From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S934452AbaGXSrV (ORCPT ); Thu, 24 Jul 2014 14:47:21 -0400 Received: from mail-la0-f53.google.com ([209.85.215.53]:51940 "EHLO mail-la0-f53.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751250AbaGXSrT convert rfc822-to-8bit (ORCPT ); Thu, 24 Jul 2014 14:47:19 -0400 MIME-Version: 1.0 In-Reply-To: <53D064C7.5050807@daenzer.net> References: <20140723155526.GW3935@laptop> <20140723170324.GZ3935@laptop> <20140723182518.GD3935@laptop> <20140723184111.GG3935@laptop> <20140723190230.GH3935@laptop> <53D064C7.5050807@daenzer.net> Date: Thu, 24 Jul 2014 11:47:17 -0700 X-Google-Sender-Auth: EaF-S4E4Rl_u3xIauTxJyYkoCQA Message-ID: Subject: Re: Random panic in load_balance() with 3.16-rc From: Linus Torvalds To: =?UTF-8?Q?Michel_D=C3=A4nzer?= , Jakub Jelinek Cc: Peter Zijlstra , Dietmar Eggemann , Ingo Molnar , Linux Kernel Mailing List Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jul 23, 2014 at 6:43 PM, Michel Dänzer wrote: >> >> Michel, mind doing >> >> make kernel/sched/fair.s >> >> and sending us the resulting file? > > Here it is, gzipped, hope that's okay. > > Note that my tree is now based on 3.16-rc6. Ok, so I'm looking at the code generation and your compiler is pure and utter *shit*. Adding Jakub to the cc, because gcc-4.9.0 seems to be terminally broken. Lookie here, your compiler does some absolutely insane things with the spilling, including spilling a *constant*. For chrissake, that compiler shouldn't have been allowed to graduate from kindergarten. We're talking "sloth that was dropped on the head as a baby" level retardation levels here: ... movq $load_balance_mask, -136(%rbp) #, %sfp subq $184, %rsp #, movq (%rdx), %rax # sd_22(D)->parent, sd_parent movl %edi, -144(%rbp) # this_cpu, %sfp movl %ecx, -140(%rbp) # idle, %sfp movq %r8, -200(%rbp) # continue_balancing, %sfp movq %rax, -184(%rbp) # sd_parent, %sfp movq -136(%rbp), %rax # %sfp, tcp_ptr__ #APP add %gs:this_cpu_off, %rax # this_cpu_off, tcp_ptr__ #NO_APP ... Note the contents of -136(%rbp). Seriously. That's an _immediate_constant_ that the compiler is spilling. Somebody needs to raise that as a gcc bug. Because it damn well is some seriously crazy shit. However, that constant spilling part just counts as "too stupid to live". The real bug is this: movq $load_balance_mask, -136(%rbp) #, %sfp subq $184, %rsp #, where gcc creates the stack frame *after* having already used it to save that constant *deep* below the stack frame. The x86-64 ABI specifies a 128-byte red-zone under the stack pointer, and this is ok by that limit. It looks like it's illegal (136 > 128), but the fact is, we've had four "pushq"s to update %rsp since loading the frame pointer, so it's just *barely* legal with the red-zoning. But we build the kernel with -mno-red-zone. We do *not* follow the x86-64 ABI wrt redzoning, because we *cannot*: interrupts while in kernel mode *will* use the stack without a redzone. So that "-mno-red-zone" is not some "optional guideline". It's a hard and harsh requirement for the kernel, and gcc-4.9 is a buggy piece of shit for ignoring it. And your bug happens becuase you happen to hit an interrupt _just_ in that single instruction window (or perhaps hit some other similar case and corrupted kernel data structures earlier). Now, I suspect that this redzoning bug might actually be related to the fact that gcc is stupid in spilling a constant. I would not be surprised if there is some liveness analysis going on to decide *when* to insert the stack decrement, and constants are being ignored because clearly liveness isn't an issue for a constant value. So the two bugs ("stupid constant spilling" and "invalid use or red zone stack") go hand in hand. But who knows. Anyway, this is not a kernel bug. This is your compiler creating completely broken code. We may need to add a warning to make sure nobody compiles with gcc-4.9.0, and the Debian people should probably downgrate their shiny new compiler. Jakub, any ideas? Linus