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 X-Spam-Level: X-Spam-Status: No, score=-4.0 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_PASS, URIBL_BLOCKED,USER_AGENT_MUTT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 4EDDAC46466 for ; Tue, 20 Nov 2018 12:05:46 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 16456208E4 for ; Tue, 20 Nov 2018 12:05:46 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=kernel.org header.i=@kernel.org header.b="yUQyf3P1" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 16456208E4 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=linuxfoundation.org Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729382AbeKTWec (ORCPT ); Tue, 20 Nov 2018 17:34:32 -0500 Received: from mail.kernel.org ([198.145.29.99]:33174 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729042AbeKTWec (ORCPT ); Tue, 20 Nov 2018 17:34:32 -0500 Received: from localhost (5356596B.cm-6-7b.dynamic.ziggo.nl [83.86.89.107]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 31D75208E3; Tue, 20 Nov 2018 12:05:42 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1542715542; bh=pr1CFEVfHeICcf3DmWZteOiFIJVO+rwLQTix+D55gzA=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=yUQyf3P1zLT9UToBnrVHJs51v/k0xviRfI/j5guMI1rf7HH5x+vBWRXt1I74hJCAE lyrv/7lNYIcbMPvxtmqF3lRL8N5xe+77bzF7BBvz3UxQSy3bGbTrFUVoZon35iegBt /tFPWDuiYsxMKjfOhqaVPqlAE1ALTRfcSYbDQ/54= Date: Tue, 20 Nov 2018 13:05:40 +0100 From: Greg Kroah-Hartman To: Dmitry Vyukov Cc: Ming Lei , Guenter Roeck , Ming Lei , Jens Axboe , Peter Zijlstra , linux-block@vger.kernel.org, LKML , Hannes Reinecke , Paolo Bonzini , Christoph Hellwig , "Martin K. Petersen" , "James E.J. Bottomley" , linux-scsi Subject: Re: kobject lifetime issues in blk-mq Message-ID: <20181120120540.GA14861@kroah.com> References: <20181112094406.GB391@ming.t460p> <20181112164848.GA1790@roeck-us.net> <20181112200236.GA4415@kroah.com> <20181113002226.GA4455@ming.t460p> <20181113004124.GC4455@ming.t460p> <20181114110827.GA31430@ming.t460p> <20181114151410.GB26378@kroah.com> <20181115003616.GA32603@ming.t460p> <20181115005658.GA24847@kroah.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Nov 20, 2018 at 12:34:40PM +0100, Dmitry Vyukov wrote: > On Thu, Nov 15, 2018 at 1:56 AM, Greg Kroah-Hartman > wrote: > > On Thu, Nov 15, 2018 at 08:36:17AM +0800, Ming Lei wrote: > >> > So even if you think the kernel is not going to do this, remember, you > >> > have no control over it. Reference counted objects are done this way > >> > for a reason, you really do not know who has a reference and you really > >> > do not care. > >> > > >> > You are just papering over the real issue here, see my previous email > >> > for how to start working on resolving it. > >> > >> IMO, there isn't real issue, and the issue is actually in 'delay release'. > > > > Nope, sorry, that is not true. > > > >> Please look at the code in block/blk-mq-sysfs.c, both q->mq_kobj and all > >> ctx->kobj share same lifetime with q->kobj, we even don't call get/put > >> on q->mq_kobj & all ctx->kobj, and all are simply released in q->kobj's > >> release handler. > > > > How do you "know" you are keeping those lifetimes in sync? The joy of a > > kobject is that _ANYTHING_ can grab a reference to your object without > > you knowing about it. That includes userspace programs. Yes, sysfs is > > now much better and it trys to release that reference "quickly" when it > > determines you are trying to delete a kobject, but it's not perfict, > > there are still races there. > > > > And that is what the delay release code is showing you. It is showing > > you that you "think" your reference counting is wrong, but it is not. > > It is showing you that if someone else grabs a reference, you are not > > correctly cleaning up for yourself. > > > > Never think that you really know the lifetime of a kobject, once you > > realize that your code gets simpler and you can then just "trust" that > > the kernel will do the right thing no matter what. > > > > Because really, you are using a kobject because you want that correct > > reference counting logic. By ignoring that logic, you are ignoring the > > reason to be using that object at all. If you don't need reference > > counting, then don't use it at all. > > > > And if you need sysfs files, then you need to use the kobject and then > > you need to handle it properly, because again, you do NOT have full > > control over the lifetime of your object. That's the basis for > > reference counting in the firstplace. > > > > So this code is broken without me evening having to look at it, please > > fix it to handle release properly. Again, the kernel tried to tell you > > this, but you hacked around the kernel core to remove that warning > > incorrectly. Please go read the kobject documentation again for even > > more details about this than what I said here. > > > > thanks, > > > > greg k-h > > Whoever is the right person to fix this, please prioritize this to the > degree possible. > This issue does not allow to use DEBUG_KOBJECT_RELEASE in any > automated testing (in particular syzbot) on both upstream and stable > trees. We have to disable it for now, so other bugs won't be noticed > and will pile up. Patches for this have already been posted :)