Add flags to asymmetric mem barrier

Summary:
Original sys_membarrier call supported an EXPEDITED flag, that was much faster than the current version, at some extra CPU cost.

The basic idea was a normal one would be used in a background thread, while EXPEDITED would be useful inline in foreground threads.

Add the flag here, and use the mprotect hack until sys_membarrier supports both.

Reviewed By: magedm

Differential Revision: D5292680

fbshipit-source-id: 30edf27514976991bbaa8e776a7138eb815ade36

folly/experimental/AsymmetricMemoryBarrier.cpp

@@ -32,8 +32,7 @@ struct DummyPageCreator {
   }
 
   static void* get() {
-    static auto ptr =
-        kIsLinux && !detail::sysMembarrierAvailable() ? create() : nullptr;
+    static auto ptr = kIsLinux ? create() : nullptr;
     return ptr;
   }
 
@@ -81,10 +80,11 @@ void mprotectMembarrier() {
 }
 }
 
-void asymmetricHeavyBarrier() {
+void asymmetricHeavyBarrier(AMBFlags flags) {
   if (kIsLinux) {
     static const bool useSysMembarrier = detail::sysMembarrierAvailable();
-    if (useSysMembarrier) {
+    // sys_membarrier currently does not support EXPEDITED
+    if (useSysMembarrier && flags != AMBFlags::EXPEDITED) {
       auto r = detail::sysMembarrier();
       checkUnixError(r, "membarrier");
     } else {

folly/experimental/AsymmetricMemoryBarrier.h

@@ -22,6 +22,11 @@
 
 namespace folly {
 
+enum class AMBFlags {
+  NORMAL,
+  EXPEDITED,
+};
+
 FOLLY_ALWAYS_INLINE void asymmetricLightBarrier() {
   if (kIsLinux) {
     asm_volatile_memory();
@@ -30,5 +35,5 @@ FOLLY_ALWAYS_INLINE void asymmetricLightBarrier() {
   }
 }
 
-void asymmetricHeavyBarrier();
+void asymmetricHeavyBarrier(AMBFlags flags = AMBFlags::NORMAL);
 }