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=-3.9 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,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 DF3B1C2D0A3 for ; Wed, 4 Nov 2020 13:26:19 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 8152E20780 for ; Wed, 4 Nov 2020 13:26:19 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=linutronix.de header.i=@linutronix.de header.b="keRKoXlK"; dkim=permerror (0-bit key) header.d=linutronix.de header.i=@linutronix.de header.b="H703sIwu" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729227AbgKDN0T (ORCPT ); Wed, 4 Nov 2020 08:26:19 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:34802 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729198AbgKDN0S (ORCPT ); Wed, 4 Nov 2020 08:26:18 -0500 Received: from galois.linutronix.de (Galois.linutronix.de [IPv6:2a0a:51c0:0:12e:550::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 96ECEC0613D3; Wed, 4 Nov 2020 05:26:18 -0800 (PST) From: Thomas Gleixner DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020; t=1604496377; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=X/y979eVJRsQpBS3uTHsEVTLszhMzUjou+ruEut+S9s=; b=keRKoXlK9BNK4il+KjFMvHwcVKtTQAY5CIRfZMZWu3ZBdswM7KDnajE9B+yNzy2ySIxPXD pdljSMaFwptkjkOOBzUx9Gm5IgeVJ7ygNXzpJpdDsyqzbk5Z+aWn97NeWfgT97TNgjXP+z LvypjSCXj/76tsMNJRFcRSzF9psbA5CwqfmoXdZIbSJyqCycHD3kl6YG3q4yQI25jkCyt4 2m/wDfJn5yVXzBNijo3v3XkJfXvBlq/xGEhHeDeUxURYqiWnmR3fB88WsjmlsJKlQVD6Wo Fqde30u9/tju2tJVTlcBCuLUb95zvoFWFn8Qe3L30QU6UmB9B8vGvLkuIrp0Uw== DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020e; t=1604496377; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=X/y979eVJRsQpBS3uTHsEVTLszhMzUjou+ruEut+S9s=; b=H703sIwu528AJqL8IC960f6xJ+Ji5K2/HvtrgPmB92y34aE2d4Kut122s6Mh/SiaC2ssWn jmCRIJMDlPFcHBCA== To: Mike Galbraith , Gratian Crisan , 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 Subject: Re: BUG_ON(!newowner) in fixup_pi_state_owner() In-Reply-To: <871rh9mkvr.fsf@nanos.tec.linutronix.de> References: <87a6w6x7bb.fsf@ni.com> <878sbixbk4.fsf@ni.com> <2376f4e71c638aee215a4911e5efed14c5adf56e.camel@gmx.de> <5f536491708682fc3b86cb5b7bc1e05ffa3521e7.camel@gmx.de> <874km5mnbf.fsf@nanos.tec.linutronix.de> <871rh9mkvr.fsf@nanos.tec.linutronix.de> Date: Wed, 04 Nov 2020 14:26:16 +0100 Message-ID: <87v9ell0cn.fsf@nanos.tec.linutronix.de> MIME-Version: 1.0 Content-Type: text/plain Precedence: bulk List-ID: X-Mailing-List: linux-rt-users@vger.kernel.org On Wed, Nov 04 2020 at 12:17, Thomas Gleixner wrote: > On Wed, Nov 04 2020 at 11:24, Thomas Gleixner wrote: >> On Wed, Nov 04 2020 at 08:42, Mike Galbraith wrote: >>> On Wed, 2020-11-04 at 01:56 +0100, Mike Galbraith wrote: >>> --- 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 = 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 = -EAGAIN; >>> + goto handle_err; >> >> Yes, that cures it, but does not really explain why newowner is >> NULL. Lemme stare more. > > Aside of that it's going to create inconsistent state in the worst > case. There is something really fishy in the trace Gratian provided.... Ok. With some more info added to the trace I could convince myself, that Gratians trace is not so fishy at all and that Mike's fix is actually the right thing to do. I'll post that with a proper comment and changelog. Mike, can I add your Signed-off-by to the thing?