Fix the ExposedLocalsNumbering test (#105037)

* Fix the ExposedLocalsNumbering test

The test had a stack corrupting race condition:

1) Main thread returns from "Main".
2) Main thread calls some runtime code on the stack of former "Main".
3) Mutator thread writes to the "safeIndex" variable, which is now
   part of that runtime's code stack.
4) Things crash.

Fix by 'parking' the mutated index in some dynamic memory instead.

* issues.targets de-exclusion

src/tests/JIT/opt/ValueNumbering/ExposedLocalsNumbering.cs

@@ -9,26 +9,23 @@
 
 public unsafe class ExposedLocalsNumbering
 {
-    private static volatile bool s_mutateIndex;
+    private const int UnsafeIndex = 1;
+
     private static volatile bool s_finished;
     private static int* s_pIndex = (int*)NativeMemory.Alloc(4);
 
     [Fact]
     public static int TestEntryPoint()
     {
         const int RetryCount = 100;
-        const int UnsafeIndex = 1;
 
         try
         {
             new Thread(_ =>
             {
                 while (!s_finished)
                 {
-                    if (s_mutateIndex)
-                    {
-                        *s_pIndex = UnsafeIndex;
-                    }
+                    *s_pIndex = UnsafeIndex;
                 }
             }).Start();
         }
@@ -40,12 +37,11 @@ public static int TestEntryPoint()
         int[] array = new int[UnsafeIndex + 1];
         array[UnsafeIndex] = 1;
 
-        int safeIndex = 0;
         for (int i = 0; i < RetryCount; i++)
         {
             try
             {
-                if (RunBoundsChecks(array.AsSpan(0, UnsafeIndex), &safeIndex) != 0)
+                if (RunBoundsChecks(array.AsSpan(0, UnsafeIndex)) != 0)
                 {
                     s_finished = true;
                     return 101;
@@ -59,7 +55,7 @@ public static int TestEntryPoint()
     }
 
     [MethodImpl(MethodImplOptions.NoInlining)]
-    private static int RunBoundsChecks(Span<int> span, int* pSafeIndex)
+    private static int RunBoundsChecks(Span<int> span)
     {
         int result = 0;
         int index = 0;
@@ -80,8 +76,12 @@ private static int RunBoundsChecks(Span<int> span, int* pSafeIndex)
         result += span[index];
         result += span[index];
 
+        int* pSafeIndex = (int*)NativeMemory.AllocZeroed(4);
         s_pIndex = pSafeIndex;
-        s_mutateIndex = false;
+
+        // Wait until the mutator thread sees the switch, so that it
+        // doesn't write to the stack of a method that has returned.
+        while (Volatile.Read(ref *pSafeIndex) != UnsafeIndex) { }
 
         return result;
     }
@@ -90,6 +90,5 @@ private static int RunBoundsChecks(Span<int> span, int* pSafeIndex)
     private static void CaptureIndex(int* pIndex)
     {
         s_pIndex = pIndex;
-        s_mutateIndex = true;
     }
 }

src/tests/issues.targets

@@ -11,9 +11,6 @@
         <ExcludeList Include="$(XunitTestBinBase)/baseservices/finalization/CriticalFinalizer/*">
             <Issue>https://github.com/dotnet/runtime/issues/76041</Issue>
         </ExcludeList>
-        <ExcludeList Include = "$(XunitTestBinBase)/JIT/opt/ValueNumbering/ExposedLocalsNumbering/**">
-            <Issue>https://github.com/dotnet/runtime/issues/80184</Issue>
-        </ExcludeList>
         <ExcludeList Include = "$(XunitTestBinBase)/JIT/opt/SSA/MemorySsa/**">
             <Issue>https://github.com/dotnet/runtime/issues/86112</Issue>
         </ExcludeList>