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=-19.0 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, 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 9E9D6C433DB for ; Mon, 29 Mar 2021 08:05:38 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 78F6061999 for ; Mon, 29 Mar 2021 08:05:36 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231614AbhC2IF1 (ORCPT ); Mon, 29 Mar 2021 04:05:27 -0400 Received: from mail.kernel.org ([198.145.29.99]:45110 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232026AbhC2IDV (ORCPT ); Mon, 29 Mar 2021 04:03:21 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id 5319861981; Mon, 29 Mar 2021 08:03:20 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1617005001; bh=lmEPBLVgIU6iXSGIJhGttAIrGiQ3KcEsOLjkepM9wZE=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=QxoQhzZya+Myn5mH/dopFw8LzwbY0olrlsUF7kKfa3lsgEFF2FG/7iGlGhSENlCQd 4auetahzkINyzff0gw1x9bFZQp/tlZy/NAFPq3I+da4gIbckLCFoY/k+7XAMZ9Z7RX la3ozXXTWHqCIcvIz2r5jvueLQRG8IubwpbCxZ4c= From: Greg Kroah-Hartman To: linux-kernel@vger.kernel.org, stable@vger.kernel.org Cc: Greg Kroah-Hartman , "Peter Zijlstra (Intel)" , Linus Torvalds , Thomas Gleixner , Ingo Molnar , Sasha Levin , Ben Hutchings Subject: [PATCH 4.9 44/53] futex: Fix (possible) missed wakeup Date: Mon, 29 Mar 2021 09:58:19 +0200 Message-Id: <20210329075608.956552582@linuxfoundation.org> X-Mailer: git-send-email 2.31.1 In-Reply-To: <20210329075607.561619583@linuxfoundation.org> References: <20210329075607.561619583@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: Peter Zijlstra commit b061c38bef43406df8e73c5be06cbfacad5ee6ad upstream. We must not rely on wake_q_add() to delay the wakeup; in particular commit: 1d0dcb3ad9d3 ("futex: Implement lockless wakeups") moved wake_q_add() before smp_store_release(&q->lock_ptr, NULL), which could result in futex_wait() waking before observing ->lock_ptr == NULL and going back to sleep again. Signed-off-by: Peter Zijlstra (Intel) Cc: Linus Torvalds Cc: Peter Zijlstra Cc: Thomas Gleixner Fixes: 1d0dcb3ad9d3 ("futex: Implement lockless wakeups") Signed-off-by: Ingo Molnar Signed-off-by: Sasha Levin Signed-off-by: Ben Hutchings Signed-off-by: Greg Kroah-Hartman --- kernel/futex.c | 13 ++++++++----- 1 file changed, 8 insertions(+), 5 deletions(-) --- a/kernel/futex.c +++ b/kernel/futex.c @@ -1553,11 +1553,7 @@ static void mark_wake_futex(struct wake_ if (WARN(q->pi_state || q->rt_waiter, "refusing to wake PI futex\n")) return; - /* - * Queue the task for later wakeup for after we've released - * the hb->lock. wake_q_add() grabs reference to p. - */ - wake_q_add(wake_q, p); + get_task_struct(p); __unqueue_futex(q); /* * The waiting task can free the futex_q as soon as @@ -1566,6 +1562,13 @@ static void mark_wake_futex(struct wake_ * store to lock_ptr from getting ahead of the plist_del. */ smp_store_release(&q->lock_ptr, NULL); + + /* + * Queue the task for later wakeup for after we've released + * the hb->lock. wake_q_add() grabs reference to p. + */ + wake_q_add(wake_q, p); + put_task_struct(p); } /*