkh> On Thu, Sep 03, 2026 at 07:41:53AM +0200, Greg Kroah-Hartman wrote: > On Wed, Sep 02, 2026 at 10:37:21PM -0700, Shakeel Butt wrote: > > On Thu, Sep 03, 2026 at 06:31:58AM +0200, Greg Kroah-Hartman wrote: > > > On Wed, Sep 02, 2026 at 09:02:51PM -0700, Shakeel Butt wrote: > > > > kernfs_rename_ns() only takes kernfs_rename_lock when the rename moves > > > > the node to a new parent. A rename that keeps the same parent, like > > > > renaming a network interface, changes kernfs_node::name with only > > > > kernfs_rwsem held. So the lock protects ->__parent but not ->name, and > > > > a reader that wants a stable name has to take kernfs_rwsem, the same > > > > lock every path lookup needs. > > > > > > > > That also makes for a small but real bug. kernfs_path_from_node() > > > > takes kernfs_rename_lock for reading, and kernfs_path_from_node_locked() > > > > then reads the name of each ancestor. It reads each one once, so a > > > > single same-parent rename only moves the answer from the old path to the > > > > new one, but two of them landing inside one walk build a path that never > > > > existed: > > > > > > > > CPU0 CPU1 > > > > kernfs_path_from_node() on /a/b/c > > > > reads the name of a, gets "a" > > > > renames a to a2 > > > > renames b to b2 > > > > reads the name of b, gets "b2" > > > > returns "/a/b2/c" > > > > > > > > This hits roots without KERNFS_ROOT_INVARIANT_PARENT: sysfs, where the > > > > bad path can reach sysfs_warn_dup() and pr_cont_kernfs_path(), and > > > > resctrl, which renames a mon group inside its mon_groups directory. > > > > cgroup sets the flag, so it skips the lock and reads names under RCU > > > > alone; that case needs something else and is not addressed here. > > > > > > > > So take the lock in both cases, and let kernfs_rcu_name() accept it the > > > > way kernfs_parent() already does for ->__parent. Same-parent renames > > > > are rare, the lock is per filesystem, and the locked section is at most > > > > three stores. It also gives a future rename sequence counter one place > > > > to sit that covers every rename. > > > > > > > > Fixes: 741c10b096bc ("kernfs: Use RCU to access kernfs_node::name.") > > > > Signed-off-by: Shakeel Butt > > > > --- > > > > fs/kernfs/dir.c | 28 +++++++++++++++------------- > > > > fs/kernfs/kernfs-internal.h | 9 ++++++++- > > > > 2 files changed, 23 insertions(+), 14 deletions(-) > > > > > > How was this found and tested? Did you forget an Assisted-by: tag? > > > > I am working on a series to improve kernfs_rwsem and going through > > review-prompt with AI to review my series and these were existing > > issues AI found. I have created reproducers with AI for these and > > tested that these patches those. > > Then please read our documentation for how to properly document this > usage of a LLM tool. Sure > > If you have reproducers, please add them to the kernfs tests as well as > patches part of this series when you resend them. > The reproducers are like stress tests and are targeting race conditions. In one case delay was added to fully expose the race. I am not sure selftests is the right place for this kind of tests. I can just publish the reproducer on the list to have them on record if that is what you are looking for.