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

[Josh Poimboeuf]

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:
> >>
> >> <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@xxxxxxxxxx>
> >> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> >> Cc: Thomas Graf <tgraf@xxxxxxx>
> >> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> >> Cc: David Rientjes <rientjes@xxxxxxxxxx>
> >> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> >> Cc: linux-kernel@xxxxxxxxxxxxxxx
> >> ---
> >>  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.

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