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=-0.8 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 6F0CCC433E0 for ; Sat, 27 Jun 2020 18:31:06 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 4A71020760 for ; Sat, 27 Jun 2020 18:31:06 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="C5RrWz/f" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1725900AbgF0SbF (ORCPT ); Sat, 27 Jun 2020 14:31:05 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:46620 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725867AbgF0SbF (ORCPT ); Sat, 27 Jun 2020 14:31:05 -0400 Received: from merlin.infradead.org (unknown [IPv6:2001:8b0:10b:1231::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id ECAE4C061794; Sat, 27 Jun 2020 11:31:04 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=merlin.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=EqUqgITG7ecsHU1e88clY3vesSjbEZhx3MeoTPdWfIg=; b=C5RrWz/fA6p/G3PTFtiwvmczon 4mz/MmxNzJKyeo7VvdUJMBSd9TcPYjJG4+zfpLX4uw3yZdQ0WYKKZxpYYJ5IiJ40812UD0FoAnysS UlzrnQHO72zQ4UIuUymqAVVzNyP68SdX7PTQprTpl3MhKO1pEptFBAvdAxwbuV4C55KcgdZ2VfF9O 1ImievnmyEYFw89GohdJXs1fH06ZBus1NZBNhWlhiSb8mKl2syj0g0XeizGlsO2DEBPjj7mIrBIIV MmmJ3Up5lcQxmx3DSuNHlt9zRLvGYf1YiQbISb7ddq5BjWFE5n5cGNiWk5LpBJ+JrbaMVm0yL4CkA pgXgZnIg==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=noisy.programming.kicks-ass.net) by merlin.infradead.org with esmtpsa (Exim 4.92.3 #3 (Red Hat Linux)) id 1jpFbB-0004Ve-DO; Sat, 27 Jun 2020 18:30:45 +0000 Received: from hirez.programming.kicks-ass.net (hirez.programming.kicks-ass.net [192.168.1.225]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id BB45F301A7A; Sat, 27 Jun 2020 20:30:42 +0200 (CEST) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id A3C7E22B8B6C1; Sat, 27 Jun 2020 20:30:42 +0200 (CEST) Date: Sat, 27 Jun 2020 20:30:42 +0200 From: Peter Zijlstra To: Dave Chinner Cc: linux-kernel@vger.kernel.org, linux-xfs@vger.kernel.org, Ingo Molnar Subject: Re: [Bug, sched, 5.8-rc2]: PREEMPT kernels crashing in check_preempt_wakeup() running fsx on XFS Message-ID: <20200627183042.GK4817@hirez.programming.kicks-ass.net> References: <20200626004722.GF2005@dread.disaster.area> <20200626073345.GI4800@hirez.programming.kicks-ass.net> <20200626223254.GH2005@dread.disaster.area> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200626223254.GH2005@dread.disaster.area> Sender: linux-xfs-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-xfs@vger.kernel.org On Sat, Jun 27, 2020 at 08:32:54AM +1000, Dave Chinner wrote: > Observation from the outside: > > "However I'm having trouble convincing myself that's actually > possible on x86_64.... " Using the weaker rules of LKMM (as relevant to Power) I could in fact make it happen, the 'problem' is that it's being observed on the much stronger x86_64. So possibly I did overlook a more 'sensible' scenario, but I'm pretty confident the problem holds as it fully explains the failure mode. > This scheduler code has fallen off a really high ledge on the memory > barrier cliff, hasn't it? Just a wee bit.. I did need pen and paper and a fair amount of scribbling for this one. > Having looked at this code over the past 24 hours and the recent > history, I know that understanding it - let alone debugging and > fixing problem in it - is way beyond my capabilities. And I say > that as an experienced kernel developer with a pretty good grasp of > concurrent programming and a record of implementing a fair number of > non-trivial lockless algorithms over the years.... All in the name of making it go fast, I suppose. It used to be much simpler... like much of the kernel. The biggest problem I had with this thing was that the reproduction case we had (Paul's rcutorture) wouldn't readily trigger on my machines (altough it did, but at a much lower rate, just twice in a week's worth of runtime). Also; I'm sure you can spot a problem in the I/O layer much faster than I possibly could :-) Anyway, let me know if you still observe any problems.