Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757906AbcDMMgM (ORCPT ); Wed, 13 Apr 2016 08:36:12 -0400 Received: from mx1.redhat.com ([209.132.183.28]:58938 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750891AbcDMMgK (ORCPT ); Wed, 13 Apr 2016 08:36:10 -0400 Date: Wed, 13 Apr 2016 07:36:07 -0500 From: Josh Poimboeuf To: Denys Vlasenko Cc: Ingo Molnar , Thomas Graf , Peter Zijlstra , David Rientjes , Andrew Morton , linux-kernel@vger.kernel.org, Arnd Bergmann Subject: Re: This patch triggers a bad gcc bug (was Re: [PATCH] force inlining of some byteswap operations) Message-ID: <20160413123607.3y3v5esq3myto4kc@treble> References: <1454615136-32308-1-git-send-email-dvlasenk@redhat.com> <1454615136-32308-2-git-send-email-dvlasenk@redhat.com> <20160413033649.7r3msnmo3trtq47z@treble> <570E37A9.2050503@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <570E37A9.2050503@redhat.com> User-Agent: Mutt/1.6.0.1 (2016-04-01) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3889 Lines: 99 On Wed, Apr 13, 2016 at 02:12:25PM +0200, Denys Vlasenko wrote: > On 04/13/2016 05:36 AM, Josh Poimboeuf wrote: > > On Thu, Feb 04, 2016 at 08:45:35PM +0100, Denys Vlasenko wrote: > >> Sometimes gcc mysteriously doesn't inline > >> very small functions we expect to be inlined. See > >> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66122 > >> > >> With this .config: > >> http://busybox.net/~vda/kernel_config_OPTIMIZE_INLINING_and_Os, > >> the following functions get deinlined many times. > >> Examples of disassembly: > >> > >> (12 copies, 51 calls): > >> 66 8b 07 mov (%rdi),%ax > >> 55 push %rbp > >> 48 89 e5 mov %rsp,%rbp > >> 86 e0 xchg %ah,%al > >> 5d pop %rbp > >> c3 retq > ... > >> This patch fixes this via s/inline/__always_inline/. > >> Code size decrease after the patch is ~4.5k: > >> > >> text data bss dec hex filename > >> 92202377 20826112 36417536 149446025 8e85d89 vmlinux > >> 92197848 20826112 36417536 149441496 8e84bd8 vmlinux5_swap_after > >> > >> Signed-off-by: Denys Vlasenko > >> Cc: Ingo Molnar > >> Cc: Thomas Graf > >> Cc: Peter Zijlstra > >> Cc: David Rientjes > >> Cc: Andrew Morton > >> Cc: linux-kernel@vger.kernel.org > >> --- > >> include/uapi/linux/byteorder/big_endian.h | 24 ++++++++++++------------ > >> include/uapi/linux/byteorder/little_endian.h | 24 ++++++++++++------------ > >> include/uapi/linux/swab.h | 10 +++++----- > >> 3 files changed, 29 insertions(+), 29 deletions(-) > > > > Hi, > > > > This patch seems to trigger a gcc bug which can produce corrupt code. I > > discovered it when investigating an objtool warning reported by kbuild > > bot: > > > > http://www.spinics.net/lists/linux-scsi/msg95481.html > > > > From the disassembly of drivers/scsi/qla2xxx/qla_attr.o: > > > > 0000000000002f53 : > > 2f53: 55 push %rbp > > 2f54: 48 89 e5 mov %rsp,%rbp > > > > 0000000000002f57 : > > 2f57: 55 push %rbp > > 2f58: b9 e8 00 00 00 mov $0xe8,%ecx > > 2f5d: 48 89 e5 mov %rsp,%rbp > > ... > > > > Note that qla2x00_get_host_fabric_name() is inexplicably truncated after > > setting up the frame pointer. It falls through to the next function, which is > > very wrong. > > Wow, that's ... interesting. > > > > I can recreate it with either gcc 5.3.1 or gcc 6.0 on linus/master with > > the .config from the above link. > > > > The call chain which appears to trigger the problem is: > > > > qla2x00_get_host_fabric_name() > > wwn_to_u64() > > get_unaligned_be64() > > be64_to_cpup() > > __be64_to_cpup() <- changed to __always_inline by this patch > > > > It occurs with the combination of the following two recent commits: > > > > - bc27fb68aaad ("include/uapi/linux/byteorder, swab: force inlining of some byteswap operations") > > - ef3fb2422ffe ("scsi: fc: use get/put_unaligned64 for wwn access") > > > > I can confirm that reverting either patch makes the problem go away. > > I'm planning on opening a gcc bug tomorrow. > > > Note that if CONFIG_OPTIMIZE_INLINING is not set, _all_ "inline" > keywords are in fact __always_inline, so the bug must be triggering > even without the patch. Makes sense in theory, but the bug doesn't actually trigger when I revert the patch and set CONFIG_OPTIMIZE_INLINING=n. Perhaps even more surprising, it doesn't trigger *with* the patch and CONFIG_OPTIMIZE_INLINING=n. -- Josh