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=-18.8 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,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 31324C4361A for ; Fri, 5 Mar 2021 12:41:55 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 11F0C6501C for ; Fri, 5 Mar 2021 12:41:55 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233118AbhCEMlj (ORCPT ); Fri, 5 Mar 2021 07:41:39 -0500 Received: from mail.kernel.org ([198.145.29.99]:56986 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231852AbhCEMlD (ORCPT ); Fri, 5 Mar 2021 07:41:03 -0500 Received: by mail.kernel.org (Postfix) with ESMTPSA id 7BE2264F44; Fri, 5 Mar 2021 12:41:02 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1614948063; bh=Ua8uMB+3J2yuKK65fDFSM/1F0CTq/Uj2iq29WEfOcho=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=oV5d2bcHcskUZNJWt4klchT+lkB8PgLKTWc/NKCCbVh0ArlDG/kuzNHCjxQQl08LH 4i0Y74H2cL0cbglMwn7edyYTYx6zT5J5opqKg6B75dRk4tobY72aMfH9w0vlVc6Ahp GSSX/9nz7t+G6IefIXoucs9JKUkXueNOqKcL43WE= From: Greg Kroah-Hartman To: linux-kernel@vger.kernel.org, stable@vger.kernel.org Cc: Greg Kroah-Hartman , "Peter Zijlstra (Intel)" , juri.lelli@arm.com, bigeasy@linutronix.de, xlpang@redhat.com, rostedt@goodmis.org, mathieu.desnoyers@efficios.com, jdesfossez@efficios.com, dvhart@infradead.org, bristot@redhat.com, Thomas Gleixner , Ben Hutchings Subject: [PATCH 4.9 04/41] futex: Futex_unlock_pi() determinism Date: Fri, 5 Mar 2021 13:22:11 +0100 Message-Id: <20210305120851.486188046@linuxfoundation.org> X-Mailer: git-send-email 2.30.1 In-Reply-To: <20210305120851.255002428@linuxfoundation.org> References: <20210305120851.255002428@linuxfoundation.org> User-Agent: quilt/0.66 MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Ben Hutchings 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 Signed-off-by: Greg Kroah-Hartman --- kernel/futex.c | 24 +++++++++++------------- 1 file changed, 11 insertions(+), 13 deletions(-) --- a/kernel/futex.c +++ b/kernel/futex.c @@ -1555,15 +1555,10 @@ static int wake_futex_pi(u32 __user *uad 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 @@ retry: 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);