From ad418f66d55ae9430ae1088c9c90ab528dce4f96 Mon Sep 17 00:00:00 2001
From: Stephen Toub <stoub@microsoft.com>
Date: Thu, 22 Jun 2023 00:10:07 -0400
Subject: [PATCH] Fix contention in LoggerFactory.CreateLogger

Some ASP.NET code paths end up calling this _a lot_, and it takes a lock on every call, even though the vast majority are satisifed by the cache and require no mutation.  We can use a ConcurrentDictionary instead of a Dictionary, with double-checked locking.  Nothing is ever removed from or overwritten in the dictionary, so there's no problem doing the hot-path read outside of the lock; we still do the mutation inside of the lock so that all of the mutation work is performed atomically and synchronized with the actions from AddProvider and RefreshFilter.
---
 .../src/LoggerFactory.cs                      | 20 +++++++++++--------
 1 file changed, 12 insertions(+), 8 deletions(-)

diff --git a/src/libraries/Microsoft.Extensions.Logging/src/LoggerFactory.cs b/src/libraries/Microsoft.Extensions.Logging/src/LoggerFactory.cs
index bff52609d62bca..305860f6e91616 100644
--- a/src/libraries/Microsoft.Extensions.Logging/src/LoggerFactory.cs
+++ b/src/libraries/Microsoft.Extensions.Logging/src/LoggerFactory.cs
@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System;
+using System.Collections.Concurrent;
 using System.Collections.Generic;
 using System.Diagnostics.CodeAnalysis;
 using Microsoft.Extensions.DependencyInjection;
@@ -14,7 +15,7 @@ namespace Microsoft.Extensions.Logging
     /// </summary>
     public class LoggerFactory : ILoggerFactory
     {
-        private readonly Dictionary<string, Logger> _loggers = new Dictionary<string, Logger>(StringComparer.Ordinal);
+        private readonly ConcurrentDictionary<string, Logger> _loggers = new ConcurrentDictionary<string, Logger>(StringComparer.Ordinal);
         private readonly List<ProviderRegistration> _providerRegistrations = new List<ProviderRegistration>();
         private readonly object _sync = new object();
         private volatile bool _disposed;
@@ -138,19 +139,22 @@ public ILogger CreateLogger(string categoryName)
                 throw new ObjectDisposedException(nameof(LoggerFactory));
             }
 
-            lock (_sync)
+            if (!_loggers.TryGetValue(categoryName, out Logger? logger))
             {
-                if (!_loggers.TryGetValue(categoryName, out Logger? logger))
+                lock (_sync)
                 {
-                    logger = new Logger(categoryName, CreateLoggers(categoryName));
+                    if (!_loggers.TryGetValue(categoryName, out logger))
+                    {
+                        logger = new Logger(categoryName, CreateLoggers(categoryName));
 
-                    (logger.MessageLoggers, logger.ScopeLoggers) = ApplyFilters(logger.Loggers);
+                        (logger.MessageLoggers, logger.ScopeLoggers) = ApplyFilters(logger.Loggers);
 
-                    _loggers[categoryName] = logger;
+                        _loggers[categoryName] = logger;
+                    }
                 }
-
-                return logger;
             }
+
+            return logger;
         }
 
         /// <summary>