Threading issues

Question

getvariana: tpp.c:63: __pthread_tpp_change_priority: Assertion `new_prio == -1 || (new_prio >= __sched_fifo_min_prio && new_prio <= __sched_fifo_max_prio)' failed.

Hi all,

I am trying to re-run a program which creates 5 threads and after pthread_join(), I do a return, based on which, I re-run the entire program i.e., it is in while(1) loop.

When I run the program for the second time, I get an error as you can see above. I am unable to trace its origin. Can anyone please explain why is this error caused ?

FYI: I dont use any mutex locks or semaphore. I wait for the threads to join after which I re-run the entire program. Does it have anything to do with race conditions ? I am assuming, that when I wait for all the 5 threads to join, only then I can move out of the pthread

main
{
    while(1)
    {
         test();
    }
}//main

test()
{
    for( i = 0; i < 5; i++ )
        pthread_create( &th[i], NULL, tfunc, &some_struct);

    for( i = 0; i < 5, i++ )
        pthread_join( th[i], NULL);
}

void * tfunc( void * ptr )
{
    // waiting for a callback function to set a global counter to a value
    // sleep until then
    if( g_count == value_needed )
        pthread_exit(NULL);
}

Answer 1

Here is your program cleaned up. It runs without the above assertion:

#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <string.h>

static pthread_t th[5];

void *
tfunc (void *ptr)
{
  sleep (5);                    /* remove this to test it without the sleep */
  pthread_exit (NULL);
}

void
test ()
{
  int i;
  memset (th, 0, 5 * sizeof (pthread_t));

  for (i = 0; i < 5; i++)
    {
      if (pthread_create (&th[i], NULL, tfunc, NULL) < 0)
        perror ("pthread_create");
    }

  for (i = 0; i < 5; i++)
    {
      if (pthread_join (th[i], NULL) < 0)
        perror ("pthread_join");
    }
}

int
main (int argc, char **argv)
{
  while (1)
    {
      test ();
    }
  exit (0);
}

Here's what I noticed when cleaning it up:

• for( i = 0; i < 5, i++ ) comma not semicolon means loop may not have been working

• in test(), th was not zeroed meaning any failed pthread_create was using an old thread reference.

• In tfunc, you did a pthread_join if ( g_count == value_needed ), but then you exited anyway, i.e. you were always immediately doing the pthread_join or the equivalent. Note I also tested the version below without the sleep(), so exiting immediately now works.

• various other orthographic issues.

• no error handling

As there were a few compilation problems, I suspect that you may not have compiled the code you pasted above, but something more complicated. And I suspect it's part of that that's causing the issue.

If you post a minimal example of compilable code that actually causes the issue, I might be able to help you further.

Answer 2

tpp.c:63: __pthread_tpp_change_priority: Assertion is a known problem and solved:
https://sourceware.org/ml/libc-help/2008-05/msg00071.html
in brief, the problem is caused by repeated locking of a fast mutex, and solved by using a recursive mutex, and the default pthread_mutex_t is not recursive. Is it possible that there's pthread_mutex_t deeply inside the thread running code ??
BTW, to make the mutex recursive, plz set the mutex attribute with attribute PTHREAD_MUTEX_RECURSIVE_NP.