From 2e4b02e34783ecc6d4596d9bdd4368df0b85f9ac Mon Sep 17 00:00:00 2001
From: James Kovacs <jkovacs@post.harvard.edu>
Date: Wed, 24 Mar 2021 13:43:14 -0600
Subject: [PATCH 1/2] CSHARP-2450: Improved deserialization performance by
 switching from HashSet<T> protected by a ReaderWriterLockSlim to a
 ConcurrentDictionary<K,V> outside the ReaderWriterLockSlim.

---
 .../Serialization/BsonSerializer.cs           | 19 ++++++-------------
 1 file changed, 6 insertions(+), 13 deletions(-)

diff --git a/src/MongoDB.Bson/Serialization/BsonSerializer.cs b/src/MongoDB.Bson/Serialization/BsonSerializer.cs
index 52dabe5e7f6..4082dea45f1 100644
--- a/src/MongoDB.Bson/Serialization/BsonSerializer.cs
+++ b/src/MongoDB.Bson/Serialization/BsonSerializer.cs
@@ -14,6 +14,7 @@
 */
 
 using System;
+using System.Collections.Concurrent;
 using System.Collections.Generic;
 using System.IO;
 using System.Linq;
@@ -41,7 +42,7 @@ public static class BsonSerializer
         private static HashSet<Type> __discriminatedTypes = new HashSet<Type>();
         private static BsonSerializerRegistry __serializerRegistry;
         private static TypeMappingSerializationProvider __typeMappingSerializationProvider;
-        private static HashSet<Type> __typesWithRegisteredKnownTypes = new HashSet<Type>();
+        private static ConcurrentDictionary<Type, Type> __typesWithRegisteredKnownTypes = new ConcurrentDictionary<Type, Type>();
 
         private static bool __useNullIdChecker = false;
         private static bool __useZeroIdChecker = false;
@@ -679,23 +680,15 @@ public static void Serialize(
         // internal static methods
         internal static void EnsureKnownTypesAreRegistered(Type nominalType)
         {
-            __configLock.EnterReadLock();
-            try
-            {
-                if (__typesWithRegisteredKnownTypes.Contains(nominalType))
-                {
-                    return;
-                }
-            }
-            finally
+            if (__typesWithRegisteredKnownTypes.ContainsKey(nominalType))
             {
-                __configLock.ExitReadLock();
+                return;
             }
 
             __configLock.EnterWriteLock();
             try
             {
-                if (!__typesWithRegisteredKnownTypes.Contains(nominalType))
+                if (!__typesWithRegisteredKnownTypes.ContainsKey(nominalType))
                 {
                     // only call LookupClassMap for classes with a BsonKnownTypesAttribute
 #if NET452
@@ -709,7 +702,7 @@ internal static void EnsureKnownTypesAreRegistered(Type nominalType)
                         LookupSerializer(nominalType);
                     }
 
-                    __typesWithRegisteredKnownTypes.Add(nominalType);
+                    __typesWithRegisteredKnownTypes[nominalType] = nominalType;
                 }
             }
             finally

From 440c55997fd607232747b1b4fb2ddbaf8b6637ad Mon Sep 17 00:00:00 2001
From: James Kovacs <jkovacs@post.harvard.edu>
Date: Mon, 29 Mar 2021 11:07:48 -0600
Subject: [PATCH 2/2] Incorporated PR feedback.

---
 src/MongoDB.Bson/Serialization/BsonSerializer.cs | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/src/MongoDB.Bson/Serialization/BsonSerializer.cs b/src/MongoDB.Bson/Serialization/BsonSerializer.cs
index 4082dea45f1..0f493a00f21 100644
--- a/src/MongoDB.Bson/Serialization/BsonSerializer.cs
+++ b/src/MongoDB.Bson/Serialization/BsonSerializer.cs
@@ -42,7 +42,8 @@ public static class BsonSerializer
         private static HashSet<Type> __discriminatedTypes = new HashSet<Type>();
         private static BsonSerializerRegistry __serializerRegistry;
         private static TypeMappingSerializationProvider __typeMappingSerializationProvider;
-        private static ConcurrentDictionary<Type, Type> __typesWithRegisteredKnownTypes = new ConcurrentDictionary<Type, Type>();
+        // ConcurrentDictionary<Type, object> is being used as a concurrent set of Type. The values will always be null.
+        private static ConcurrentDictionary<Type, object> __typesWithRegisteredKnownTypes = new ConcurrentDictionary<Type, object>();
 
         private static bool __useNullIdChecker = false;
         private static bool __useZeroIdChecker = false;
@@ -702,7 +703,10 @@ internal static void EnsureKnownTypesAreRegistered(Type nominalType)
                         LookupSerializer(nominalType);
                     }
 
-                    __typesWithRegisteredKnownTypes[nominalType] = nominalType;
+                    // NOTE: The nominalType MUST be added to __typesWithRegisteredKnownTypes after all registration
+                    //       work is done to ensure that other threads don't access a partially registered nominalType
+                    //       when performing the initial check above outside the __config lock.
+                    __typesWithRegisteredKnownTypes[nominalType] = null;
                 }
             }
             finally