how to write flash in ble interrupt handler(on_evt_ble)

Question

Hi, 
 I want to write flash using received data in BLE interrupt handler(on_ble_evt()).
When i received some data from BLE, i try to write it to flash and wait until done. 
 
 But, It's waiting forever... what's the problem? 
 How much data can be written in flash without pending? 
 Is there any good example, writing (ble received)data in on_ble_evt ? 
 
 Here is my flash write function 
 void write_flash(...) {
 // erase 
 app_flash_erase((uint32_t *)dst, num_pages));
 // store
 app_flash_store(ptr_dest, ptr_src, FS_MAX_WRITE_SIZE_WORDS)
 // wait
 app_flash_wait();
}
 
 In app_flash_wait(), there are some flags for operation done & check it until done 
 static fs_ret_t app_flash_wait(void)
{
 if ((m_flags & FLASH_FLAG_SD_ENABLED) != 0)
 {
 while ((m_flags & FLASH_FLAG_OPER) != 0)
 {
 (void)sd_app_evt_wait(); 
 }

 if ((m_flags & FLASH_FLAG_FAILURE_SINCE_LAST) != 0)
 {
 return FS_ERR_FAILURE_SINCE_LAST;
 }
 } 
 return FS_SUCCESS;
}
 
 and make a callback(app_fs_event_handler) to set flags done & register it like this 
 // Our fstorage configuration.
FS_REGISTER_CFG(fs_config_t app_fs_config) =
{
 .callback = app_fs_event_handler,
 .num_pages = 3,
 .priority = 0xFE // 1 lower than the peer manager
};
 
 
 edit 1 
 AS-IS 
 on_evt_ble() {
 // receive some data

 // write received data to flash 

 (before call library, We should guarantee flash operation has done)
 // call 3rd party library 
}
 
 TO-BE (is this ok?) 
 on_evt_ble() {
 // receive some data

 // put event 
 app_sched_event_put(p_event_data, event_size, handler) 
}

handler() {
 // write received data to flash 
 
 (before call library, We should guarantee flash operation has done)
 // call 3rd party library 
}

main() 
{
 while() {
 app_sched_execute()
 }
}