Revert D4832473: [Folly] Disable EnvUtil::setAsCurrentEnvironment() on platforms...

[folly.git] / folly / ThreadLocal.h

diff --git a/folly/ThreadLocal.h b/folly/ThreadLocal.h
index 55b7a246e9e00d94dbb45d92f7165a9ad5e4c516..3565c210a25e0279bd6d1ce614cf684a8fe3852e 100644 (file)
--- a/folly/ThreadLocal.h
+++ b/folly/ThreadLocal.h
@@ -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.
@@ -23,6 +23,10 @@
  * objects of a parent.  accessAllThreads() initializes an accessor which holds
  * a global lock *that blocks all creation and destruction of ThreadLocal
  * objects with the same Tag* and can be used as an iterable container.
+ * accessAllThreads() can race with destruction of thread-local elements. We
+ * provide a strict mode which is dangerous because it requires the access lock
+ * to be held while destroying thread-local elements which could cause
+ * deadlocks. We gate this mode behind the AccessModeStrict template parameter.
  *
  * Intended use is for frequent write, infrequent read data access patterns such
  * as counters.
@@ -49,15 +53,17 @@ enum class TLPDestructionMode {
   THIS_THREAD,
   ALL_THREADS
 };
+struct AccessModeStrict {};
 }  // namespace
 
 #include <folly/detail/ThreadLocalDetail.h>
 
 namespace folly {
 
-template<class T, class Tag> class ThreadLocalPtr;
+template <class T, class Tag, class AccessMode>
+class ThreadLocalPtr;
 
-template<class T, class Tag=void>
+template <class T, class Tag = void, class AccessMode = void>
 class ThreadLocal {
  public:
   constexpr ThreadLocal() : constructor_([]() {
@@ -90,7 +96,7 @@ class ThreadLocal {
     tlp_.reset(newPtr);
   }
 
-  typedef typename ThreadLocalPtr<T,Tag>::Accessor Accessor;
+  typedef typename ThreadLocalPtr<T, Tag, AccessMode>::Accessor Accessor;
   Accessor accessAllThreads() const {
     return tlp_.accessAllThreads();
   }
@@ -110,7 +116,7 @@ class ThreadLocal {
     return ptr;
   }
 
-  mutable ThreadLocalPtr<T,Tag> tlp_;
+  mutable ThreadLocalPtr<T, Tag, AccessMode> tlp_;
   std::function<T*()> constructor_;
 };
 
@@ -140,10 +146,11 @@ class ThreadLocal {
  *       with __declspec(thread)
  */
 
-template<class T, class Tag=void>
+template <class T, class Tag = void, class AccessMode = void>
 class ThreadLocalPtr {
  private:
-  typedef threadlocal_detail::StaticMeta<Tag> StaticMeta;
+  typedef threadlocal_detail::StaticMeta<Tag, AccessMode> StaticMeta;
+
  public:
   constexpr ThreadLocalPtr() : id_() {}
 
@@ -247,7 +254,7 @@ class ThreadLocalPtr {
   // Can be used as an iterable container.
   // Use accessAllThreads() to obtain one.
   class Accessor {
-    friend class ThreadLocalPtr<T,Tag>;
+    friend class ThreadLocalPtr<T, Tag, AccessMode>;
 
     threadlocal_detail::StaticMetaBase& meta_;
     SharedMutex* accessAllThreadsLock_;
@@ -348,14 +355,14 @@ class ThreadLocalPtr {
     }
 
     Accessor()
-        : meta_(threadlocal_detail::StaticMeta<Tag>::instance()),
+        : meta_(threadlocal_detail::StaticMeta<Tag, AccessMode>::instance()),
           accessAllThreadsLock_(nullptr),
           lock_(nullptr),
           id_(0) {}
 
    private:
     explicit Accessor(uint32_t id)
-        : meta_(threadlocal_detail::StaticMeta<Tag>::instance()),
+        : meta_(threadlocal_detail::StaticMeta<Tag, AccessMode>::instance()),
           accessAllThreadsLock_(&meta_.accessAllThreadsLock_),
           lock_(&meta_.lock_) {
       accessAllThreadsLock_->lock();