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When compiling an i386_defconfig kernel with
gcc-4.6.1-9.fc15.i686, I noticed a warning about the asm operand
for test_bit in kprobes' can_boost. I discovered that this
caused only the first long of twobyte_is_boostable[] to be
output.
Jakub filed and fixed gcc PR50571 to correct the warning and
this output issue. But to solve it for less current gcc, we can
make kprobes' twobyte_is_boostable[] volatile, and it won't be
optimized out.
Before:
CC arch/x86/kernel/kprobes.o
In file included from include/linux/bitops.h:22:0,
from include/linux/kernel.h:17,
from [...]/arch/x86/include/asm/percpu.h:44,
from [...]/arch/x86/include/asm/current.h:5,
from [...]/arch/x86/include/asm/processor.h:15,
from [...]/arch/x86/include/asm/atomic.h:6,
from include/linux/atomic.h:4,
from include/linux/mutex.h:18,
from include/linux/notifier.h:13,
from include/linux/kprobes.h:34,
from arch/x86/kernel/kprobes.c:43:
[...]/arch/x86/include/asm/bitops.h: In function ‘can_boost.part.1’:
[...]/arch/x86/include/asm/bitops.h:319:2: warning: use of memory input without lvalue in asm operand 1 is deprecated [enabled by default]
$ objdump -rd arch/x86/kernel/kprobes.o | grep -A1 -w bt
551: 0f a3 05 00 00 00 00 bt %eax,0x0
554: R_386_32 .rodata.cst4
$ objdump -s -j .rodata.cst4 -j .data arch/x86/kernel/kprobes.o
arch/x86/kernel/kprobes.o: file format elf32-i386
Contents of section .data:
0000 48000000 00000000 00000000 00000000 H...............
Contents of section .rodata.cst4:
0000 4c030000 L...
Only a single long of twobyte_is_boostable[] is in the object
file.
After, with volatile:
$ objdump -rd arch/x86/kernel/kprobes.o | grep -A1 -w bt
551: 0f a3 05 20 00 00 00 bt %eax,0x20
554: R_386_32 .data
$ objdump -s -j .rodata.cst4 -j .data arch/x86/kernel/kprobes.o
arch/x86/kernel/kprobes.o: file format elf32-i386
Contents of section .data:
0000 48000000 00000000 00000000 00000000 H...............
0010 00000000 00000000 00000000 00000000 ................
0020 4c030000 0f000200 ffff0000 ffcff0c0 L...............
0030 0000ffff 3bbbfff8 03ff2ebb 26bb2e77 ....;.......&..w
Now all 32 bytes are output into .data instead.
Signed-off-by: Josh Stone <jistone@redhat.com>
Acked-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Cc: Jakub Jelinek <jakub@redhat.com>
Link: http://lkml.kernel.org/r/1318899645-4068-1-git-send-email-jistone@redhat.com
Signed-off-by: Ingo Molnar <mingo@elte.hu>
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