X-Git-Url: http://plrg.eecs.uci.edu/git/?a=blobdiff_plain;f=folly%2FAtomicIntrusiveLinkedList.h;h=c0ee1b144274a8252f924355140e95a12b4c7612;hb=61fbd6698139d3f797401da6350371c7afc4030c;hp=e74554220123a1bf7dc93a6edffdefdfe2e66a43;hpb=36f174f920a35c7de58906b216008f8b56bd7231;p=folly.git

diff --git a/folly/AtomicIntrusiveLinkedList.h b/folly/AtomicIntrusiveLinkedList.h
index e7455422..c0ee1b14 100644
--- a/folly/AtomicIntrusiveLinkedList.h
+++ b/folly/AtomicIntrusiveLinkedList.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.
@@ -18,6 +18,7 @@
 
 #include <atomic>
 #include <cassert>
+#include <utility>
 
 namespace folly {
 
@@ -104,15 +105,31 @@ class AtomicIntrusiveLinkedList {
   void sweep(F&& func) {
     while (auto head = head_.exchange(nullptr)) {
       auto rhead = reverse(head);
-      while (rhead != nullptr) {
-        auto t = rhead;
-        rhead = next(t);
-        next(t) = nullptr;
-        func(t);
-      }
+      unlinkAll(rhead, std::forward<F>(func));
     }
   }
 
+  /**
+   * Similar to sweep() but calls func() on elements in LIFO order.
+   *
+   * func() is called for all elements in the list at the moment
+   * reverseSweep() is called.  Unlike sweep() it does not loop to ensure the
+   * list is empty at some point after the last invocation.  This way callers
+   * can reason about the ordering: elements inserted since the last call to
+   * reverseSweep() will be provided in LIFO order.
+   *
+   * Example: if elements are inserted in the order 1-2-3, the callback is
+   * invoked 3-2-1.  If the callback moves elements onto a stack, popping off
+   * the stack will produce the original insertion order 1-2-3.
+   */
+  template <typename F>
+  void reverseSweep(F&& func) {
+    // We don't loop like sweep() does because the overall order of callbacks
+    // would be strand-wise LIFO which is meaningless to callers.
+    auto head = head_.exchange(nullptr);
+    unlinkAll(head, std::forward<F>(func));
+  }
+
  private:
   std::atomic<T*> head_{nullptr};
 
@@ -132,6 +149,18 @@ class AtomicIntrusiveLinkedList {
     }
     return rhead;
   }
+
+  /* Unlinks all elements in the linked list fragment pointed to by `head',
+   * calling func() on every element */
+  template <typename F>
+  void unlinkAll(T* head, F&& func) {
+    while (head != nullptr) {
+      auto t = head;
+      head = next(t);
+      next(t) = nullptr;
+      func(t);
+    }
+  }
 };
 
 } // namespace folly