diff options
Diffstat (limited to 'arch/mips/include/asm/barrier.h')
| -rw-r--r-- | arch/mips/include/asm/barrier.h | 36 |
1 files changed, 36 insertions, 0 deletions
diff --git a/arch/mips/include/asm/barrier.h b/arch/mips/include/asm/barrier.h index a5eb1bb199a7..b7f6ac5e513c 100644 --- a/arch/mips/include/asm/barrier.h +++ b/arch/mips/include/asm/barrier.h @@ -222,6 +222,42 @@ #define __smp_mb__before_atomic() __smp_mb__before_llsc() #define __smp_mb__after_atomic() smp_llsc_mb() +/* + * Some Loongson 3 CPUs have a bug wherein execution of a memory access (load, + * store or pref) in between an ll & sc can cause the sc instruction to + * erroneously succeed, breaking atomicity. Whilst it's unusual to write code + * containing such sequences, this bug bites harder than we might otherwise + * expect due to reordering & speculation: + * + * 1) A memory access appearing prior to the ll in program order may actually + * be executed after the ll - this is the reordering case. + * + * In order to avoid this we need to place a memory barrier (ie. a sync + * instruction) prior to every ll instruction, in between it & any earlier + * memory access instructions. Many of these cases are already covered by + * smp_mb__before_llsc() but for the remaining cases, typically ones in + * which multiple CPUs may operate on a memory location but ordering is not + * usually guaranteed, we use loongson_llsc_mb() below. + * + * This reordering case is fixed by 3A R2 CPUs, ie. 3A2000 models and later. + * + * 2) If a conditional branch exists between an ll & sc with a target outside + * of the ll-sc loop, for example an exit upon value mismatch in cmpxchg() + * or similar, then misprediction of the branch may allow speculative + * execution of memory accesses from outside of the ll-sc loop. + * + * In order to avoid this we need a memory barrier (ie. a sync instruction) + * at each affected branch target, for which we also use loongson_llsc_mb() + * defined below. + * + * This case affects all current Loongson 3 CPUs. + */ +#ifdef CONFIG_CPU_LOONGSON3_WORKAROUNDS /* Loongson-3's LLSC workaround */ +#define loongson_llsc_mb() __asm__ __volatile__(__WEAK_LLSC_MB : : :"memory") +#else +#define loongson_llsc_mb() do { } while (0) +#endif + #include <asm-generic/barrier.h> #endif /* __ASM_BARRIER_H */ |