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 mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id EB816C43219 for ; Wed, 20 Oct 2021 08:28:58 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id CF29561212 for ; Wed, 20 Oct 2021 08:28:58 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229677AbhJTIbL (ORCPT ); Wed, 20 Oct 2021 04:31:11 -0400 Received: from mail.kernel.org ([198.145.29.99]:40668 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229632AbhJTIbK (ORCPT ); Wed, 20 Oct 2021 04:31:10 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id 3E5CF61183; Wed, 20 Oct 2021 08:28:56 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1634718536; bh=8nW1a1YxwhHLkokHLmbAPUvEy8Sro976Y9vnhAQOL7U=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=b5OhBK9vzw98Pra6K1wa2zJ0J5EzHVm1XCT+E1fH6mcqn/K9AU7vR477/yH5qRf2Y SwAeIQb04+15xwtWK1Tt7kdQGvlEAp4HnJ10Ae/gbnnLHh9ON0GTMtaH9GB8htEXdC PEBT4jNWWKAtQNxw7vujFQeR5PU+4RP0ONCEkVl4= Date: Wed, 20 Oct 2021 10:28:54 +0200 From: Greg KH To: Miroslav Benes Cc: Ming Lei , Luis Chamberlain , Benjamin Herrenschmidt , Paul Mackerras , tj@kernel.org, akpm@linux-foundation.org, minchan@kernel.org, jeyu@kernel.org, shuah@kernel.org, bvanassche@acm.org, dan.j.williams@intel.com, joe@perches.com, tglx@linutronix.de, keescook@chromium.org, rostedt@goodmis.org, linux-spdx@vger.kernel.org, linux-doc@vger.kernel.org, linux-block@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-kernel@vger.kernel.org, live-patching@vger.kernel.org Subject: Re: [PATCH v8 11/12] zram: fix crashes with cpu hotplug multistate Message-ID: References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On Wed, Oct 20, 2021 at 10:19:27AM +0200, Miroslav Benes wrote: > On Wed, 20 Oct 2021, Ming Lei wrote: > > > On Wed, Oct 20, 2021 at 08:43:37AM +0200, Miroslav Benes wrote: > > > On Tue, 19 Oct 2021, Ming Lei wrote: > > > > > > > On Tue, Oct 19, 2021 at 08:23:51AM +0200, Miroslav Benes wrote: > > > > > > > By you only addressing the deadlock as a requirement on approach a) you are > > > > > > > forgetting that there *may* already be present drivers which *do* implement > > > > > > > such patterns in the kernel. I worked on addressing the deadlock because > > > > > > > I was informed livepatching *did* have that issue as well and so very > > > > > > > likely a generic solution to the deadlock could be beneficial to other > > > > > > > random drivers. > > > > > > > > > > > > In-tree zram doesn't have such deadlock, if livepatching has such AA deadlock, > > > > > > just fixed it, and seems it has been fixed by 3ec24776bfd0. > > > > > > > > > > I would not call it a fix. It is a kind of ugly workaround because the > > > > > generic infrastructure lacked (lacks) the proper support in my opinion. > > > > > Luis is trying to fix that. > > > > > > > > What is the proper support of the generic infrastructure? I am not > > > > familiar with livepatching's model(especially with module unload), you mean > > > > livepatching have to do the following way from sysfs: > > > > > > > > 1) during module exit: > > > > > > > > mutex_lock(lp_lock); > > > > kobject_put(lp_kobj); > > > > mutex_unlock(lp_lock); > > > > > > > > 2) show()/store() method of attributes of lp_kobj > > > > > > > > mutex_lock(lp_lock) > > > > ... > > > > mutex_unlock(lp_lock) > > > > > > Yes, this was exactly the case. We then reworked it a lot (see > > > 958ef1e39d24 ("livepatch: Simplify API by removing registration step"), so > > > now the call sequence is different. kobject_put() is basically offloaded > > > to a workqueue scheduled right from the store() method. Meaning that > > > Luis's work would probably not help us currently, but on the other hand > > > the issues with AA deadlock were one of the main drivers of the redesign > > > (if I remember correctly). There were other reasons too as the changelog > > > of the commit describes. > > > > > > So, from my perspective, if there was a way to easily synchronize between > > > a data cleanup from module_exit callback and sysfs/kernfs operations, it > > > could spare people many headaches. > > > > kobject_del() is supposed to do so, but you can't hold a shared lock > > which is required in show()/store() method. Once kobject_del() returns, > > no pending show()/store() any more. > > > > The question is that why one shared lock is required for livepatching to > > delete the kobject. What are you protecting when you delete one kobject? > > I think it boils down to the fact that we embed kobject statically to > structures which livepatch uses to maintain data. That is discouraged > generally, but all the attempts to implement it correctly were utter > failures. Sounds like this is the real problem that needs to be fixed. kobjects should always control the lifespan of the structure they are embedded in. If not, then that is a design flaw of the user of the kobject :( Where in the kernel is this happening? And where have been the attempts to fix this up? thanks, greg k-h