From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Date: Mon, 28 Jan 2019 14:44:10 +0100 From: Peter Zijlstra Subject: Re: WARN_ON_ONCE(!new_owner) within wake_futex_pi() triggered Message-ID: <20190128134410.GA28485@hirez.programming.kicks-ass.net> References: <20181127081115.GB3625@osiris> <20181129112321.GB3449@osiris> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20181129112321.GB3449@osiris> Sender: linux-kernel-owner@vger.kernel.org List-Archive: List-Post: To: Heiko Carstens Cc: Thomas Gleixner , Ingo Molnar , Martin Schwidefsky , linux-kernel@vger.kernel.org, linux-s390@vger.kernel.org, Stefan Liebler List-ID: On Thu, Nov 29, 2018 at 12:23:21PM +0100, Heiko Carstens wrote: > And indeed, if I run only this test case in an endless loop and do > some parallel work (like kernel compile) it currently seems to be > possible to reproduce the warning: > > while true; do time ./testrun.sh nptl/tst-robustpi8 --direct ; done > > within the build directory of glibc (2.28). Right; so that reproduces for me. After staring at all that for a while; trying to remember how it all worked (or supposed to work rather), I became suspiscous of commit: 56222b212e8e ("futex: Drop hb->lock before enqueueing on the rtmutex") And indeed, when I revert that; the above reproducer no longer works (as in, it no longer triggers in minutes and has -- so far -- held up for an hour+ or so). That patch in particular allows futex_unlock_pi() to 'start' early: futex_lock_pi() futex_unlock_pi() lock hb queue lock wait_lock unlock hb lock hb futex_top_waiter get_pi_state lock wait_lock rt_mutex_proxy_start // fail unlock wait_lock // acquired wait_lock wake_futex_pi() rt_mutex_next_owner() // whoops, no waiter WARN lock hb unqueue_me_pi So reverting that patch should cure things, because then there is no hb lock break between queue/unqueue and futex_unlock_pi() cannot observe this half-arsed state. Now obviously reverting that makes RT unhappy; let me see what the options are. (concurrently tglx generated a trace that corroborates)