From a355313e020684c98969d07ae87c3f9bca6516df Mon Sep 17 00:00:00 2001
From: Russell Gray <russgray@gmail.com>
Date: Wed, 4 Feb 2015 15:18:30 +0000
Subject: [PATCH] Call TryGetValue in overload of ConcurrentDictionary.GetOrAdd
 to avoid acquiring lock.

There are two overloads of ConcurrentDictionary.GetOrAdd. One accepts a
factory method to generate the value if the key is not found, and calls
TryGetValue for the existence check which is a lock-free operation.

The other overload, however, does not call TryGetValue - it simply calls
straight through to TryAddInternal, which will obtain a bucket lock *before it
does the existence check*. This violates the documented claim of
ConcurrentDictionary of lock-free reads. This commit simply adds the same
TryGetValue check, making the two overloads consistent.
---
 .../src/System/Collections/Concurrent/ConcurrentDictionary.cs | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/src/mscorlib/src/System/Collections/Concurrent/ConcurrentDictionary.cs b/src/mscorlib/src/System/Collections/Concurrent/ConcurrentDictionary.cs
index ef18182c0cf6..ee66cb289399 100644
--- a/src/mscorlib/src/System/Collections/Concurrent/ConcurrentDictionary.cs
+++ b/src/mscorlib/src/System/Collections/Concurrent/ConcurrentDictionary.cs
@@ -1064,6 +1064,10 @@ public TValue GetOrAdd(TKey key, TValue value)
             if (key == null) throw new ArgumentNullException("key");
 
             TValue resultingValue;
+            if (TryGetValue(key, out resultingValue))
+            {
+                return resultingValue;
+            }
             TryAddInternal(key, value, false, true, out resultingValue);
             return resultingValue;
         }