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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 272BFC433F5 for ; Mon, 3 Oct 2022 23:57:06 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229942AbiJCX5E (ORCPT ); Mon, 3 Oct 2022 19:57:04 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:38742 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229950AbiJCX4u (ORCPT ); Mon, 3 Oct 2022 19:56:50 -0400 Received: from bombadil.infradead.org (bombadil.infradead.org [IPv6:2607:7c80:54:3::133]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 6077F27CDB; Mon, 3 Oct 2022 16:56:49 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=bombadil.20210309; h=Sender:In-Reply-To:Content-Type: MIME-Version:References:Message-ID:Subject:Cc:To:From:Date:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=G37LnXkC8VTdsMvruTDa2+n50xVxzxAz9mNwqVDycIY=; b=XTOqMBWME2UGab0kwjo1gzrMRW gB+DuURn9NS4zcZSmoeMs18rQmqAsTNNa6uuhzopJE7oJUzHFGX+qVY7cdxQlfhPuwqLeXoDvetBs CcsdjMo+/vbb/SNGM9xa8ArXmcyHMYiLJbgqoh53RFxan4Zh6rxIM0KwecTCGmVw7g/2QE2XOPexA zmbGEaVvP9cKgoIuClztMdTyR+b3wnCwudN9YEIqi1v33/NbiEad2VssWmt64gBQQnGlv4v+/ejDr cvjhjQ2lZjbHT91+3gOMqh0aGmftMrWfQC/SD1FT/gQY7QqVITbhlMYVu9rTYP2v6z59AYKycxcnF 095W7VAA==; Received: from mcgrof by bombadil.infradead.org with local (Exim 4.94.2 #2 (Red Hat Linux)) id 1ofVIh-0086q0-0V; Mon, 03 Oct 2022 23:56:43 +0000 Date: Mon, 3 Oct 2022 16:56:42 -0700 From: Luis Chamberlain To: Bart Van Assche Cc: Mauro Carvalho Chehab , "Martin K . Petersen" , linux-scsi@vger.kernel.org, Christoph Hellwig , Ming Lei , Hannes Reinecke , John Garry , Mike Christie , Krzysztof Kozlowski , Greg Kroah-Hartman , linux-modules@vger.kernel.org, linux-kernel@vger.kernel.org, "James E.J. Bottomley" , Tejun Heo Subject: Re: [PATCH v5 6/7] module: Improve support for asynchronous module exit code Message-ID: References: <20220914225621.415631-1-bvanassche@acm.org> <20220914225621.415631-7-bvanassche@acm.org> <33ee1c3a-d37a-b81f-68be-d1901c7dd6e8@acm.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: Luis Chamberlain Precedence: bulk List-ID: On Fri, Sep 30, 2022 at 12:39:27PM -0700, Luis Chamberlain wrote: > On Wed, Sep 28, 2022 at 11:17:02AM -0700, Bart Van Assche wrote: > > On 9/27/22 17:02, Luis Chamberlain wrote: > > > On Tue, Sep 20, 2022 at 10:13:40AM -0700, Bart Van Assche wrote: > > > > On 9/14/22 15:56, Bart Van Assche wrote: > > > > > Some kernel modules call device_del() from their module exit code and > > > > > schedule asynchronous work from inside the .release callback without waiting > > > > > until that callback has finished. As an example, many SCSI LLD drivers call > > > > > scsi_remove_host() from their module exit code. scsi_remove_host() may > > > > > invoke scsi_device_dev_release_usercontext() asynchronously. > > > > > scsi_device_dev_release_usercontext() uses the host template pointer and > > > > > that pointer usually exists in static storage in the SCSI LLD. Support > > > > > using the module reference count to keep the module around until > > > > > asynchronous module exiting has completed by waiting in the delete_module() > > > > > system call until the module reference count drops to zero. > > > > > > > > Hi Luis, > > > > > > > > I'd like to know your opinion about this patch since you are the maintainer > > > > of the kernel module system. > > > > > > See this patch which extends the documentation of try_module_get(): > > > > > > https://lkml.kernel.org/r/20211029184500.2821444-7-mcgrof@kernel.org > > > > > > You can ignore discussion around the thread as sadly it is just > > > irrelevant stuff not about that patch. But the logic it spells out > > > is still true. > > > > > > So, in short, using try_module_get() on exit is actually the wrong > > > thing to do and it is no surprise it would fail. I haven't gotten > > > yet around to reviewing Mauro's driver API which let's you unbind > > > drivers, but it sounds related so I CC'd you on that. > > > > > > So I'd like to ask instead if an alternative to using try_module_get() > > > on exit would be better here and for the future. > > > > Hi Luis, > > > > The extended documentation of try_module_get() is very helpful. But please > > note that this patch is not related to try_module_get() at all. See also > > patch 7/7 in this series (https://lore.kernel.org/linux-scsi/20220914225621.415631-8-bvanassche@acm.org/). > > I cannot see how this patch set is no way related to try_module_get() > given the 7/7 patch you posted replaces try_module_get() with __module_get(). > My point, and hint, is that the original construct that added try_module_get() > on removal was flawed and I'm not sure trying to expand on that idea would > or even *should* be fruitful given the issues / tribal knowledge I tried > extending documentation for. > > It would beg the question if instead re-evaluating the goal could be > done in such a way that the new documentation I suggested on try_module_get() > would be seriously taken into account. Yeah I've gone ahead and re-read your original patch again the issue with that is it waits *once* for the refcnt to go to 0, but that does not forbit it from going back up, at which point you have a race which can still create the situation. Every subsystem is different, but for instance simply running a loop opening a device block file should suffice to bump the refcnt of a respective block driver module. So at least the patch itself won't ultimately address this issue I'm afraid. Luis