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 B7ADAC433EF for ; Wed, 3 Nov 2021 00:02:17 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 84C0660F02 for ; Wed, 3 Nov 2021 00:02:17 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230293AbhKCAEw (ORCPT ); Tue, 2 Nov 2021 20:04:52 -0400 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:30143 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230059AbhKCAEv (ORCPT ); Tue, 2 Nov 2021 20:04:51 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1635897735; 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=cqJQisP1QPS+4wMPSGYJMWn3HlqeTze3DoRoDxtXcVY=; b=HREh8hWlhCgMqnmW/rKxDYygvwAt8SSNUhm4H8YMAx63BGkzY6M+DiHyuytBICRUvXaJd7 wrlLV9KA407LrIwk5Lr2aSi9c6oYdSBIBORSW92Es0R9hZzRB/xlpF5E0e5+UD+VE261vK p01/+OTvkf6cX0p9W7AgLS9JPVJzJ60= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-469-bzhDjEpxNyW8kBJ_BBDKpQ-1; Tue, 02 Nov 2021 20:02:14 -0400 X-MC-Unique: bzhDjEpxNyW8kBJ_BBDKpQ-1 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.phx2.redhat.com [10.5.11.12]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id E769A8066EB; Wed, 3 Nov 2021 00:02:09 +0000 (UTC) Received: from T590 (ovpn-8-17.pek2.redhat.com [10.72.8.17]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 9128C60C0F; Wed, 3 Nov 2021 00:01:49 +0000 (UTC) Date: Wed, 3 Nov 2021 08:01:45 +0800 From: Ming Lei To: Luis Chamberlain Cc: Petr Mladek , Miroslav Benes , Julia Lawall , Benjamin Herrenschmidt , Paul Mackerras , tj@kernel.org, gregkh@linuxfoundation.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, ming.lei@redhat.com 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: X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 Precedence: bulk List-ID: X-Mailing-List: linux-spdx@vger.kernel.org On Tue, Nov 02, 2021 at 09:25:44AM -0700, Luis Chamberlain wrote: > On Tue, Nov 02, 2021 at 04:24:06PM +0100, Petr Mladek wrote: > > On Wed 2021-10-27 13:57:40, Miroslav Benes wrote: > > > >From my perspective, it is quite easy to get it wrong due to either a lack > > > of generic support, or missing rules/documentation. So if this thread > > > leads to "do not share locks between a module removal and a sysfs > > > operation" strict rule, it would be at least something. In the same > > > manner as Luis proposed to document try_module_get() expectations. > > > > The rule "do not share locks between a module removal and a sysfs > > operation" is not clear to me. > > That's exactly it. It *is* not. The test_sysfs selftest will hopefully > help with this. But I'll wait to take a final position on whether or not > a generic fix should be merged until the Coccinelle patch which looks > for all uses cases completes. > > So I think that once that Coccinelle hunt is done for the deadlock, we > should also remind folks of the potential deadlock and some of the rules > you mentioned below so that if we take a position that we don't support > this, we at least inform developers why and what to avoid. If Coccinelle > finds quite a bit of cases, then perhaps evaluating the generic fix > might be worth evaluating. > > > IMHO, there are the following rules: > > > > 1. rule: kobject_del() or kobject_put() must not be called under a lock that > > is used by store()/show() callbacks. > > > > reason: kobject_del() waits until the sysfs interface is destroyed. > > It has to wait until all store()/show() callbacks are finished. > > Right, this is what actually started this entire conversation. > > Note that as Ming pointed out, the generic kernfs fix I proposed would > only cover the case when kobject_del() ends up being called on module > exit, so it would not cover the cases where perhaps kobject_del() might > be called outside of module exit, and so the cope of the possible > deadlock then increases in scope. > > Likewise, the Coccinelle hunt I'm trying would only cover the module > exit case. I'm a bit of afraid of the complexity of a generic hunt > as expresed in rule 1. Question is that why one shared lock is required between kobject_del() and its show()/store(), both zram and livepatch needn't that. Is it one common usage? > > > > > 2. rule: kobject_del()/kobject_put() must not be called from the > > related store() callbacks. > > > > reason: same as in 1st rule. > > Sensible corollary. > > Given tha the exact kobjet_del() / kobject_put() which must not be > called from the respective sysfs ops depends on which kobject is > underneath the device for which the sysfs ops is being created, > it would make this hunt in Coccinelle a bit tricky. My current iteration > of a coccinelle hunt cheats and looks at any sysfs looking op and > ensures a module exit exists. Actually kernfs/sysfs provides interface for supporting deleting kobject/attr from the attr's show()/store(), see example of sdev_store_delete(), and the livepatch example: https://lore.kernel.org/lkml/20211102145932.3623108-4-ming.lei@redhat.com/ > > > 3. rule: module_exit() must wait until all release() callbacks are called > > when kobject are static. > > > > reason: kobject_put() must be called to clean up internal > > dependencies. The clean up might be done asynchronously > > and need access to the kobject structure. > > This might be an easier rule to implement a respective Coccinelle rule > for. If kobject_del() is done in module_exit() or before module_exit(), kobject should have been freed in module_exit() via kobject_put(). But yes, it can be asynchronously because of CONFIG_DEBUG_KOBJECT_RELEASE, seems like one real issue. Thanks, Ming