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 <dvlas...@redhat.com> > >> Cc: Ingo Molnar <mi...@kernel.org> > >> Cc: Thomas Graf <tg...@suug.ch> > >> Cc: Peter Zijlstra <pet...@infradead.org> > >> Cc: David Rientjes <rient...@google.com> > >> Cc: Andrew Morton <a...@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. -- Josh
