messageBlackHole: fix deadlock bug caused by a missing 'volatile'

[ghc-hetmet.git] / includes / stg / SMP.h

diff --git a/includes/stg/SMP.h b/includes/stg/SMP.h
index 8297f57..ad8c0ba 100644 (file)
--- a/includes/stg/SMP.h
+++ b/includes/stg/SMP.h
@@ -64,6 +64,13 @@ EXTERN_INLINE StgWord atomic_inc(StgVolatilePtr p);
  */
 EXTERN_INLINE StgWord atomic_dec(StgVolatilePtr p);
 
+/*
+ * Busy-wait nop: this is a hint to the CPU that we are currently in a
+ * busy-wait loop waiting for another CPU to change something.  On a
+ * hypertreaded CPU it should yield to another thread, for example.
+ */
+EXTERN_INLINE void busy_wait_nop(void);
+
 #endif // !IN_STG_CODE
 
 /*
@@ -216,6 +223,17 @@ atomic_dec(StgVolatilePtr p)
 #endif
 }
 
+EXTERN_INLINE void
+busy_wait_nop(void)
+{
+#if defined(i386_HOST_ARCH) || defined(x86_64_HOST_ARCH)
+    __asm__ __volatile__ ("rep; nop");
+    //
+#else
+    // nothing
+#endif
+}
+
 #endif // !IN_STG_CODE
 
 /*
@@ -275,6 +293,12 @@ load_load_barrier(void) {
 #endif
 }
 
+// Load a pointer from a memory location that might be being modified
+// concurrently.  This prevents the compiler from optimising away
+// multiple loads of the memory location, as it might otherwise do in
+// a busy wait loop for example.
+#define VOLATILE_LOAD(p) (*((StgVolatilePtr)(p)))
+
 /* ---------------------------------------------------------------------- */
 #else /* !THREADED_RTS */
 
@@ -313,6 +337,8 @@ atomic_dec(StgVolatilePtr p)
     return --(*p);
 }
 
+#define VOLATILE_LOAD(p) ((StgWord)*((StgWord*)(p)))
+
 #endif /* !THREADED_RTS */
 
 #endif /* SMP_H */