Fix comments around static constructor memory model (dotnet#105911)

- Fix comments around static constructor memory model
- Delete unnecessary MemoryBarrier

Contributes to dotnet#81151

src/coreclr/nativeaot/System.Private.CoreLib/src/System/Runtime/CompilerServices/ClassConstructorRunner.cs

@@ -54,7 +54,7 @@ public static unsafe void EnsureClassConstructorRun(StaticClassConstructionConte
             IntPtr pfnCctor = pContext->cctorMethodAddress;
             NoisyLog("EnsureClassConstructorRun, context={0}, thread={1}", pContext, CurrentManagedThreadId);
 
-            // If we were called from MRT, this check is redundant but harmless. This is in case someone within classlib
+            // If we were called from JIT helper, this check is redundant but harmless. This is in case someone within classlib
             // (cough, Reflection) needs to call this explicitly.
             if (pfnCctor == 0)
             {
@@ -87,13 +87,6 @@ public static unsafe void EnsureClassConstructorRun(StaticClassConstructionConte
 
                                 ((delegate*<void>)pfnCctor)();
 
-                                // Insert a memory barrier here to order any writes executed as part of static class
-                                // construction above with respect to the initialized flag update we're about to make
-                                // below. This is important since the fast path for checking the cctor uses a normal read
-                                // and doesn't come here so without the barrier it could observe initialized == 1 but
-                                // still see uninitialized static fields on the class.
-                                Interlocked.MemoryBarrier();
-
                                 NoisyLog("Set type inited, context={0}, thread={1}", pContext, currentManagedThreadId);
 
                                 pContext->cctorMethodAddress = 0;

src/coreclr/nativeaot/System.Private.CoreLib/src/System/Runtime/CompilerServices/StaticClassConstructionContext.cs

@@ -16,6 +16,8 @@ namespace System.Runtime.CompilerServices
     public struct StaticClassConstructionContext
     {
         // Pointer to the code for the static class constructor method. Set to 0 once the cctor has run.
+        // Volatile to insert memory barriers to order any writes executed as part of static class
+        // constructor with respect to this flag.
         public volatile IntPtr cctorMethodAddress;
     }
 }

src/coreclr/nativeaot/Test.CoreLib/src/System/Runtime/CompilerServices/ClassConstructorRunner.cs

@@ -66,13 +66,6 @@ private static unsafe void CheckStaticClassConstruction(ref StaticClassConstruct
 
                     ((delegate*<void>)oldInitializationState)();
 
-                    // Insert a memory barrier here to order any writes executed as part of static class
-                    // construction above with respect to the initialized flag update we're about to make
-                    // below. This is important since the fast path for checking the cctor uses a normal read
-                    // and doesn't come here so without the barrier it could observe initialized == 1 but
-                    // still see uninitialized static fields on the class.
-                    Interlocked.MemoryBarrier();
-
                     // Set the cctorMethodAddress to 0 to indicate to the runtime and other threads that this cctor has now
                     // been run.
                     context.cctorMethodAddress = (IntPtr)0;

src/coreclr/nativeaot/Test.CoreLib/src/System/Runtime/CompilerServices/StaticClassConstructionContext.cs

@@ -12,7 +12,9 @@ namespace System.Runtime.CompilerServices
     [StructLayout(LayoutKind.Sequential)]
     public struct StaticClassConstructionContext
     {
-        // Pointer to the code for the static class constructor method. Set to 0 .
+        // Pointer to the code for the static class constructor method. Set to 0 once the cctor has run.
+        // Volatile to insert memory barriers to order any writes executed as part of static class
+        // constructor with respect to this flag.
         public volatile IntPtr cctorMethodAddress;
     }
 }