On Wed, Apr 13, 2016 at 09:55:09AM -0700, James Bottomley wrote: > 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@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. > > > > [ 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. The bug isn't specific to a compiler version. I've seen it with gcc 5.3.1 and gcc 6.0. I haven't tried any older versions. And the gcc bug hasn't been resolved (or even investigated) yet. The bug is triggered by a combination of the above two commits from the 4.6 merge window, so presumably we'd need to revert one of them to avoid crashes in 4.6. -- Josh -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
