Update

src/Grpc.Net.Client/Balancer/Internal/ConnectionManager.cs

@@ -31,6 +31,7 @@ internal sealed class ConnectionManager : IDisposable, IChannelControlHelper
     private static readonly ChannelIdProvider _channelIdProvider = new ChannelIdProvider();
 
     private readonly object _lock;
+    private readonly object _subChannelStateChangedLock;
     internal readonly Resolver _resolver;
     private readonly ISubchannelTransportFactory _subchannelTransportFactory;
     private readonly List<Subchannel> _subchannels;
@@ -57,6 +58,7 @@ internal ConnectionManager(
         LoadBalancerFactory[] loadBalancerFactories)
     {
         _lock = new object();
+        _subChannelStateChangedLock = new object();
         _nextPickerTcs = new TaskCompletionSource<SubchannelPicker>(TaskCreationOptions.RunContinuationsAsynchronously);
         _resolverStartedTcs = new TaskCompletionSource<object?>(TaskCreationOptions.RunContinuationsAsynchronously);
         _channelId = _channelIdProvider.GetNextChannelId();
@@ -204,7 +206,13 @@ internal void OnSubchannelStateChange(Subchannel subchannel, ConnectivityState s
             }
         }
 
-        subchannel.RaiseStateChanged(state, status);
+        // Lock to avoid parallel state change events.
+        // This is here so consumers of the state changed API don't need to worry about synchronization.
+        // Use a new lock to avoid deadlocks. See https://github.com/grpc/grpc-dotnet/issues/2589.
+        lock (_subChannelStateChangedLock)
+        {
+            subchannel.RaiseStateChanged(state, status);
+        }
     }
 
     public async Task ConnectAsync(bool waitForReady, CancellationToken cancellationToken)