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=-14.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH, MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED 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 AF6E7C388F7 for ; Sat, 7 Nov 2020 21:08:45 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 70A6020723 for ; Sat, 7 Nov 2020 21:08:45 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=linutronix.de header.i=@linutronix.de header.b="OdHBpqRn"; dkim=permerror (0-bit key) header.d=linutronix.de header.i=@linutronix.de header.b="ZGnaczaf" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728717AbgKGVIZ (ORCPT ); Sat, 7 Nov 2020 16:08:25 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:46796 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725838AbgKGVIY (ORCPT ); Sat, 7 Nov 2020 16:08:24 -0500 Received: from galois.linutronix.de (Galois.linutronix.de [IPv6:2a0a:51c0:0:12e:550::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 7AF3CC0613CF; Sat, 7 Nov 2020 13:08:24 -0800 (PST) Date: Sat, 07 Nov 2020 21:08:21 -0000 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020; t=1604783302; h=from:from:sender:sender:reply-to:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=WmfDsakZIWHD+NwFPXvMcaDmIMMEaMz35ydaZGU4Npo=; b=OdHBpqRnq2pngGGtp4TDB6+NdExoHQIkMiKk8p5CCLrXlWfF6ynCBLh1Hm7CbFR8hTQZ0d JMJVAauYmrUUU5k0WGlsR2tGEbSNU2Qi0DOnnas397TXJwrPR19/mkWsXvTLb/GIKgw6Dp Yv+o4zDUm8zHmaVC1M/hB/RhYglckIEyFcnHsZgg2AhrnSJoYeLki+iWW5BVW9rX7mMoqV 8TdJyJ7jLG9lMVp/1OtdYt8WkHLLUjiQXRKhvf/wMXyHKiEvcQ4ZNBw6cVV9q8INeS0G3d EeTSr+HrDlZgstAVE62ByYotMbpg4pqYsi650iAXHh8NhCxa9pEnCwEbqgTflg== DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020e; t=1604783302; h=from:from:sender:sender:reply-to:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=WmfDsakZIWHD+NwFPXvMcaDmIMMEaMz35ydaZGU4Npo=; b=ZGnaczaf5mxaTTb8NcztQhTFfYXv9Cm3zpJU4ih4EDFug8abLZN23kaWg3Dw14P0nYHwU4 GqSW7m0vUkb7J0Bg== From: "tip-bot2 for Mike Galbraith" Sender: tip-bot2@linutronix.de Reply-to: linux-kernel@vger.kernel.org To: linux-tip-commits@vger.kernel.org Subject: [tip: locking/urgent] futex: Handle transient "ownerless" rtmutex state correctly Cc: Gratian Crisan , Mike Galbraith , Thomas Gleixner , stable@vger.kernel.org, x86@kernel.org, linux-kernel@vger.kernel.org In-Reply-To: <87a6w6x7bb.fsf@ni.com> References: <87a6w6x7bb.fsf@ni.com> MIME-Version: 1.0 Message-ID: <160478330115.11244.17107406724104832304.tip-bot2@tip-bot2> Robot-ID: Robot-Unsubscribe: Contact to get blacklisted from these emails Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org The following commit has been merged into the locking/urgent branch of tip: Commit-ID: 9f5d1c336a10c0d24e83e40b4c1b9539f7dba627 Gitweb: https://git.kernel.org/tip/9f5d1c336a10c0d24e83e40b4c1b9539f7dba627 Author: Mike Galbraith AuthorDate: Wed, 04 Nov 2020 16:12:44 +01:00 Committer: Thomas Gleixner CommitterDate: Sat, 07 Nov 2020 22:07:04 +01:00 futex: Handle transient "ownerless" rtmutex state correctly 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 --- kernel/futex.c | 16 ++++++++++++++-- 1 file changed, 14 insertions(+), 2 deletions(-) diff --git a/kernel/futex.c b/kernel/futex.c index f8614ef..ac32887 100644 --- a/kernel/futex.c +++ b/kernel/futex.c @@ -2380,10 +2380,22 @@ 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_err; + } } else { WARN_ON_ONCE(argowner != current); if (oldowner == current) {