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=-3.1 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_PASS, USER_AGENT_MUTT autolearn=unavailable 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 AF09EC04EBC for ; Thu, 15 Nov 2018 00:57:02 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 77F6222360 for ; Thu, 15 Nov 2018 00:57:02 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=kernel.org header.i=@kernel.org header.b="DsJhtO6m" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 77F6222360 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 S1728303AbeKOLCh (ORCPT ); Thu, 15 Nov 2018 06:02:37 -0500 Received: from mail.kernel.org ([198.145.29.99]:58158 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726943AbeKOLCh (ORCPT ); Thu, 15 Nov 2018 06:02:37 -0500 Received: from localhost (unknown [64.114.255.97]) (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 0FAFF208E7; Thu, 15 Nov 2018 00:57:00 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1542243420; bh=kycEJDTnQ3efzpmgmTLjKFGm9sBL8UnyW8pLuhE3gtI=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=DsJhtO6mTXq5BRgWeUAr6WmikJToRnjoZYB2rztoQMxux7sk4a0JoGvBODxUfQgSZ AR4ISx168MuKIT2M234fEvi1lKn6KqXEQ4fVEFVsuCNuuebjoxVg6vannCjwh8n5+Z pNaKJ6npQwAWlva37cegeXQ4MVlW2j4Id13NYlYc= Date: Wed, 14 Nov 2018 16:56:58 -0800 From: Greg Kroah-Hartman To: Ming Lei Cc: Guenter Roeck , Ming Lei , Jens Axboe , Peter Zijlstra , linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, Hannes Reinecke , Paolo Bonzini , Christoph Hellwig , "Martin K. Petersen" , "James E.J. Bottomley" , linux-scsi@vger.kernel.org Subject: Re: kobject lifetime issues in blk-mq Message-ID: <20181115005658.GA24847@kroah.com> References: <20181109203518.GA7130@roeck-us.net> <20181112092051.GA391@ming.t460p> <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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20181115003616.GA32603@ming.t460p> 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 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