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=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no 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 44AD2C43331 for ; Wed, 13 Nov 2019 09:04:23 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 1D21921A49 for ; Wed, 13 Nov 2019 09:04:23 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727032AbfKMJEV (ORCPT ); Wed, 13 Nov 2019 04:04:21 -0500 Received: from Galois.linutronix.de ([193.142.43.55]:36972 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726086AbfKMJEV (ORCPT ); Wed, 13 Nov 2019 04:04:21 -0500 Received: from p5b06da22.dip0.t-ipconnect.de ([91.6.218.34] helo=nanos) by Galois.linutronix.de with esmtpsa (TLS1.2:DHE_RSA_AES_256_CBC_SHA256:256) (Exim 4.80) (envelope-from ) id 1iUoZT-0006K4-Gd; Wed, 13 Nov 2019 10:04:15 +0100 Date: Wed, 13 Nov 2019 10:04:14 +0100 (CET) From: Thomas Gleixner To: "Harris, Robert" cc: Ingo Molnar , "peterz@infradead.org" , "dvhart@infradead.org" , "linux-kernel@vger.kernel.org" Subject: Re: Help requested: futex(..., FUTEX_WAIT_PRIVATE, ...) returns EPERM In-Reply-To: Message-ID: References: User-Agent: Alpine 2.21 (DEB 202 2017-01-01) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII X-Linutronix-Spam-Score: -1.0 X-Linutronix-Spam-Level: - X-Linutronix-Spam-Status: No , -1.0 points, 5.0 required, ALL_TRUSTED=-1,SHORTCIRCUIT=-0.0001 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 12 Nov 2019, Harris, Robert wrote: > I am investigating an issue on 4.9.184 in which futex() returns EPERM > intermittently for > > futex(uaddr, FUTEX_WAIT_PRIVATE, val, &timeout, NULL, 0) > > The failure affects an application in an AWS lambda; traditional > debugging approaches vary from difficult to impossible. I cannot > reproduce the problem at will, instrument the kernel, install a new > kernel or get an application core dump. > > Understanding the circumstances under which EPERM can be returned for > FUTEX_WAIT_PRIVATE would be useful but it is not a documented failure > mode. I have spent some time looking through futex.c but have not > found anything yet. I would be grateful for a hint from someone more > knowledgeable. sys_futex(FUTEX_WAIT_PRIVATE) does not return -EPERM. Only the PI variants do that. Thanks, tglx