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    SubjectRe: This patch triggers a bad gcc bug (was Re: [PATCH] force inlining of some byteswap operations)
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    On Wed, 2016-04-13 at 10:15 -0500, Josh Poimboeuf wrote:
    > 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_O
    > > > > > s,
    > > > > > 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@redhat.com>
    > > > > > Cc: Ingo Molnar <mingo@kernel.org>
    > > > > > Cc: Thomas Graf <tgraf@suug.ch>
    > > > > > Cc: Peter Zijlstra <peterz@infradead.org>
    > > > > > Cc: David Rientjes <rientjes@google.com>
    > > > > > Cc: Andrew Morton <akpm@linux-foundation.org>
    > > > > > 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 <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
    >


    Actually, adding me doesn't help, I've added linux-scsi. The summary
    is that there's a but in gcc-5.3.1 which is miscompiling qla_attr.c ...
    this means we're going to have to ask the compiler version of reported
    crashes.

    James

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