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 052BCC433EF for ; Tue, 2 Nov 2021 14:51:45 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id DE97060F36 for ; Tue, 2 Nov 2021 14:51:44 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233685AbhKBOyR (ORCPT ); Tue, 2 Nov 2021 10:54:17 -0400 Received: from smtp-out1.suse.de ([195.135.220.28]:54500 "EHLO smtp-out1.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233657AbhKBOyM (ORCPT ); Tue, 2 Nov 2021 10:54:12 -0400 Received: from relay2.suse.de (relay2.suse.de [149.44.160.134]) by smtp-out1.suse.de (Postfix) with ESMTP id C6FDC218DF; Tue, 2 Nov 2021 14:51:35 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1635864695; h=from:from:reply-to: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=UFNoUWc0AZc9WUSca1S+eN9tmYLlYuOWyX2traTgthA=; b=REnrQBUIZHLPwnPamGCk1suZ2rJ/dU4SDyZTSsXR9/abMy14g0KAbcxRcodPwYgf3CCebu LZhw3EhN8IRUyla4yH4fr7xb0yFWGsQYo30lZZW7uL/XywOIfPaoMdhqTyuZVFVnGvDeDE yaa2kF8VchFwSkTFJf3yoOD+VKyZKJ4= Received: from suse.cz (unknown [10.100.216.66]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by relay2.suse.de (Postfix) with ESMTPS id 519D4A3B85; Tue, 2 Nov 2021 14:51:35 +0000 (UTC) Date: Tue, 2 Nov 2021 15:51:33 +0100 From: Petr Mladek To: Ming Lei 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 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 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. 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. > 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. Best Regards, Petr