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authorMark Rutland <mark.rutland@arm.com>2017-05-03 16:09:33 +0100
committerCatalin Marinas <catalin.marinas@arm.com>2017-05-09 17:44:50 +0100
commitfee960bed5e857eb126c4e56dd9ff85938356579 (patch)
tree804eb813637d43f169304c4ca0da4317bbd9295c /arch
parentf0e421b1bf7af97f026e1bb8bfe4c5a7a8c08f42 (diff)
arm64: xchg: hazard against entire exchange variable
The inline assembly in __XCHG_CASE() uses a +Q constraint to hazard against other accesses to the memory location being exchanged. However, the pointer passed to the constraint is a u8 pointer, and thus the hazard only applies to the first byte of the location. GCC can take advantage of this, assuming that other portions of the location are unchanged, as demonstrated with the following test case: union u { unsigned long l; unsigned int i[2]; }; unsigned long update_char_hazard(union u *u) { unsigned int a, b; a = u->i[1]; asm ("str %1, %0" : "+Q" (*(char *)&u->l) : "r" (0UL)); b = u->i[1]; return a ^ b; } unsigned long update_long_hazard(union u *u) { unsigned int a, b; a = u->i[1]; asm ("str %1, %0" : "+Q" (*(long *)&u->l) : "r" (0UL)); b = u->i[1]; return a ^ b; } The linaro 15.08 GCC 5.1.1 toolchain compiles the above as follows when using -O2 or above: 0000000000000000 <update_char_hazard>: 0: d2800001 mov x1, #0x0 // #0 4: f9000001 str x1, [x0] 8: d2800000 mov x0, #0x0 // #0 c: d65f03c0 ret 0000000000000010 <update_long_hazard>: 10: b9400401 ldr w1, [x0,#4] 14: d2800002 mov x2, #0x0 // #0 18: f9000002 str x2, [x0] 1c: b9400400 ldr w0, [x0,#4] 20: 4a000020 eor w0, w1, w0 24: d65f03c0 ret This patch fixes the issue by passing an unsigned long pointer into the +Q constraint, as we do for our cmpxchg code. This may hazard against more than is necessary, but this is better than missing a necessary hazard. Fixes: 305d454aaa29 ("arm64: atomics: implement native {relaxed, acquire, release} atomics") Cc: <stable@vger.kernel.org> # 4.4.x- Acked-by: Will Deacon <will.deacon@arm.com> Signed-off-by: Mark Rutland <mark.rutland@arm.com> Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Diffstat (limited to 'arch')
-rw-r--r--arch/arm64/include/asm/cmpxchg.h2
1 files changed, 1 insertions, 1 deletions
diff --git a/arch/arm64/include/asm/cmpxchg.h b/arch/arm64/include/asm/cmpxchg.h
index 91b26d26af8a..ae852add053d 100644
--- a/arch/arm64/include/asm/cmpxchg.h
+++ b/arch/arm64/include/asm/cmpxchg.h
@@ -46,7 +46,7 @@ static inline unsigned long __xchg_case_##name(unsigned long x, \
" swp" #acq_lse #rel #sz "\t%" #w "3, %" #w "0, %2\n" \
__nops(3) \
" " #nop_lse) \
- : "=&r" (ret), "=&r" (tmp), "+Q" (*(u8 *)ptr) \
+ : "=&r" (ret), "=&r" (tmp), "+Q" (*(unsigned long *)ptr) \
: "r" (x) \
: cl); \
\