make extent_hooks static.

[folly.git] / folly / experimental / AsymmetricMemoryBarrier.cpp

diff --git a/folly/experimental/AsymmetricMemoryBarrier.cpp b/folly/experimental/AsymmetricMemoryBarrier.cpp
index 31c34cb7aea10a389372316da55ae1fd5d34a8f1..55bec5f141bdc17d35d6347c1477b8d8dc9b4ac4 100644 (file)
--- a/folly/experimental/AsymmetricMemoryBarrier.cpp
+++ b/folly/experimental/AsymmetricMemoryBarrier.cpp
@@ -1,5 +1,5 @@
 /*
- * Copyright 2016 Facebook, Inc.
+ * Copyright 2017 Facebook, Inc.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -40,18 +40,20 @@ struct DummyPageCreator {
  private:
   static void* create() {
     auto ptr = mmap(nullptr, 1, PROT_READ, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
-    checkUnixError(reinterpret_cast<uintptr_t>(ptr), "mmap");
+    checkUnixError(reinterpret_cast<ssize_t>(ptr), "mmap");
 
-    // Lock the memory so it can't get paged out. If it gets paged out, changing
-    // its protection won't accomplish anything.
+    // Optimistically try to lock the page so it stays resident. Could make
+    // the heavy barrier faster.
     auto r = mlock(ptr, 1);
-    checkUnixError(r, "mlock");
+    if (r != 0) {
+      // Do nothing.
+    }
 
     return ptr;
   }
 };
 
-// Make sure dummy page is always initialized before shutdown
+// Make sure dummy page is always initialized before shutdown.
 DummyPageCreator dummyPageCreator;
 
 void mprotectMembarrier() {
@@ -63,9 +65,17 @@ void mprotectMembarrier() {
   std::lock_guard<std::mutex> lg(*mprotectMutex);
 
   int r = 0;
+
+  // We want to downgrade the page while it is resident. To do that, it must
+  // first be upgraded and forced to be resident.
   r = mprotect(dummyPage, 1, PROT_READ | PROT_WRITE);
   checkUnixError(r, "mprotect");
 
+  // Force the page to be resident. If it is already resident, almost no-op.
+  *static_cast<char*>(dummyPage) = 0;
+
+  // Downgrade the page. Forces a memory barrier in every core running any
+  // of the process's threads. On a sane platform.
   r = mprotect(dummyPage, 1, PROT_READ);
   checkUnixError(r, "mprotect");
 }