linux-kernel - Re: This patch triggers a bad gcc bug (was Re: [PATCH] force inlining of some byteswap operations)

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Date:	Wed, 13 Apr 2016 14:12:25 +0200
From:	Denys Vlasenko <dvlasenk@...hat.com>
To:	Josh Poimboeuf <jpoimboe@...hat.com>
CC:	Ingo Molnar <mingo@...nel.org>, Thomas Graf <tgraf@...g.ch>,
	Peter Zijlstra <peterz@...radead.org>,
	David Rientjes <rientjes@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, Arnd Bergmann <arnd@...db.de>
Subject: Re: This patch triggers a bad gcc bug (was Re: [PATCH] force inlining
 of some byteswap operations)

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:
>>
>> <get_unaligned_be16> (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 <dvlasenk@...hat.com>
>> Cc: Ingo Molnar <mingo@...nel.org>
>> Cc: Thomas Graf <tgraf@...g.ch>
>> Cc: Peter Zijlstra <peterz@...radead.org>
>> Cc: David Rientjes <rientjes@...gle.com>
>> Cc: Andrew Morton <akpm@...ux-foundation.org>
>> Cc: linux-kernel@...r.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 <qla2x00_get_host_fabric_name>:
>     2f53:       55                      push   %rbp
>     2f54:       48 89 e5                mov    %rsp,%rbp
> 
> 0000000000002f57 <qla2x00_get_fc_host_stats>:
>     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.