Remove unnecessary check for channel disconnection upon LB pick failure (#27330)

* Remove unnecessary check for channel disconnection upon LB pick failure

* don't use atomic for disconnect_error_

src/core/ext/filters/client_channel/client_channel.cc

@@ -719,8 +719,7 @@ class ClientChannel::SubchannelWrapper : public SubchannelInterface {
     // in chand_->pending_subchannel_updates_.  So we don't want to add
     // entries there that will never be processed, since that would
     // leave dangling refs to the channel and prevent its destruction.
-    grpc_error_handle disconnect_error = chand_->disconnect_error();
+    if (chand_->disconnect_error_ != GRPC_ERROR_NONE) return;
-    if (disconnect_error != GRPC_ERROR_NONE) return;
     // Not shutting down, so do the update.
     if (connected_subchannel_ != connected_subchannel) {
       connected_subchannel_ = std::move(connected_subchannel);
@@ -985,9 +984,8 @@ class ClientChannel::ClientChannelControlHelper
       std::unique_ptr<LoadBalancingPolicy::SubchannelPicker> picker) override
       ABSL_EXCLUSIVE_LOCKS_REQUIRED(chand_->work_serializer_) {
     if (chand_->resolver_ == nullptr) return;  // Shutting down.
-    grpc_error_handle disconnect_error = chand_->disconnect_error();
     if (GRPC_TRACE_FLAG_ENABLED(grpc_client_channel_routing_trace)) {
-      const char* extra = disconnect_error == GRPC_ERROR_NONE
+      const char* extra = chand_->disconnect_error_ == GRPC_ERROR_NONE
                               ? ""
                               : " (ignoring -- channel shutting down)";
       gpr_log(GPR_INFO, "chand=%p: update: state=%s status=(%s) picker=%p%s",
@@ -995,7 +993,7 @@ class ClientChannel::ClientChannelControlHelper
               picker.get(), extra);
     }
     // Do update only if not shutting down.
-    if (disconnect_error == GRPC_ERROR_NONE) {
+    if (chand_->disconnect_error_ == GRPC_ERROR_NONE) {
       chand_->UpdateStateAndPickerLocked(state, status, "helper",
                                          std::move(picker));
     }
@@ -1086,8 +1084,7 @@ ClientChannel::ClientChannel(grpc_channel_element_args* args,
       interested_parties_(grpc_pollset_set_create()),
       work_serializer_(std::make_shared<WorkSerializer>()),
       state_tracker_("client_channel", GRPC_CHANNEL_IDLE),
-      subchannel_pool_(GetSubchannelPool(args->channel_args)),
+      subchannel_pool_(GetSubchannelPool(args->channel_args)) {
-      disconnect_error_(GRPC_ERROR_NONE) {
   if (GRPC_TRACE_FLAG_ENABLED(grpc_client_channel_routing_trace)) {
     gpr_log(GPR_INFO, "chand=%p: creating client_channel for channel stack %p",
             this, owning_stack_);
@@ -1158,7 +1155,7 @@ ClientChannel::~ClientChannel() {
   // Stop backup polling.
   grpc_client_channel_stop_backup_polling(interested_parties_);
   grpc_pollset_set_destroy(interested_parties_);
-  GRPC_ERROR_UNREF(disconnect_error_.load(std::memory_order_relaxed));
+  GRPC_ERROR_UNREF(disconnect_error_);
 }
 
 OrphanablePtr<ClientChannel::LoadBalancedCall>
@@ -1793,7 +1790,7 @@ void ClientChannel::StartTransportOpLocked(grpc_transport_op* op) {
     if (grpc_error_get_int(op->disconnect_with_error,
                            GRPC_ERROR_INT_CHANNEL_CONNECTIVITY_STATE, &value) &&
         static_cast<grpc_connectivity_state>(value) == GRPC_CHANNEL_IDLE) {
-      if (disconnect_error() == GRPC_ERROR_NONE) {
+      if (disconnect_error_ == GRPC_ERROR_NONE) {
         // Enter IDLE state.
         UpdateStateAndPickerLocked(GRPC_CHANNEL_IDLE, absl::Status(),
                                    "channel entering IDLE", nullptr);
@@ -1801,10 +1798,8 @@ void ClientChannel::StartTransportOpLocked(grpc_transport_op* op) {
       GRPC_ERROR_UNREF(op->disconnect_with_error);
     } else {
       // Disconnect.
-      GPR_ASSERT(disconnect_error_.load(std::memory_order_relaxed) ==
+      GPR_ASSERT(disconnect_error_ == GRPC_ERROR_NONE);
-                 GRPC_ERROR_NONE);
+      disconnect_error_ = op->disconnect_with_error;
-      disconnect_error_.store(op->disconnect_with_error,
-                              std::memory_order_release);
       UpdateStateAndPickerLocked(
           GRPC_CHANNEL_SHUTDOWN, absl::Status(), "shutdown from API",
           absl::make_unique<LoadBalancingPolicy::TransientFailurePicker>(
@@ -3149,13 +3144,6 @@ bool ClientChannel::LoadBalancedCall::PickSubchannelLocked(
               gpr_log(GPR_INFO, "chand=%p lb_call=%p: LB pick failed: %s",
                       chand_, this, fail_pick->status.ToString().c_str());
             }
-            // If we're shutting down, fail all RPCs.
-            grpc_error_handle disconnect_error = chand_->disconnect_error();
-            if (disconnect_error != GRPC_ERROR_NONE) {
-              MaybeRemoveCallFromLbQueuedCallsLocked();
-              *error = GRPC_ERROR_REF(disconnect_error);
-              return true;
-            }
             // If wait_for_ready is false, then the error indicates the RPC
             // attempt's final status.
             if ((send_initial_metadata_flags &

src/core/ext/filters/client_channel/client_channel.h

@@ -207,11 +207,6 @@ class ClientChannel {
   static void GetChannelInfo(grpc_channel_element* elem,
                              const grpc_channel_info* info);
 
-  // Note: Does NOT return a new ref.
-  grpc_error_handle disconnect_error() const {
-    return disconnect_error_.load(std::memory_order_acquire);
-  }
-
   // Note: All methods with "Locked" suffix must be invoked from within
   // work_serializer_.
 
@@ -343,12 +338,8 @@ class ClientChannel {
   std::map<RefCountedPtr<SubchannelWrapper>, RefCountedPtr<ConnectedSubchannel>>
       pending_subchannel_updates_ ABSL_GUARDED_BY(work_serializer_);
   int keepalive_time_ ABSL_GUARDED_BY(work_serializer_) = -1;
-
+  grpc_error_handle disconnect_error_ ABSL_GUARDED_BY(work_serializer_) =
-  //
+      GRPC_ERROR_NONE;
-  // Fields accessed from both data plane mutex and control plane
-  // work_serializer.
-  //
-  std::atomic<grpc_error_handle> disconnect_error_{GRPC_ERROR_NONE};
 
   //
   // Fields guarded by a mutex, since they need to be accessed