Linux Kernel : Not able to load simple linux kernel module with workqueues

Question

I'm having an issue with using the workqueues in my linux kernel module. My modules compiles without giving any error but at the time of loading it fails. I'm not able to load the following module and getting following error in dmesg.

[root@nanderson test_mod]# insmod  workqueue_test.ko 
insmod: ERROR: could not insert module workqueue_test.ko: Unknown symbol in module
[root@nanderson test_mod]# dmesg -c
[50404.453417] workqueue_test: Unknown symbol destroy_workqueue (err 0)
[50404.453437] workqueue_test: Unknown symbol __alloc_workqueue_key (err 0)
[root@nanderson test_mod]# 

Following is the module code :-

  1 #include <linux/module.h>
  2 #include <linux/kernel.h>
  3 #include <linux/kthread.h>
  4 #include <linux/blkdev.h>
  5 #include <linux/fs.h> 
  6 #include <linux/delay.h>
  7 #include <linux/workqueue.h>
  8 #include <linux/completion.h>
  9
 10 
 11 #define LOG_ENTRY() \
 12     do {\
 13         printk(KERN_INFO "++ %s %d %s\n", __func__, __LINE__,\
 14                                         current->comm);\
 15     } while (0);
 16 
 17 #define LOG_INFO() \
 18     do {\
 19         printk(KERN_INFO "%s %d %s\n", __func__, __LINE__,\
 20                                         current->comm); mdelay(1000);\
 21     } while (0);
 22 
 23 #define LOG_EXIT() \
 24     do {\
 25         printk(KERN_INFO "-- %s %d %s\n", __func__, __LINE__,\
 26                                         current->comm);\
 27     } while (0);
 28 
 29 
 30 void
 31 async_callback(void *data)
 32 {
 33 
 34 }
 35 
 36 int
 37 init_module(void)
 38 {
 39     struct workqueue_struct *async_queue;
 40 
 41     LOG_ENTRY();
 42     if ((async_queue = create_workqueue("HGST_WORKQUEUE")) == NULL) {
 43          printk(KERN_ERR "failed to create workqueue\n");
 44          return -1;
 45     }    
 46          
 47     mdelay(10000);
 48     destroy_workqueue(async_queue);
 49     LOG_EXIT();
 50     return 0;
 51 }   
 52     
 53 
 54 void
 55 cleanup_module(void)
 56 {
 57     printk(KERN_INFO "Unloading MOdule..\n");
 58 }

I also looked into /proc/kallsysm for the unknown symbols reposted by insmod looks like symbols are available following is the output:-

[root@nanderson test_mod]# cat /proc/kallsyms  | grep __alloc_workqueue_key
ffffffff81084a10 T __alloc_workqueue_key
ffffffff8187a090 r __ksymtab___alloc_workqueue_key
ffffffff8188bd70 r __kcrctab___alloc_workqueue_key
ffffffff81892ba0 r __kstrtab___alloc_workqueue_key
[root@nanderson test_mod]# 

Can someone tell me what might be the problem or I'm missing something ?

Thanks.

Answer 1

You need

MODULE_LICENSE("GPL");

in you code for use GPL symbols (exported using EXPORT_SYMBOL_GPL).

Otherwise module loader simply does not see such symbols.