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=-3.5 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no 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 EA941C2D0A3 for ; Wed, 4 Nov 2020 07:42:59 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 8217A2224E for ; Wed, 4 Nov 2020 07:42:59 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=gmx.net header.i=@gmx.net header.b="aaSBXC8b" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728711AbgKDHm6 (ORCPT ); Wed, 4 Nov 2020 02:42:58 -0500 Received: from mout.gmx.net ([212.227.17.22]:42385 "EHLO mout.gmx.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726152AbgKDHm6 (ORCPT ); Wed, 4 Nov 2020 02:42:58 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=gmx.net; s=badeba3b8450; t=1604475758; bh=BVK0qslWs4eh+9VxmukAHWfu/UJGdfJZRyuedwiAwrY=; h=X-UI-Sender-Class:Subject:From:To:Cc:Date:In-Reply-To:References; b=aaSBXC8bQHgF3Hb6U3FWYjZwDkz6UsKIr3sAxcSV4eRPFS8zXVs+YRYgTJiwiywsq HdRYLUaG4WV2UtoiquNs2HK9SeQ4niYyRGguWtgdI3XMaQfkJSsBajDnPRnDqCY9Vo 3jYs7gqQJf7CifjMZlafNgn3PgwWBNOWaX8q+yRM= X-UI-Sender-Class: 01bb95c1-4bf8-414a-932a-4f6e2808ef9c Received: from homer.fritz.box ([185.221.148.80]) by mail.gmx.com (mrgmx105 [212.227.17.168]) with ESMTPSA (Nemesis) id 1M5wLT-1kgVld1flT-007UXA; Wed, 04 Nov 2020 08:42:38 +0100 Message-ID: <5f536491708682fc3b86cb5b7bc1e05ffa3521e7.camel@gmx.de> Subject: Re: BUG_ON(!newowner) in fixup_pi_state_owner() From: Mike Galbraith To: Gratian Crisan , Thomas Gleixner , Peter Zijlstra Cc: linux-kernel@vger.kernel.org, linux-rt-users@vger.kernel.org, Brandon Streiff , Ingo Molnar , Darren Hart , James Minor , Sebastian Andrzej Siewior Date: Wed, 04 Nov 2020 08:42:36 +0100 In-Reply-To: <2376f4e71c638aee215a4911e5efed14c5adf56e.camel@gmx.de> References: <87a6w6x7bb.fsf@ni.com> <878sbixbk4.fsf@ni.com> <2376f4e71c638aee215a4911e5efed14c5adf56e.camel@gmx.de> Content-Type: text/plain; charset="ISO-8859-15" User-Agent: Evolution 3.34.4 MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable X-Provags-ID: V03:K1:iU4vDL9MUhMTGUXWJ4e1rT7hD5VTd8KbKyZcJZz7OqA1HI4i9ZX CsKoK1o5aL9K/22h4mx9sh945ITXt9dVX/FDsa+FhsODoymHOXfAXMTzzQ0wp2ccgIrYw08 owOjB3Gj0ZvfpLAsBme5NzXZp9sijOe/UWw+lCkHDFlL4kWFtv4SRnEVC3crbPwhH6OSSC9 qNy+Bsyxs89xdi1UL9VyQ== X-UI-Out-Filterresults: notjunk:1;V03:K0:/BkHvbh99PQ=:xIJWas2soeUe8xa4m9uGtU dbC7CvA9E5jF6vyoE5lcmrVsAT3E6zJu2kmpq/HngYNN0kG3UazOMKxNiSu0nezH65SQQykWj Wbmg0VNivsQD2ZMCha4O9w9wcQ0Fyt7Pku2RIY0XFt+XU9lxp3ncs0TI3eEGttwO8cyoic0Aq 5x9PyFVSmjpGSiGoRW1QoQhIQJBuePLiIzutPW1Exuo98qB9Y7ZJAZETpvROf2JKUcO+tL9q3 9+nR8I+dmhzfprNocHPE/KlZutZcIfIoebojw0EYRBb5axr4dRg7gX0ygYsQ8c0BiHaKCYgML Thpw61TPXR1czLGqV9OTq4rh2Y5gPJFfIqbqTKECeEV8Wefmy4vb/MLoO1diC0p/RAJsRwMRE O++Qjxvn2+ZtAaqopgILDazWOSmd4EqyK7VfpCiCtpFbW9uzTmYYcO38+D72Yx1ktPnJhYlkN VuqCMiJgBVEj5JhR34cZKruDSX2CHFh+d8sguhr22ugWVGuSSpWhux6RSQXO7T7rwPMCQKGct pjbDCjbh9ZBOpXELyB1lTJcc4IJdSfprLlTAETmz0bvzHY0mGVrvZMAmRgQe9IUndh4yFgMru 3g5KmZjAGHmd5ppzXfYsGoWZWUqAtrPRbdB9WTo/gEkxcbEZMxBRx7SLmgYDOKs4rU+oNZYhw WGFCH4QaqzD+qmvr6sAR2EuLZ2Dj18oE5HVtECAVfkqj4H43SzYMhFVHp3an+aZ7lRMxlg0PI hgKrftBhMEGXTnyS+g5FE2SpJfuqY8MuEFW1+XgiLirw/IJmhQB9CoiqoGAqSH4m2+uhxLmC5 r3+sogjpoFFwrIOX/wAkYr8li/6hjIMRK4UlB24QzL5wFvJFQMy9h7DiZ8fDIGtA4XBD/ufjz YwonV3Mv1R3WRKLeEWFQ== Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 2020-11-04 at 01:56 +0100, Mike Galbraith wrote: > On Tue, 2020-11-03 at 17:31 -0600, Gratian Crisan wrote: > > Hi all, > > > > I apologize for waking up the futex demons (and replying to my own > > email), but ... > > > > Gratian Crisan writes: > > > > > > Brandon and I have been debugging a nasty race that leads to > > > BUG_ON(!newowner) in fixup_pi_state_owner() in futex.c. So far > > > we've only been able to reproduce the issue on 4.9.y-rt kernels. > > > We are still testing if this is a problem for later RT branches. > > > > I was able to reproduce the BUG_ON(!newowner) in fixup_pi_state_owner(= ) > > with a 5.10.0-rc1-rt1 kernel (currently testing 5.10.0-rc2-rt4). > > My box says it's generic. =2D-- kernel/futex.c | 13 ++++++++++++- 1 file changed, 12 insertions(+), 1 deletion(-) =2D-- a/kernel/futex.c +++ b/kernel/futex.c @@ -2383,7 +2383,18 @@ static int fixup_pi_state_owner(u32 __us * Since we just failed the trylock; there must be an owner. */ newowner =3D rt_mutex_owner(&pi_state->pi_mutex); - BUG_ON(!newowner); + + /* + * Why? Because I know what I'm doing with these beasts? Nope, + * but what the hell, a busy restart loop let f_boosted become + * owner, so go for it. Box still boots, works, no longer makes + * boom with fbomb_v2, and as an added bonus, didn't even blow + * futextests all up. Maybe it'll help... or not, we'll see. + */ + if (unlikely(!newowner)) { + err =3D -EAGAIN; + goto handle_err; + } } else { WARN_ON_ONCE(argowner !=3D current); if (oldowner =3D=3D current) {