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=-10.1 required=3.0 tests=DKIMWL_WL_MED,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_PASS,URIBL_BLOCKED,USER_IN_DEF_DKIM_WL 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 94946C28CF8 for ; Tue, 20 Nov 2018 12:54:10 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 5AF712087E for ; Tue, 20 Nov 2018 12:54:10 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="pUvVq/NJ" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 5AF712087E Authentication-Results: mail.kernel.org; dmarc=fail (p=reject dis=none) header.from=google.com 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 S1729496AbeKTXXI (ORCPT ); Tue, 20 Nov 2018 18:23:08 -0500 Received: from mail-it1-f196.google.com ([209.85.166.196]:36306 "EHLO mail-it1-f196.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728320AbeKTXXI (ORCPT ); Tue, 20 Nov 2018 18:23:08 -0500 Received: by mail-it1-f196.google.com with SMTP id c9so3312823itj.1 for ; Tue, 20 Nov 2018 04:54:08 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=mime-version:in-reply-to:references:from:date:message-id:subject:to :cc; bh=VF1vQMfRkcB5pYEIP10m2Fp1BBbVbneNsZsyp3kJaoM=; b=pUvVq/NJn0TtWERB3bNtaiqeNqyjeCdrQSWxjEvtbU8c0T0j+dS+L0vVu4Vic39Wyk mdGXJO0u6rsAB+GGk5TwjW4O4Ehp/D0abrWKgpavajXkIYc+vLUCSbEGHAsnZRgtzeoj sSO8uMCS7edS6wJyjrCuxYduMLCQly71j8HshCrxY3EqooQzJZX3vWMZatHHFxZgJXr/ jVQ1e4kAv561NW5a+duWBbmo1ZIvKZkb7ko2DKHtKUAOUX5IvO/WVQv43GjEMCt+DDtV +pSeXf6mYy47GNjMxBalP1FerCaNScz/dzL9fONEngBM3oXCOJAdMQ99OKv1EHlIxIYV FA2Q== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:in-reply-to:references:from:date :message-id:subject:to:cc; bh=VF1vQMfRkcB5pYEIP10m2Fp1BBbVbneNsZsyp3kJaoM=; b=YmYJR1YEN/vjToAafTs90MvCMoIqxV0TaR5B91rb3NzQrFqfYX6rZgljjib656Bou0 SLhDTiWiuGVKRFCZGOQzWfy1PF3dDMn0df991WGKtQtF8xcV+RFkqbMDfTuzW/WEpdE1 ImP++embzgRmPrHB5XO4b2ISVjE8BIixjjbL41XDlDAzpgwGzAyusMsnPpBA/hv2yo5e nT4ZhkQvcgkgoBRTnrj+se73Yrxj3ct0F9KiK3vA4Mlzo1IYmqCXLPWTouRbJk7Kx6Kc FTrJWBlUxHhPurHdqWF428HynLdfrTEoEnjAXcM58wR8gfA9cIKBnWN4/Uv6W57dfsaF 0VVQ== X-Gm-Message-State: AA+aEWY+Jwf9513oFgQ+sLkHVRBbLxwkuRfiUUP3LX4uvHbtp7i1n6ho qHdoPioUPywUraLyBLVX88qDf2eM/NJ0ekF4TQn/kA== X-Google-Smtp-Source: AJdET5fsSgyDix+wGz7Pwz/bmX4Fi7+C6s769z1SuioZVB2gUG4OXkJW02qQW9+AVQ4M5NAvOsoDFEuj+iEVsdHPEls= X-Received: by 2002:a02:516:: with SMTP id e22mr1590689jad.82.1542718447809; Tue, 20 Nov 2018 04:54:07 -0800 (PST) MIME-Version: 1.0 Received: by 2002:a02:b003:0:0:0:0:0 with HTTP; Tue, 20 Nov 2018 04:53:47 -0800 (PST) In-Reply-To: <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> <20181120120540.GA14861@kroah.com> From: Dmitry Vyukov Date: Tue, 20 Nov 2018 13:53:47 +0100 Message-ID: Subject: Re: kobject lifetime issues in blk-mq To: Greg Kroah-Hartman 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 Content-Type: text/plain; charset="UTF-8" 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 1:05 PM, Greg Kroah-Hartman wrote: > 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 :) This is great. What is the patch name? I can't find anything that looks relevant on LKML searching by kobject.