sd_ble_gatts_hvx() freezes (doesn't return)

Question

Hello, 
 After a characteristic (with write and notify properties) is written, the nRF51 sends some data through the UART. Then it receives data, also through the UART, and send a characteristic notification using sd_ble_gatts_hvx() function. 
 However this function never return. Since I don't have access to its code, can someone tell me what's going wrong? 
 This is how I add the characteristic: 
 static uint32_t vm_char_add(ble_pks_t * p_pks, const ble_paykey_init_t * p_pks_init)
{
	ble_gatts_char_md_t char_md;
	ble_gatts_attr_md_t cccd_md;
	ble_gatts_attr_t attr_char_value;
	ble_uuid_t ble_uuid;
	ble_gatts_attr_md_t attr_md;

 memset(&cccd_md, 0, sizeof(cccd_md));

 BLE_GAP_CONN_SEC_MODE_SET_OPEN(&cccd_md.read_perm);
 BLE_GAP_CONN_SEC_MODE_SET_OPEN(&cccd_md.write_perm);
 cccd_md.vloc = BLE_GATTS_VLOC_STACK;

	memset(&char_md, 0, sizeof(char_md));

	char_md.char_props.read = 0;
	char_md.char_props.write = 1;
	char_md.char_props.write_wo_resp = 0;
	char_md.char_props.notify = 1;
	char_md.p_char_user_desc = NULL;
	char_md.p_char_pf = NULL;
	char_md.p_user_desc_md = NULL;
	char_md.p_cccd_md = &cccd_md;
	char_md.p_sccd_md = NULL;

	ble_uuid.type = p_pks->uuid_type;
	ble_uuid.uuid = BLE_UUID_PKS_VM_CHAR;

	memset(&attr_md, 0, sizeof(attr_md));

	BLE_GAP_CONN_SEC_MODE_SET_OPEN(&attr_md.read_perm);
	BLE_GAP_CONN_SEC_MODE_SET_OPEN(&attr_md.write_perm);

	attr_md.vloc = BLE_GATTS_VLOC_STACK;
	attr_md.rd_auth = 0;
	attr_md.wr_auth = 0;
	attr_md.vlen = 1;

	memset(&attr_char_value, 0, sizeof(attr_char_value));

	attr_char_value.p_uuid = &ble_uuid;
	attr_char_value.p_attr_md = &attr_md;
	attr_char_value.init_len = sizeof(uint8_t);
	attr_char_value.init_offs = 0;
	attr_char_value.max_len = BLE_PKS_MAX_DATA_LEN;
	attr_char_value.p_value = NULL;

	return sd_ble_gatts_characteristic_add(p_pks->service_handle,
 &char_md,
 &attr_char_value,
 &p_pks->vm_handles);
}
 
 And this is how I send a notification: 
 static uint32_t send_notification(ble_pks_t *p_pks, uint16_t ch_value_handle, uint8_t *p_data, uint16_t length)
{
 ble_gatts_hvx_params_t hvx_params;

 VERIFY_PARAM_NOT_NULL(p_pks);

 if ((p_pks->conn_handle == BLE_CONN_HANDLE_INVALID))
 {
 return NRF_ERROR_INVALID_STATE;
 }
 if (length > BLE_PKS_MAX_DATA_LEN)
 {
 return NRF_ERROR_INVALID_PARAM;
 }

 memset(&hvx_params, 0, sizeof(hvx_params));
 hvx_params.handle = ch_value_handle;
 hvx_params.p_data = p_data;
 hvx_params.p_len = &length;
 hvx_params.type = BLE_GATT_HVX_NOTIFICATION;

 uint32_t err_code = sd_ble_gatts_hvx(p_pks->conn_handle, &hvx_params);
 if(err_code != NRF_SUCCESS)
 {
 NRF_LOG_ERROR("failed to notify %d
",err_code);
 }
 return err_code;
}
 
 Since the code hangs (infinite loop somewhere?) I end up losing the connection and I can only recover the nRF51 by reprograming / resetting it. 
 Why?

Mario Ribeiro · Accepted Answer

I indeed end up finding the solution. UART IRQ priority was the cause of my problem since I syncronously wait for data available in the buffers in a specific part of my code. To solve the problem all I had to do was to use APP_IRQ_PRIORITY_HIGH, instead of APP_IRQ_PRIORITY_LOW.