From fb13f3c7d83c233126121e557cd96d7ea5e4b22b Mon Sep 17 00:00:00 2001
From: Jaideep Pyne <25899506+jaideeppyne@users.noreply.github.com>
Date: Sat, 8 Aug 2026 14:58:30 +0530
Subject: [PATCH] Fix PerformanceCounterLib cache races

Capture initialized cache tables before returning so concurrent invalidation cannot turn a successful lookup into null. Synchronize shared library-table enumeration with creation and replacement.

Fix #90803
---
 .../Diagnostics/PerformanceCounterLib.cs      | 30 ++++++++++++-------
 1 file changed, 19 insertions(+), 11 deletions(-)

diff --git a/src/libraries/System.Diagnostics.PerformanceCounter/src/System/Diagnostics/PerformanceCounterLib.cs b/src/libraries/System.Diagnostics.PerformanceCounter/src/System/Diagnostics/PerformanceCounterLib.cs
index 34f317c812be5f..c47f75f065535d 100644
--- a/src/libraries/System.Diagnostics.PerformanceCounter/src/System/Diagnostics/PerformanceCounterLib.cs
+++ b/src/libraries/System.Diagnostics.PerformanceCounter/src/System/Diagnostics/PerformanceCounterLib.cs
@@ -124,7 +124,8 @@ internal Hashtable CategoryTable
         {
             get
             {
-                if (_categoryTable == null)
+                Hashtable categoryTable = _categoryTable;
+                if (categoryTable == null)
                 {
                     lock (_categoryTableLock)
                     {
@@ -194,10 +195,12 @@ internal Hashtable CategoryTable
 
                             _categoryTable = tempCategoryTable;
                         }
+
+                        categoryTable = _categoryTable;
                     }
                 }
 
-                return _categoryTable;
+                return categoryTable;
             }
         }
 
@@ -205,15 +208,16 @@ internal Hashtable HelpTable
         {
             get
             {
-                if (_helpTable == null)
+                Hashtable helpTable = _helpTable;
+                if (helpTable == null)
                 {
                     lock (_helpTableLock)
                     {
-                        _helpTable ??= GetStringTable(true);
+                        helpTable = _helpTable ??= GetStringTable(true);
                     }
                 }
 
-                return _helpTable;
+                return helpTable;
             }
         }
 
@@ -246,15 +250,16 @@ internal Hashtable NameTable
         {
             get
             {
-                if (_nameTable == null)
+                Hashtable nameTable = _nameTable;
+                if (nameTable == null)
                 {
                     lock (_nameTableLock)
                     {
-                        _nameTable ??= GetStringTable(false);
+                        nameTable = _nameTable ??= GetStringTable(false);
                     }
                 }
 
-                return _nameTable;
+                return nameTable;
             }
         }
 
@@ -329,10 +334,13 @@ internal static void CloseAllLibraries()
 
         internal static void CloseAllTables()
         {
-            if (s_libraryTable != null)
+            lock (InternalSyncObject)
             {
-                foreach (PerformanceCounterLib library in s_libraryTable.Values)
-                    library.CloseTables();
+                if (s_libraryTable != null)
+                {
+                    foreach (PerformanceCounterLib library in s_libraryTable.Values)
+                        library.CloseTables();
+                }
             }
         }