MAC/RLC executor stalls during ZMQ UE attach in VirtualBox
## Description
While testing OCUDU with the ZMQ radio backend inside a VirtualBox VM, we observed a reproducible UE attach failure during the MAC/RLC stages of connection establishment.
Investigation suggests the behavior is related to work dispatched through `cell_exec` in MAC and `pcell_executor` in RLC. With diagnostic patches that bypass a few selected executor handoffs, the UE successfully completes Random Access, RRC Setup, RRC Reconfiguration, NAS Registration, and PDU Session Establishment.
## Setup Details
- Open5GS (fdea4cc18), OCUDU (8e48b78bf6ea89e83e93dcee6f4ad0e17da132e1), srsRAN_4G (6bcbd9e5).
- Single FDD cell, band 3, 20 MHz bandwidth, 15 kHz SCS
- Tested with both direct ZMQ connection and using OCUDU [ZMQ broker](https://gitlab.com/ocudu/ocudu_docs/-/blob/main/docs/tutorials/srsue/assets/multi_ue_scenario.grc).
- Dell XPS 13 with Windows 11 running VirtualBox (reproduced on Ubuntu 20.04 LTS, 22.04 LTS, 24.04 LTS, and 26.04 LTS).
## Actual Behavior
Using the default ZMQ configuration in VirtualBox, the UE does not reliably complete connection setup.
During testing, progress stopped at different stages depending on which diagnostic patches were applied:
1. Without the MAC patch, OCUDU did not complete startup.
2. With the MAC patch applied, OCUDU started successfully, but the UE did not complete the full connection procedure.
3. With the MAC and RLC TM patches applied, the UE completed Random Access and reached `RRC Connected`, but PDU session establishment did not complete.
4. With the MAC, RLC TM, and RLC AM patches applied, the UE completed PDU session establishment.
The RLC AM patch is not suitable as a final fix because it changes the RLC AM execution behavior and causes 58 `rlc_tx_am_test_each_sn_size` unit tests to fail.
## Expected Behavior
The UE should complete Random Access, RRC Setup, RRC Reconfiguration, NAS Registration, and PDU Session Establishment when using the default ZMQ configuration in this environment.
## Steps to Reproduce
1. Build OCUDU from commit `8e48b78bf6`.
2. Configure the gNB to use the ZMQ radio backend.
3. Configure srsRAN_4G/srsUE using the default ZMQ configuration.
4. Run OCUDU gNB, Open5GS, FlexRIC, and the UE inside a VirtualBox VM.
5. Observe that the UE attach procedure stalls before completing PDU session establishment.
6. Apply the diagnostic patches listed below.
7. Rebuild OCUDU and rerun the same test.
8. Observe that the UE completes PDU session establishment, but the RLC AM workaround causes RLC AM unit-test failures.
## Diagnostic Files
The diagnostic patches are included below. Logs and configuration files from the failing and successful runs can also be provided if helpful.
## Possible Solution
<details>
<summary>Patch for lib/mac/mac_dl/mac_cell_processor.cpp</summary>
```diff
diff --git a/lib/mac/mac_dl/mac_cell_processor.cpp b/lib/mac/mac_dl/mac_cell_processor.cpp
index c5ecf0492b..432618aa5a 100644
--- a/lib/mac/mac_dl/mac_cell_processor.cpp
+++ b/lib/mac/mac_dl/mac_cell_processor.cpp
@@ -11,6 +11,7 @@
#include "ocudu/ran/band_helper.h"
#include "ocudu/ran/pdsch/pdsch_constants.h"
#include "ocudu/scheduler/result/sched_result.h"
+#include "ocudu/support/async/async_no_op_task.h"
#include "ocudu/support/async/async_timer.h"
#include "ocudu/support/async/execute_on_blocking.h"
#include "ocudu/support/executors/execute_until_success.h"
@@ -72,23 +73,14 @@ async_task<void> mac_cell_processor::start()
// Note: This is done in the control executor context to avoid concurrency with other CTRL procedures.
sched.handle_cell_activation(cell_cfg.cell_index);
- return execute_and_continue_on_blocking(
- cell_exec,
- ctrl_exec,
- timers,
- [this]() noexcept OCUDU_RTSAN_NONBLOCKING {
- if (state != cell_state::inactive) {
- // No-op.
- return;
- }
+ if (state != cell_state::inactive) {
+ return launch_no_op_task();
+ }
- state = cell_state::active;
- logger.info("cell={}: Cell was activated", fmt::underlying(cell_cfg.cell_index));
- },
- [this]() {
- logger.warning("cell={}: Postponed cell start operation. Cause: Task queue is full",
- fmt::underlying(cell_cfg.cell_index));
- });
+ state = cell_state::active;
+ logger.info("cell={}: Cell was activated", fmt::underlying(cell_cfg.cell_index));
+
+ return launch_no_op_task();
}
async_task<void> mac_cell_processor::stop()
@@ -215,32 +207,21 @@ async_task<bool> mac_cell_processor::add_ue(const mac_ue_create_request& request
// > Create a MAC UE DL context.
mac_dl_ue_context ue_inst(request);
- // > Update the UE context inside the cell executor.
- return defer_and_continue_on_blocking(
- cell_exec,
- ctrl_exec,
- timers,
- [this, ue_inst = std::move(ue_inst)]() mutable noexcept OCUDU_RTSAN_NONBLOCKING {
- // > Insert UE and DL bearers.
-
- // Note: Ensure we only do so if the cell is active.
- if (state != cell_state::active) {
- // Note: We do not need to call dl_harq_buffers.deallocate_ue_buffers(...) because the buffers were reset
- // on cell deactivation.
- return false;
- }
+ // > Insert UE and DL bearers.
+ // Note: Ensure we only do so if the cell is active.
+ if (state != cell_state::active) {
+ // Note: We do not need to call dl_harq_buffers.deallocate_ue_buffers(...) because the buffers were reset on cell
+ // deactivation.
+ return launch_no_op_task(false);
+ }
- const du_ue_index_t ue_idx = ue_inst.get_ue_index();
- const rnti_t rnti = ue_inst.get_rnti();
- if (ue_mng.add_ue(std::move(ue_inst))) {
- logger.debug("ue={} rnti={} proc=\"MAC UE Creation\": MAC UE DL context created successfully.", ue_idx, rnti);
- return true;
- }
- return false;
- },
- [this, ue_index = request.ue_index]() {
- logger.warning("ue={}: Postponed UE creation. Cause: Task queue is full", fmt::underlying(ue_index));
- });
+ const du_ue_index_t ue_idx = ue_inst.get_ue_index();
+ const rnti_t rnti = ue_inst.get_rnti();
+ if (ue_mng.add_ue(std::move(ue_inst))) {
+ logger.debug("ue={} rnti={} proc=\"MAC UE Creation\": MAC UE DL context created successfully.", ue_idx, rnti);
+ return launch_no_op_task(true);
+ }
+ return launch_no_op_task(false);
}
async_task<void> mac_cell_processor::remove_ue(const mac_ue_delete_request& request)
@@ -279,18 +260,7 @@ async_task<bool> mac_cell_processor::addmod_bearers(du_ue_index_t
{
// Note: logical_channels must outlive the returned async_task completion.
- return defer_and_continue_on_blocking(
- cell_exec,
- ctrl_exec,
- timers,
- [this, ue_index, logical_channels]() noexcept OCUDU_RTSAN_NONBLOCKING {
- // Configure logical channels.
- return state == cell_state::active and ue_mng.addmod_bearers(ue_index, logical_channels);
- },
- [this, ue_index]() {
- logger.warning("ue={}: Postponed UE bearer add/mod operation. Cause: Task queue is full",
- fmt::underlying(ue_index));
- });
+ return launch_no_op_task(state == cell_state::active and ue_mng.addmod_bearers(ue_index, logical_channels));
}
async_task<bool> mac_cell_processor::remove_bearers(du_ue_index_t ue_index, span<const lcid_t> lcids_to_rem)
```
</details>
> After applying the above patch, OCUDU now starts successfully in VirtualBox (`==== gNB started ===`). All tests pass.
<details>
<summary>Patch for lib/rlc/rlc_tx_tm_entity.cpp</summary>
```diff
diff --git a/lib/rlc/rlc_tx_tm_entity.cpp b/lib/rlc/rlc_tx_tm_entity.cpp
index cec4cc91e2..0c60f245e0 100644
--- a/lib/rlc/rlc_tx_tm_entity.cpp
+++ b/lib/rlc/rlc_tx_tm_entity.cpp
@@ -134,10 +134,7 @@ void rlc_tx_tm_entity::handle_changed_buffer_state()
{
if (not pending_buffer_state.test_and_set(std::memory_order_seq_cst)) {
logger.log_debug("Triggering buffer state update to lower layer");
- // Redirect handling of status to pcell_executor
- if (not pcell_executor.defer([this]() { update_mac_buffer_state(); })) {
- logger.log_error("Failed to enqueue buffer state update");
- }
+ update_mac_buffer_state();
} else {
logger.log_debug("Avoiding redundant buffer state update to lower layer");
}
```
</details>
> After applying the above two patches, Random Access Transmission completes and UEs reach "RRC Connected", but PDU session establishment does not complete. All tests pass.
<details>
<summary>Patch for lib/rlc/rlc_tx_am_entity.cpp</summary>
```diff
diff --git a/lib/rlc/rlc_tx_am_entity.cpp b/lib/rlc/rlc_tx_am_entity.cpp
index 199c8e311a..98e466d2be 100644
--- a/lib/rlc/rlc_tx_am_entity.cpp
+++ b/lib/rlc/rlc_tx_am_entity.cpp
@@ -624,11 +624,7 @@ size_t rlc_tx_am_entity::build_retx_pdu(span<uint8_t> rlc_pdu_buf)
void rlc_tx_am_entity::on_status_pdu(rlc_am_status_pdu status)
{
- // Redirect handling of status to pcell_executor
- if (not pcell_executor.execute(
- [this, status = std::move(status)]() mutable { handle_status_pdu(std::move(status)); })) {
- logger.log_error("Unable to handle status report");
- }
+ handle_status_pdu(std::move(status));
}
void rlc_tx_am_entity::handle_status_pdu(rlc_am_status_pdu status) noexcept OCUDU_RTSAN_NONBLOCKING
@@ -969,10 +965,7 @@ void rlc_tx_am_entity::handle_changed_buffer_state()
{
if (not pending_buffer_state.test_and_set(std::memory_order_seq_cst)) {
logger.log_debug("Triggering buffer state update to lower layer");
- // Redirect handling of status to pcell_executor
- if (not pcell_executor.defer([this]() { update_mac_buffer_state(/* force_notify */ false); })) {
- logger.log_error("Failed to enqueue buffer state update");
- }
+ update_mac_buffer_state(/* force_notify */ false);
} else {
logger.log_debug("Avoiding redundant buffer state update to lower layer");
}
```
</details>
> After applying the above three patches, the UEs complete PDU session establishment. However, the `rlc_tx_am_entity.cpp` patch is not suitable as-is because it changes the RLC AM execution behavior and causes 58 `rlc_tx_am_test_each_sn_size` unit tests to fail.
<details>
<summary>Tests that fail from the patch to lib/rlc/rlc_tx_am_entity.cpp</summary>
```console
The following tests FAILED:
5939 - rlc_tx_am_test_each_sn_size/rlc_tx_am_test.tx_without_segmentation/12bit#GetParam()=2-byteobject<0C-00> (Failed)
5940 - rlc_tx_am_test_each_sn_size/rlc_tx_am_test.tx_without_segmentation/18bit#GetParam()=2-byteobject<12-00> (Failed)
5941 - rlc_tx_am_test_each_sn_size/rlc_tx_am_test.tx_small_grant_/12bit#GetParam()=2-byteobject<0C-00> (Failed)
5942 - rlc_tx_am_test_each_sn_size/rlc_tx_am_test.tx_small_grant_/18bit#GetParam()=2-byteobject<12-00> (Failed)
5947 - rlc_tx_am_test_each_sn_size/rlc_tx_am_test.sdu_discard/12bit#GetParam()=2-byteobject<0C-00> (Failed)
5948 - rlc_tx_am_test_each_sn_size/rlc_tx_am_test.sdu_discard/18bit#GetParam()=2-byteobject<12-00> (Failed)
5949 - rlc_tx_am_test_each_sn_size/rlc_tx_am_test.sdu_discard_with_pdcp_sn_wraparound/12bit#GetParam()=2-byteobject<0C-00> (Failed)
5950 - rlc_tx_am_test_each_sn_size/rlc_tx_am_test.sdu_discard_with_pdcp_sn_wraparound/18bit#GetParam()=2-byteobject<12-00> (Failed)
5951 - rlc_tx_am_test_each_sn_size/rlc_tx_am_test.invalid_status_report_ack_sn_larger_than_tx_next/12bit#GetParam()=2-byteobject<0C-00> (Failed)
5952 - rlc_tx_am_test_each_sn_size/rlc_tx_am_test.invalid_status_report_ack_sn_larger_than_tx_next/18bit#GetParam()=2-byteobject<12-00> (Failed)
5953 - rlc_tx_am_test_each_sn_size/rlc_tx_am_test.retx_pdu_without_segmentation/12bit#GetParam()=2-byteobject<0C-00> (Failed)
5954 - rlc_tx_am_test_each_sn_size/rlc_tx_am_test.retx_pdu_without_segmentation/18bit#GetParam()=2-byteobject<12-00> (Failed)
5955 - rlc_tx_am_test_each_sn_size/rlc_tx_am_test.retx_pdu_with_segmentation/12bit#GetParam()=2-byteobject<0C-00> (Failed)
5956 - rlc_tx_am_test_each_sn_size/rlc_tx_am_test.retx_pdu_with_segmentation/18bit#GetParam()=2-byteobject<12-00> (Failed)
5957 - rlc_tx_am_test_each_sn_size/rlc_tx_am_test.retx_pdu_first_segment_without_segmentation/12bit#GetParam()=2-byteobject<0C-00> (Failed)
5958 - rlc_tx_am_test_each_sn_size/rlc_tx_am_test.retx_pdu_first_segment_without_segmentation/18bit#GetParam()=2-byteobject<12-00> (Failed)
5959 - rlc_tx_am_test_each_sn_size/rlc_tx_am_test.retx_pdu_middle_segment_without_segmentation/12bit#GetParam()=2-byteobject<0C-00> (Failed)
5960 - rlc_tx_am_test_each_sn_size/rlc_tx_am_test.retx_pdu_middle_segment_without_segmentation/18bit#GetParam()=2-byteobject<12-00> (Failed)
5961 - rlc_tx_am_test_each_sn_size/rlc_tx_am_test.retx_pdu_last_segment_without_segmentation/12bit#GetParam()=2-byteobject<0C-00> (Failed)
5962 - rlc_tx_am_test_each_sn_size/rlc_tx_am_test.retx_pdu_last_segment_without_segmentation/18bit#GetParam()=2-byteobject<12-00> (Failed)
5963 - rlc_tx_am_test_each_sn_size/rlc_tx_am_test.retx_of_sn_under_segmentation_is_trimmed_to_already_sent_bytes/12bit#GetParam()=2-byteobject<0C-00> (Failed)
5964 - rlc_tx_am_test_each_sn_size/rlc_tx_am_test.retx_of_sn_under_segmentation_is_trimmed_to_already_sent_bytes/18bit#GetParam()=2-byteobject<12-00> (Failed)
5965 - rlc_tx_am_test_each_sn_size/rlc_tx_am_test.retx_of_sn_under_segmentation_is_ignored_for_unsent_bytes/12bit#GetParam()=2-byteobject<0C-00> (Failed)
5966 - rlc_tx_am_test_each_sn_size/rlc_tx_am_test.retx_of_sn_under_segmentation_is_ignored_for_unsent_bytes/18bit#GetParam()=2-byteobject<12-00> (Failed)
5967 - rlc_tx_am_test_each_sn_size/rlc_tx_am_test.retx_only_sent_bytes_of_sn_under_segmentation/12bit#GetParam()=2-byteobject<0C-00> (Failed)
5968 - rlc_tx_am_test_each_sn_size/rlc_tx_am_test.retx_only_sent_bytes_of_sn_under_segmentation/18bit#GetParam()=2-byteobject<12-00> (Failed)
5969 - rlc_tx_am_test_each_sn_size/rlc_tx_am_test.retx_pdu_segment_invalid_so_start_and_so_end/12bit#GetParam()=2-byteobject<0C-00> (Failed)
5970 - rlc_tx_am_test_each_sn_size/rlc_tx_am_test.retx_pdu_segment_invalid_so_start_and_so_end/18bit#GetParam()=2-byteobject<12-00> (Failed)
5971 - rlc_tx_am_test_each_sn_size/rlc_tx_am_test.retx_pdu_segment_invalid_so_start_larger_than_so_end/12bit#GetParam()=2-byteobject<0C-00> (Failed)
5972 - rlc_tx_am_test_each_sn_size/rlc_tx_am_test.retx_pdu_segment_invalid_so_start_larger_than_so_end/18bit#GetParam()=2-byteobject<12-00> (Failed)
5973 - rlc_tx_am_test_each_sn_size/rlc_tx_am_test.tx_huge_bursts_report_buffer_state_correctly/12bit#GetParam()=2-byteobject<0C-00> (Failed)
5974 - rlc_tx_am_test_each_sn_size/rlc_tx_am_test.tx_huge_bursts_report_buffer_state_correctly/18bit#GetParam()=2-byteobject<12-00> (Failed)
5975 - rlc_tx_am_test_each_sn_size/rlc_tx_am_test.retx_many_pdus_and_notify_mac/12bit#GetParam()=2-byteobject<0C-00> (Failed)
5976 - rlc_tx_am_test_each_sn_size/rlc_tx_am_test.retx_many_pdus_and_notify_mac/18bit#GetParam()=2-byteobject<12-00> (Failed)
5977 - rlc_tx_am_test_each_sn_size/rlc_tx_am_test.retx_hol_toa_has_priority/12bit#GetParam()=2-byteobject<0C-00> (Failed)
5978 - rlc_tx_am_test_each_sn_size/rlc_tx_am_test.retx_hol_toa_has_priority/18bit#GetParam()=2-byteobject<12-00> (Failed)
5979 - rlc_tx_am_test_each_sn_size/rlc_tx_am_test.invalid_nack_nack_sn_outside_rx_window/12bit#GetParam()=2-byteobject<0C-00> (Failed)
5980 - rlc_tx_am_test_each_sn_size/rlc_tx_am_test.invalid_nack_nack_sn_outside_rx_window/18bit#GetParam()=2-byteobject<12-00> (Failed)
5981 - rlc_tx_am_test_each_sn_size/rlc_tx_am_test.invalid_nack_sn_larger_than_ack_sn/12bit#GetParam()=2-byteobject<0C-00> (Failed)
5982 - rlc_tx_am_test_each_sn_size/rlc_tx_am_test.invalid_nack_sn_larger_than_ack_sn/18bit#GetParam()=2-byteobject<12-00> (Failed)
5983 - rlc_tx_am_test_each_sn_size/rlc_tx_am_test.retx_insufficient_space/12bit#GetParam()=2-byteobject<0C-00> (Failed)
5984 - rlc_tx_am_test_each_sn_size/rlc_tx_am_test.retx_insufficient_space/18bit#GetParam()=2-byteobject<12-00> (Failed)
5985 - rlc_tx_am_test_each_sn_size/rlc_tx_am_test.retx_pdu_range_without_segmentation/12bit#GetParam()=2-byteobject<0C-00> (Failed)
5986 - rlc_tx_am_test_each_sn_size/rlc_tx_am_test.retx_pdu_range_without_segmentation/18bit#GetParam()=2-byteobject<12-00> (Failed)
5987 - rlc_tx_am_test_each_sn_size/rlc_tx_am_test.retx_pdu_range_wraparound/12bit#GetParam()=2-byteobject<0C-00> (Failed)
5988 - rlc_tx_am_test_each_sn_size/rlc_tx_am_test.retx_pdu_range_wraparound/18bit#GetParam()=2-byteobject<12-00> (Failed)
5995 - rlc_tx_am_test_each_sn_size/rlc_tx_am_test.expired_poll_retransmit_timer_triggers_retx_when_queues_are_empty_and_poll_in_full_pdu/12bit#GetParam()=2-byteobject<0C-00> (Failed)
5996 - rlc_tx_am_test_each_sn_size/rlc_tx_am_test.expired_poll_retransmit_timer_triggers_retx_when_queues_are_empty_and_poll_in_full_pdu/18bit#GetParam()=2-byteobject<12-00> (Failed)
5997 - rlc_tx_am_test_each_sn_size/rlc_tx_am_test.expired_poll_retransmit_timer_triggers_retx_when_queues_are_empty_and_poll_in_last_segment/12bit#GetParam()=2-byteobject<0C-00> (Failed)
5998 - rlc_tx_am_test_each_sn_size/rlc_tx_am_test.expired_poll_retransmit_timer_triggers_retx_when_queues_are_empty_and_poll_in_last_segment/18bit#GetParam()=2-byteobject<12-00> (Failed)
5999 - rlc_tx_am_test_each_sn_size/rlc_tx_am_test.expired_poll_retransmit_timer_triggers_no_retx_when_fresh_sdu_can_be_sent/12bit#GetParam()=2-byteobject<0C-00> (Failed)
6000 - rlc_tx_am_test_each_sn_size/rlc_tx_am_test.expired_poll_retransmit_timer_triggers_no_retx_when_fresh_sdu_can_be_sent/18bit#GetParam()=2-byteobject<12-00> (Failed)
6001 - rlc_tx_am_test_each_sn_size/rlc_tx_am_test.expired_poll_retransmit_increments_retx_counter/12bit#GetParam()=2-byteobject<0C-00> (Failed)
6002 - rlc_tx_am_test_each_sn_size/rlc_tx_am_test.expired_poll_retransmit_increments_retx_counter/18bit#GetParam()=2-byteobject<12-00> (Failed)
6003 - rlc_tx_am_test_each_sn_size/rlc_tx_am_test.retx_count_ignores_pending_retx/12bit#GetParam()=2-byteobject<0C-00> (Failed)
6004 - rlc_tx_am_test_each_sn_size/rlc_tx_am_test.retx_count_ignores_pending_retx/18bit#GetParam()=2-byteobject<12-00> (Failed)
6005 - rlc_tx_am_test_each_sn_size/rlc_tx_am_test.retx_count_trigger_max_retx_without_segmentation/12bit#GetParam()=2-byteobject<0C-00> (Failed)
6006 - rlc_tx_am_test_each_sn_size/rlc_tx_am_test.retx_count_trigger_max_retx_without_segmentation/18bit#GetParam()=2-byteobject<12-00> (Failed)
```
</details>
## Additional Context
Possibly related OCUDU issues:
- [#476: UE stuck at Random Access transmission](https://gitlab.com/ocudu/ocudu/-/issues/476)
- [#343: RLC: revisit update_mac_buffer_state defer to the cell_executor](https://gitlab.com/ocudu/ocudu/-/issues/343)
Related downstream testbed issue:
- [usnistgov/O-RAN-Testbed-Automation#15: UE number 3 doesn't connect](https://github.com/usnistgov/O-RAN-Testbed-Automation/issues/15)
issue
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