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=-16.7 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_GIT 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 87DE2C4338F for ; Mon, 2 Aug 2021 13:49:38 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 72EF4610FD for ; Mon, 2 Aug 2021 13:49:38 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234531AbhHBNtq (ORCPT ); Mon, 2 Aug 2021 09:49:46 -0400 Received: from szxga01-in.huawei.com ([45.249.212.187]:7776 "EHLO szxga01-in.huawei.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234366AbhHBNrg (ORCPT ); Mon, 2 Aug 2021 09:47:36 -0400 Received: from dggemv703-chm.china.huawei.com (unknown [172.30.72.56]) by szxga01-in.huawei.com (SkyGuard) with ESMTP id 4GdfSc6vxwzYkNQ; Mon, 2 Aug 2021 21:47:12 +0800 (CST) Received: from dggpemm500006.china.huawei.com (7.185.36.236) by dggemv703-chm.china.huawei.com (10.3.19.46) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2176.2; Mon, 2 Aug 2021 21:47:16 +0800 Received: from thunder-town.china.huawei.com (10.174.179.0) by dggpemm500006.china.huawei.com (7.185.36.236) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2176.2; Mon, 2 Aug 2021 21:47:15 +0800 From: Zhen Lei To: Greg Kroah-Hartman , stable CC: Zhen Lei , Anna-Maria Gleixner , Mike Galbraith , Sasha Levin , Ingo Molnar , Peter Zijlstra , Thomas Gleixner , linux-kernel Subject: [PATCH 4.4 08/11] futex: Handle transient "ownerless" rtmutex state correctly Date: Mon, 2 Aug 2021 21:46:21 +0800 Message-ID: <20210802134624.1934-9-thunder.leizhen@huawei.com> X-Mailer: git-send-email 2.26.0.windows.1 In-Reply-To: <20210802134624.1934-1-thunder.leizhen@huawei.com> References: <20210802134624.1934-1-thunder.leizhen@huawei.com> MIME-Version: 1.0 Content-Transfer-Encoding: 7BIT Content-Type: text/plain; charset=US-ASCII X-Originating-IP: [10.174.179.0] X-ClientProxiedBy: dggems704-chm.china.huawei.com (10.3.19.181) To dggpemm500006.china.huawei.com (7.185.36.236) X-CFilter-Loop: Reflected Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Mike Galbraith [ Upstream commit 9f5d1c336a10c0d24e83e40b4c1b9539f7dba627 ] Gratian managed to trigger the BUG_ON(!newowner) in fixup_pi_state_owner(). This is one possible chain of events leading to this: Task Prio Operation T1 120 lock(F) T2 120 lock(F) -> blocks (top waiter) T3 50 (RT) lock(F) -> boosts T1 and blocks (new top waiter) XX timeout/ -> wakes T2 signal T1 50 unlock(F) -> wakes T3 (rtmutex->owner == NULL, waiter bit is set) T2 120 cleanup -> try_to_take_mutex() fails because T3 is the top waiter and the lower priority T2 cannot steal the lock. -> fixup_pi_state_owner() sees newowner == NULL -> BUG_ON() The comment states that this is invalid and rt_mutex_real_owner() must return a non NULL owner when the trylock failed, but in case of a queued and woken up waiter rt_mutex_real_owner() == NULL is a valid transient state. The higher priority waiter has simply not yet managed to take over the rtmutex. The BUG_ON() is therefore wrong and this is just another retry condition in fixup_pi_state_owner(). Drop the locks, so that T3 can make progress, and then try the fixup again. Gratian provided a great analysis, traces and a reproducer. The analysis is to the point, but it confused the hell out of that tglx dude who had to page in all the futex horrors again. Condensed version is above. [ tglx: Wrote comment and changelog ] Fixes: c1e2f0eaf015 ("futex: Avoid violating the 10th rule of futex") Reported-by: Gratian Crisan Signed-off-by: Mike Galbraith Signed-off-by: Thomas Gleixner Cc: stable@vger.kernel.org Link: https://lore.kernel.org/r/87a6w6x7bb.fsf@ni.com Link: https://lore.kernel.org/r/87sg9pkvf7.fsf@nanos.tec.linutronix.de Signed-off-by: Zhen Lei --- kernel/futex.c | 16 ++++++++++++++-- 1 file changed, 14 insertions(+), 2 deletions(-) diff --git a/kernel/futex.c b/kernel/futex.c index 8f6372d3a1feea0..e7c2e552aef4ae6 100644 --- a/kernel/futex.c +++ b/kernel/futex.c @@ -2437,10 +2437,22 @@ retry: } /* - * Since we just failed the trylock; there must be an owner. + * The trylock just failed, so either there is an owner or + * there is a higher priority waiter than this one. */ newowner = rt_mutex_owner(&pi_state->pi_mutex); - BUG_ON(!newowner); + /* + * If the higher priority waiter has not yet taken over the + * rtmutex then newowner is NULL. We can't return here with + * that state because it's inconsistent vs. the user space + * state. So drop the locks and try again. It's a valid + * situation and not any different from the other retry + * conditions. + */ + if (unlikely(!newowner)) { + err = -EAGAIN; + goto handle_fault; + } } else { WARN_ON_ONCE(argowner != current); if (oldowner == current) { -- 2.26.0.106.g9fadedd