Question

I am writing multi-threaded python application.

The main thread create a Thread pool of 5 Worker Threads.
The main thread also create a Monitor Thread.

Total: 6 threads + 1 Main Thread = 7

All Threads talk with a MySQL server (mysqldb -> libmysqlclient_r)

In my SQL wrapper i have added a Threading.Lock to the DB query function.
This Lock is a Global lock and all the Threads that query the DB, uses it.

def query(self, query):
  with lock:
   execute Query Here

Everything works well, until at some point, the main thread stuck (and so does all the threads)
I attached the GDB debugger and noticed that: (info threads)

  7 Thread 0x7f555c386700 (LWP 16077)  0x00007f5561a503c0 in sem_wait () from /lib/libpthread.so.0
  6 Thread 0x7f555bb85700 (LWP 16078)  0x00007f5561a503c0 in sem_wait () from /lib/libpthread.so.0
  5 Thread 0x7f555b384700 (LWP 16079)  0x00007f5561a503c0 in sem_wait () from /lib/libpthread.so.0
  4 Thread 0x7f555ab83700 (LWP 16080)  0x00007f5561a503c0 in sem_wait () from /lib/libpthread.so.0
  3 Thread 0x7f555a382700 (LWP 16081)  0x00007f5561a503c0 in sem_wait () from /lib/libpthread.so.0
  2 Thread 0x7f5559b81700 (LWP 16083)  0x00007f55609141a3 in select () from /lib/libc.so.6
  1 Thread 0x7f5561e6f700 (LWP 16061)  0x00007f5561a503c0 in sem_wait () from /lib/libpthread.so.0

Th1 = Main Thread, Th2 = Monitor Thread, Th3-Th7 – Worker Threads

I noticed that All the threads beside the monitor thread are waiting on sem_wait():

(gdb) bt
#0  0x00007f5561a503c0 in **sem_wait** () from /lib/libpthread.so.0
#1  0x00000000004d44e8 in **PyThread_acquire_lock** ()
#2  0x00000000004d8982 in ?? ()
#3  0x00000000004a7ba5 in PyEval_EvalFrameEx ()

However, the monitor thread is capable of acquiring and releasing the lock (it runs every 30 sec, the select() you see is due to the sleep(30)).
I do not understand , why the rest of the threads are stuck on sem_wait(), since no one acquired the lock.

Any ideas how to solve this? how to debug this?

Thank you

Answer 1

It seems like i was using the same connection using multiple threads, which by the DOCS is not allowed.

Tip: Make sure that each Thread has it’s own Connection object.