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 EA1D4C4332F for ; Tue, 2 Nov 2021 15:18:24 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id D165B603E5 for ; Tue, 2 Nov 2021 15:18:24 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234331AbhKBPU6 (ORCPT ); Tue, 2 Nov 2021 11:20:58 -0400 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:45471 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234301AbhKBPUx (ORCPT ); Tue, 2 Nov 2021 11:20:53 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1635866298; 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=6dODq+TaBd9o3SXxOkqAmjs0L7/fBuFRIhrp2FL47OI=; b=F9VS9tmcgCat/hgki00bgMT6zDvGWBADK8fLmTCEbv/Jm3uImiaIcaZk8dOKNZBCjKiavv lfGgWObkh4av7RwE+rIeZFjxuERjuS4egLD6lXWaCALYCTYiRa31zqCE8TkV+vFTqN7+Co gMTLWWcFSJaC15TDOAN8g8FCnIU6L+A= 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-339-AklUD-MXNQ26qIBOPeLekg-1; Tue, 02 Nov 2021 11:18:15 -0400 X-MC-Unique: AklUD-MXNQ26qIBOPeLekg-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 7AE5A362FC; Tue, 2 Nov 2021 15:18:11 +0000 (UTC) Received: from T590 (ovpn-8-19.pek2.redhat.com [10.72.8.19]) by smtp.corp.redhat.com (Postfix) with ESMTPS id D981F60C82; Tue, 2 Nov 2021 15:17:44 +0000 (UTC) Date: Tue, 2 Nov 2021 23:17:39 +0800 From: Ming Lei To: Petr Mladek Cc: Miroslav Benes , Luis Chamberlain , 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-kernel@vger.kernel.org On Tue, Nov 02, 2021 at 03:51:33PM +0100, Petr Mladek wrote: > On Tue 2021-11-02 15:15:19, Petr Mladek wrote: > > On Tue 2021-10-26 23:37:30, Ming Lei wrote: > > > On Tue, Oct 26, 2021 at 10:48:18AM +0200, Petr Mladek wrote: > > > > Below are more details about the livepatch code. I hope that it will > > > > help you to see if zram has similar problems or not. > > > > > > > > We have kobject in three structures: klp_func, klp_object, and > > > > klp_patch, see include/linux/livepatch.h. > > > > > > > > These structures have to be statically defined in the module sources > > > > because they define what is livepatched, see > > > > samples/livepatch/livepatch-sample.c > > > > > > > > The kobject is used there to show information about the patch, patched > > > > objects, and patched functions, in sysfs. And most importantly, > > > > the sysfs interface can be used to disable the livepatch. > > > > > > > > The problem with static structures is that the module must stay > > > > in the memory as long as the sysfs interface exists. It can be > > > > solved in module_exit() callback. It could wait until the sysfs > > > > interface is destroyed. > > > > > > > > kobject API does not support this scenario. The relase() callbacks > > > > > > kobject_delete() is for supporting this scenario, that is why we don't > > > need to grab module refcnt before calling show()/store() of the > > > kobject's attributes. > > > > > > kobject_delete() can be called in module_exit(), then any show()/store() > > > will be done after kobject_delete() returns. > > > > I am a bit confused. I do not see kobject_delete() anywhere in kernel > > sources. > > > > I see only kobject_del() and kobject_put(). AFAIK, they do _not_ > > guarantee that either the sysfs interface was destroyed or > > the release callbacks were called. For example, see > > schedule_delayed_work(&kobj->release, delay) in kobject_release(). > > Grr, I always get confused by the code. kobject_del() actually waits > until the sysfs interface gets destroyed. This is why there is > the deadlock. Right. > > But kobject_put() is _not_ synchronous. And the comment above > kobject_add() repeat 3 times that kobject_put() must be called > on success: > > * Return: If this function returns an error, kobject_put() must be > * called to properly clean up the memory associated with the > * object. Under no instance should the kobject that is passed > * to this function be directly freed with a call to kfree(), > * that can leak memory. > * > * If this function returns success, kobject_put() must also be called > * in order to properly clean up the memory associated with the object. > * > * In short, once this function is called, kobject_put() MUST be called > * when the use of the object is finished in order to properly free > * everything. > > and similar text in Documentation/core-api/kobject.rst > > After a kobject has been registered with the kobject core successfully, it > must be cleaned up when the code is finished with it. To do that, call > kobject_put(). > > > If I read the code correctly then kobject_put() calls kref_put() > that might call kobject_delayed_cleanup(). This function does a lot > of things and need to access struct kobject. Yes, then what is the problem here wrt. kobject_put() which may not be synchronous? > > > IMHO, kobject API does not support static structures and module > > removal. > > If kobject_put() has to be called also for static structures then > module_exit() must explicitly wait until the clean up is finished. Right, that is exactly how klp_patch kobject is implemented. klp_patch kobject has to be disabled first, then module refcnt can be dropped after the klp_patch kobject is released. Then module_exit() is possible. Thanks, Ming