[release/7.0] Fix AwaitableSocketAsyncEventArgs reorderings on weaker memory models (#51662)

* Fix AwaitableSocketAsyncEventArgs reorderings on weaker memory models

* add comment

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Co-authored-by: antonfirsov <antonfir@gmail.com>

Author: github-actions[bot]

src/Servers/Kestrel/Transport.Sockets/src/Internal/SocketAwaitableEventArgs.cs

@@ -18,7 +18,11 @@ internal class SocketAwaitableEventArgs : SocketAsyncEventArgs, IValueTaskSource
 
     private readonly PipeScheduler _ioScheduler;
 
-    private Action<object?>? _continuation;
+    // There are places where we read the _continuation field and then read some other state which we assume to be consistent
+    // with the value we read in _continuation. Without a fence, those secondary reads could be reordered with respect to the first.
+    // https://github.com/dotnet/runtime/pull/84432
+    // https://github.com/dotnet/aspnetcore/issues/50623
+    private volatile Action<object?>? _continuation;
 
     public SocketAwaitableEventArgs(PipeScheduler ioScheduler)
         : base(unsafeSuppressExecutionContextFlow: true)