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mips/atomic: Fix loongson_llsc_mb() wreckage

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The comment describing the loongson_llsc_mb() reorder case doesn't
make any sense what so ever. Instruction re-ordering is not an SMP
artifact, but rather a CPU local phenomenon. Clarify the comment by
explaining that these issue cause a coherence fail.

For the branch speculation case; if futex_atomic_cmpxchg_inatomic()
needs one at the bne branch target, then surely the normal
__cmpxch_asm() implementation does too. We cannot rely on the
barriers from cmpxchg() because cmpxchg_local() is implemented with
the same macro, and branch prediction and speculation are, too, CPU
local.

Fixes: e02e07e312 ("MIPS: Loongson: Introduce and use loongson_llsc_mb()")
Cc: Huacai Chen <chenhc@lemote.com>
Cc: Huang Pei <huangpei@loongson.cn>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Paul Burton <paul.burton@mips.com>
This commit is contained in:
Peter Zijlstra
2019-06-13 15:43:19 +02:00
committed by Paul Burton
parent dfc8d8de85
commit 1c6c1ca318
5 changed files with 32 additions and 16 deletions
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5
arch/mips/include/asm/cmpxchg.h
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@@ -46,6 +46,7 @@ extern unsigned long __xchg_called_with_bad_pointer(void)
__typeof(*(m)) __ret; \
\
if (kernel_uses_llsc) { \
loongson_llsc_mb(); \
__asm__ __volatile__( \
" .set push \n" \
" .set noat \n" \
@@ -117,6 +118,7 @@ static inline unsigned long __xchg(volatile void *ptr, unsigned long x,
__typeof(*(m)) __ret; \
\
if (kernel_uses_llsc) { \
loongson_llsc_mb(); \
__asm__ __volatile__( \
" .set push \n" \
" .set noat \n" \
@@ -134,6 +136,7 @@ static inline unsigned long __xchg(volatile void *ptr, unsigned long x,
: "=&r" (__ret), "=" GCC_OFF_SMALL_ASM() (*m) \
: GCC_OFF_SMALL_ASM() (*m), "Jr" (old), "Jr" (new) \
: "memory"); \
loongson_llsc_mb(); \
} else { \
unsigned long __flags; \
\
@@ -229,6 +232,7 @@ static inline unsigned long __cmpxchg64(volatile void *ptr,
*/
local_irq_save(flags);
loongson_llsc_mb();
asm volatile(
" .set push \n"
" .set " MIPS_ISA_ARCH_LEVEL " \n"
@@ -274,6 +278,7 @@ static inline unsigned long __cmpxchg64(volatile void *ptr,
"r" (old),
"r" (new)
: "memory");
loongson_llsc_mb();
local_irq_restore(flags);
return ret;