[Bug uprobes/17623] New: Sometimes probes fail to fire events when running against a multi-threaded application

["peter at peca dot dk" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=17623

            Bug ID: 17623
           Summary: Sometimes probes fail to fire events when running
                    against a multi-threaded application
           Product: systemtap
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: uprobes
          Assignee: systemtap at sourceware dot org
          Reporter: peter at peca dot dk

Created attachment 7948
  --> https://sourceware.org/bugzilla/attachment.cgi?id=7948&action=edit
The example program that triggers the behavior

I have the following test program (attached as testprog.c)

  #include <pthread.h>
  #include <unistd.h>

  #define WAIT_US 1

  void func1()
  {
  }

  void func2()
  {
  }

  void* thread_main(void *foo)
  {
    for (;;)
    {
      func1();
      usleep(WAIT_US);
      func2();
    }
  }

  int main()
  {
    pthread_t thread;
    pthread_create(&thread, NULL, thread_main, NULL);
    pthread_join(thread, NULL);
    return 0;
  }

I run this program, after compiling it with "gcc -O0 -g -Wall testprog.c -o
testprog -lpthread", and then run this systemtap script (attached as
trace_funcs.stp):

  probe process("testprog").function("func1")
  {
    printf("func1\n")
  }

  probe process("testprog").function("func2")
  {
    printf("func2\n")
  }

Expected output: An endless repetition of "func1\nfunc2\n".

Actual output: Sometimes an endless repetition of "func1\n", and at other times
the expected output.


When I compile with WAIT_US set to 1, it almost always fails. If I set it to 50
about half of the runs fails. If I set it 50000 almost all runs succeed.

If I run "testprog" via the "-c" option to stap, it always works.

If I call thread_main() directly from main() instead of using pthreads, it
always works.


I am experiencing is on a Ubuntu 14.04.1, with the Ubuntu supplies Systemtap of
version 2.3. I have compiled Systemtap version 2.6 from source and got the same
results.

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