From patchwork Mon Mar 1 17:31:39 2021 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Ben Hutchings X-Patchwork-Id: 389906 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=-13.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI, SPF_HELO_NONE, SPF_PASS autolearn=ham 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 CDB71C433E6 for ; Mon, 1 Mar 2021 17:40:49 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 8FC07652F6 for ; Mon, 1 Mar 2021 17:40:49 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234839AbhCARkZ (ORCPT ); Mon, 1 Mar 2021 12:40:25 -0500 Received: from maynard.decadent.org.uk ([95.217.213.242]:32842 "EHLO maynard.decadent.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S238317AbhCARcY (ORCPT ); Mon, 1 Mar 2021 12:32:24 -0500 Received: from [2a02:1811:d34:3700:3b8d:b310:d327:e418] (helo=deadeye) by maynard with esmtps (TLS1.3:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.92) (envelope-from ) id 1lGmOS-0002aJ-6S; Mon, 01 Mar 2021 18:31:40 +0100 Received: from ben by deadeye with local (Exim 4.94) (envelope-from ) id 1lGmOR-004iX7-BR; Mon, 01 Mar 2021 18:31:39 +0100 Date: Mon, 1 Mar 2021 18:31:39 +0100 From: Ben Hutchings To: stable@vger.kernel.org Cc: Lee Jones , "Luis Claudio R. Goncalves" Subject: [PATCH 4.9 4/7] futex: Futex_unlock_pi() determinism Message-ID: References: MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: X-SA-Exim-Connect-IP: 2a02:1811:d34:3700:3b8d:b310:d327:e418 X-SA-Exim-Mail-From: ben@decadent.org.uk X-SA-Exim-Scanned: No (on maynard); SAEximRunCond expanded to false Precedence: bulk List-ID: X-Mailing-List: stable@vger.kernel.org From: Peter Zijlstra commit bebe5b514345f09be2c15e414d076b02ecb9cce8 upstream. The problem with returning -EAGAIN when the waiter state mismatches is that it becomes very hard to proof a bounded execution time on the operation. And seeing that this is a RT operation, this is somewhat important. While in practise; given the previous patch; it will be very unlikely to ever really take more than one or two rounds, proving so becomes rather hard. However, now that modifying wait_list is done while holding both hb->lock and wait_lock, the scenario can be avoided entirely by acquiring wait_lock while still holding hb-lock. Doing a hand-over, without leaving a hole. Signed-off-by: Peter Zijlstra (Intel) Cc: juri.lelli@arm.com Cc: bigeasy@linutronix.de Cc: xlpang@redhat.com Cc: rostedt@goodmis.org Cc: mathieu.desnoyers@efficios.com Cc: jdesfossez@efficios.com Cc: dvhart@infradead.org Cc: bristot@redhat.com Link: http://lkml.kernel.org/r/20170322104152.112378812@infradead.org Signed-off-by: Thomas Gleixner Signed-off-by: Ben Hutchings --- kernel/futex.c | 24 +++++++++++------------- 1 file changed, 11 insertions(+), 13 deletions(-) diff --git a/kernel/futex.c b/kernel/futex.c index c8ec4e7f3609..77cec33ea112 100644 --- a/kernel/futex.c +++ b/kernel/futex.c @@ -1555,15 +1555,10 @@ static int wake_futex_pi(u32 __user *uaddr, u32 uval, struct futex_pi_state *pi_ WAKE_Q(wake_q); int ret = 0; - raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock); new_owner = rt_mutex_next_owner(&pi_state->pi_mutex); - if (!new_owner) { + if (WARN_ON_ONCE(!new_owner)) { /* - * Since we held neither hb->lock nor wait_lock when coming - * into this function, we could have raced with futex_lock_pi() - * such that we might observe @this futex_q waiter, but the - * rt_mutex's wait_list can be empty (either still, or again, - * depending on which side we land). + * As per the comment in futex_unlock_pi() this should not happen. * * When this happens, give up our locks and try again, giving * the futex_lock_pi() instance time to complete, either by @@ -3018,15 +3013,18 @@ static int futex_unlock_pi(u32 __user *uaddr, unsigned int flags) if (pi_state->owner != current) goto out_unlock; + get_pi_state(pi_state); /* - * Grab a reference on the pi_state and drop hb->lock. + * Since modifying the wait_list is done while holding both + * hb->lock and wait_lock, holding either is sufficient to + * observe it. * - * The reference ensures pi_state lives, dropping the hb->lock - * is tricky.. wake_futex_pi() will take rt_mutex::wait_lock to - * close the races against futex_lock_pi(), but in case of - * _any_ fail we'll abort and retry the whole deal. + * By taking wait_lock while still holding hb->lock, we ensure + * there is no point where we hold neither; and therefore + * wake_futex_pi() must observe a state consistent with what we + * observed. */ - get_pi_state(pi_state); + raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock); spin_unlock(&hb->lock); ret = wake_futex_pi(uaddr, uval, pi_state);