mips/atomic: Fix loongson_llsc_mb() wreckage

[linux.git] / arch / mips / include / asm / barrier.h

diff --git a/arch/mips/include/asm/barrier.h b/arch/mips/include/asm/barrier.h
index b865e317a14f70298ed47d77a12a277f5472156b..f9a6da96aae121f3316e3f927b07c3d57346acc6 100644 (file)
--- a/arch/mips/include/asm/barrier.h
+++ b/arch/mips/include/asm/barrier.h
@@ -238,36 +238,40 @@
 
 /*
  * 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
+ * store or prefetch) 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.
+ * 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.
+ *    In order to avoid this we need to place a memory barrier (ie. a SYNC
+ *    instruction) prior to every LL instruction, in between it and any earlier
+ *    memory access instructions.
  *
  *    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()
+ * 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.
+ *    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)
+ *    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.
+ *
+ * The above described cases cause an error in the cache coherence protocol;
+ * such that the Invalidate of a competing LL-SC goes 'missing' and SC
+ * erroneously observes its core still has Exclusive state and lets the SC
+ * proceed.
+ *
+ * Therefore the error only occurs on SMP systems.
  */
 #ifdef CONFIG_CPU_LOONGSON3_WORKAROUNDS /* Loongson-3's LLSC workaround */
-#define loongson_llsc_mb()     __asm__ __volatile__(__WEAK_LLSC_MB : : :"memory")
+#define loongson_llsc_mb()     __asm__ __volatile__("sync" : : :"memory")
 #else
 #define loongson_llsc_mb()     do { } while (0)
 #endif