From db45bd90be200692342a340e84354c26ca9e424c Mon Sep 17 00:00:00 2001 From: Josh Stone Date: Mon, 17 Oct 2011 18:00:45 -0700 Subject: [PATCH] x86, perf, kprobes: Make kprobes's twobyte_is_boostable volatile MIME-Version: 1.0 Content-Type: text/plain; charset=utf8 Content-Transfer-Encoding: 8bit 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 Acked-by: Masami Hiramatsu Cc: Srikar Dronamraju Cc: Jakub Jelinek Link: http://lkml.kernel.org/r/1318899645-4068-1-git-send-email-jistone@redhat.com Signed-off-by: Ingo Molnar --- arch/x86/kernel/kprobes.c | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-) diff --git a/arch/x86/kernel/kprobes.c b/arch/x86/kernel/kprobes.c index f1a6244..c0ed3d9 100644 --- a/arch/x86/kernel/kprobes.c +++ b/arch/x86/kernel/kprobes.c @@ -75,8 +75,10 @@ DEFINE_PER_CPU(struct kprobe_ctlblk, kprobe_ctlblk); /* * Undefined/reserved opcodes, conditional jump, Opcode Extension * Groups, and some special opcodes can not boost. + * This is volatile to keep gcc from statically optimizing it out, as + * variable_test_bit makes gcc think only *(unsigned long*) is used. */ -static const u32 twobyte_is_boostable[256 / 32] = { +static volatile const u32 twobyte_is_boostable[256 / 32] = { /* 0 1 2 3 4 5 6 7 8 9 a b c d e f */ /* ---------------------------------------------- */ W(0x00, 0, 0, 1, 1, 0, 0, 1, 0, 1, 1, 0, 0, 0, 0, 0, 0) | /* 00 */ -- 2.7.4