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 10:15:00 -0500
From:	Josh Poimboeuf <jpoimboe@...hat.com>
To:	Denys Vlasenko <dvlasenk@...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>,
	James Bottomley <James.Bottomley@...senPartnership.com>
Subject: Re: This patch triggers a bad gcc bug (was Re: [PATCH] force
 inlining of some byteswap operations)

On Wed, Apr 13, 2016 at 07:36:07AM -0500, Josh Poimboeuf wrote:
> 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@...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.
> 
> 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.

[ Adding James to CC since this bug affects scsi. ]

Here's the gcc bug:

  https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70646

-- 
Josh