From fee960bed5e857eb126c4e56dd9ff85938356579 Mon Sep 17 00:00:00 2001 From: Mark Rutland Date: Wed, 3 May 2017 16:09:33 +0100 Subject: [PATCH] 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 : 0: d2800001 mov x1, #0x0 // #0 4: f9000001 str x1, [x0] 8: d2800000 mov x0, #0x0 // #0 c: d65f03c0 ret 0000000000000010 : 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: # 4.4.x- Acked-by: Will Deacon Signed-off-by: Mark Rutland Signed-off-by: Catalin Marinas --- arch/arm64/include/asm/cmpxchg.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) 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); \ \ -- 2.20.1