simple multithread program fails on Cygwin, succeeds on Linux

[Bruno Haible <bruno at clisp dot org>]

Hi,

The attached test program for pthread_once uses the following basic POSIX
threads functions:
  pthread_create
  pthread_join
  pthread_mutex_init
  pthread_mutex_lock
  pthread_mutex_unlock
  pthread_once
  pthread_rwlock_init
  pthread_rwlock_rdlock
  pthread_rwlock_unlock
  pthread_rwlock_wrlock

On Linux with glibc 2.8:
  $ gcc bug.c -O -Wall -lpthread -o bug
  $ ./bug
  Starting test_once ... OK
The test completes in about 4 seconds.

On Cygwin 1.5.25(0.156/4/2):
  $ gcc bug.c -O -Wall -o bug.exe         
  $ ./bug.exe
  Starting test_once ...
Either it does not terminates (still running after half an hour, eating
100% CPU time on one of the two CPUs), or it crashes rather quickly:
  $ ./bug.exe
  Starting test_once ...Segmentation fault (core dumped)

This is on a machine with 2 virtual processors:

$ cat /proc/cpuinfo 
processor       : 0
vendor_id       : GenuineIntel
type            : primary processor
cpu family      : 6
model           : 12
model name      : Intel(R) Atom(TM) CPU N270   @ 1.60GHz
stepping        : 2
brand id        : 0
cpu count       : 2
apic id         : 0
cpu MHz         : 1600
fpu             : yes
flags           : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat clfl dtes acpi mmx fxsr sse sse2 ss htt tmi pbe pni monitor ds_cpl tm2 est
processor       : 1
vendor_id       : GenuineIntel
type            : primary processor
cpu family      : 6
model           : 12
model name      : Intel(R) Atom(TM) CPU N270   @ 1.60GHz
stepping        : 2
brand id        : 0
cpu count       : 2
apic id         : 1
cpu MHz         : 1599
fpu             : yes
flags           : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat clfl dtes acpi mmx fxsr sse sse2 ss htt tmi pbe pni monitor ds_cpl tm2 est

You can see what the program is doing before it crahes or while it loops,
by setting   #define ENABLE_DEBUGGING 1  in line 25.

I have other test programs which exercise pthread_mutex_*, pthread_rwlock_*,
and pthread_join. These work fine. My suspicion therefore lies on pthread_once.

Bruno


/* Enable GNU extensions on systems that have them.  */
#ifndef _GNU_SOURCE
# define _GNU_SOURCE 1
#endif
/* Enable threading extensions on Solaris.  */
#ifndef _POSIX_PTHREAD_SEMANTICS
# define _POSIX_PTHREAD_SEMANTICS 1
#endif
/* Enable general extensions on Solaris.  */
#ifndef __EXTENSIONS__
# define __EXTENSIONS__ 1
#endif

/* Whether to enable locking.
   Uncomment this to get a test program without locking, to verify that
   it crashes.  */
#define ENABLE_LOCKING 1

/* Whether to help the scheduler through explicit yield().
   Uncomment this to see if the operating system has a fair scheduler.  */
#define EXPLICIT_YIELD 1

/* Whether to print debugging messages.  */
#define ENABLE_DEBUGGING 0

/* Number of simultaneous threads.  */
#define THREAD_COUNT 10

/* Number of operations performed in each thread.
   This is quite high, because with a smaller count, say 5000, we often get
   an "OK" result even without ENABLE_LOCKING (on Linux/x86).  */
#define REPEAT_COUNT 50000

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

#include <errno.h>
#include <stdlib.h>

/* Use the POSIX threads library.  */
# include <pthread.h>

#if ENABLE_DEBUGGING
# define dbgprintf printf
#else
# define dbgprintf if (0) printf
#endif

# include <pthread.h>
# include <sched.h>
#if EXPLICIT_YIELD
# define yield() sched_yield ()
#else
# define yield()
#endif

/* Test once-only execution by having several threads attempt to grab a
   once-only task simultaneously (triggered by releasing a read-write lock).  */

static pthread_once_t fresh_once = PTHREAD_ONCE_INIT;
static int ready[THREAD_COUNT];
static pthread_mutex_t ready_lock[THREAD_COUNT];
#if ENABLE_LOCKING
static pthread_rwlock_t fire_signal[REPEAT_COUNT];
#else
static volatile int fire_signal_state;
#endif
static pthread_once_t once_control;
static int performed;
static pthread_mutex_t performed_lock = PTHREAD_MUTEX_INITIALIZER;

static void
once_execute (void)
{
  if (pthread_mutex_lock (&performed_lock))
    abort ();
  performed++;
  if (pthread_mutex_unlock (&performed_lock))
    abort ();
}

static void *
once_contender_thread (void *arg)
{
  int id = (int) (long) arg;
  int repeat;

  for (repeat = 0; repeat <= REPEAT_COUNT; repeat++)
    {
      /* Tell the main thread that we're ready.  */
      if (pthread_mutex_lock (&ready_lock[id]))
        abort ();
      ready[id] = 1;
      if (pthread_mutex_unlock (&ready_lock[id]))
        abort ();

      if (repeat == REPEAT_COUNT)
	break;

      dbgprintf ("Contender %p waiting for signal for round %d\n",
		 (void *) pthread_self (), repeat);
#if ENABLE_LOCKING
      /* Wait for the signal to go.  */
      if (pthread_rwlock_rdlock (&fire_signal[repeat]))
        abort ();
      /* And don't hinder the others (if the scheduler is unfair).  */
      if (pthread_rwlock_unlock (&fire_signal[repeat]))
        abort ();
#else
      /* Wait for the signal to go.  */
      while (fire_signal_state <= repeat)
	yield ();
#endif
      dbgprintf ("Contender %p got the     signal for round %d\n",
		 (void *) pthread_self (), repeat);

      /* Contend for execution.  */
      pthread_once (&once_control, once_execute);
    }

  return NULL;
}

void
test_once (void)
{
  int i, repeat;
  pthread_t threads[THREAD_COUNT];

  /* Initialize all variables.  */
  for (i = 0; i < THREAD_COUNT; i++)
    {
      ready[i] = 0;
      if (pthread_mutex_init (&ready_lock[i], NULL))
        abort ();
    }
#if ENABLE_LOCKING
  for (i = 0; i < REPEAT_COUNT; i++)
    if (pthread_rwlock_init (&fire_signal[i], NULL))
      abort ();
#else
  fire_signal_state = 0;
#endif

  /* Block all fire_signals.  */
  for (i = REPEAT_COUNT-1; i >= 0; i--)
    if (pthread_rwlock_wrlock (&fire_signal[i]))
      abort ();

  /* Spawn the threads.  */
  for (i = 0; i < THREAD_COUNT; i++)
    if (pthread_create (&threads[i], NULL, once_contender_thread, (void *) (long) i) != 0)
      abort ();

  for (repeat = 0; repeat <= REPEAT_COUNT; repeat++)
    {
      /* Wait until every thread is ready.  */
      dbgprintf ("Main thread before synchonizing for round %d\n", repeat);
      for (;;)
	{
	  int ready_count = 0;
	  for (i = 0; i < THREAD_COUNT; i++)
	    {
	      if (pthread_mutex_lock (&ready_lock[i]))
                abort ();
	      ready_count += ready[i];
	      if (pthread_mutex_unlock (&ready_lock[i]))
                abort ();
	    }
	  if (ready_count == THREAD_COUNT)
	    break;
	  yield ();
	}
      dbgprintf ("Main thread after  synchonizing for round %d\n", repeat);

      if (repeat > 0)
	{
	  /* Check that exactly one thread executed the once_execute()
	     function.  */
	  if (performed != 1)
	    abort ();
	}

      if (repeat == REPEAT_COUNT)
	break;

      /* Preparation for the next round: Initialize once_control.  */
      memcpy (&once_control, &fresh_once, sizeof (pthread_once_t));

      /* Preparation for the next round: Reset the performed counter.  */
      performed = 0;

      /* Preparation for the next round: Reset the ready flags.  */
      for (i = 0; i < THREAD_COUNT; i++)
	{
	  if (pthread_mutex_lock (&ready_lock[i]))
            abort ();
	  ready[i] = 0;
	  if (pthread_mutex_unlock (&ready_lock[i]))
            abort ();
	}

      /* Signal all threads simultaneously.  */
      dbgprintf ("Main thread giving signal for round %d\n", repeat);
#if ENABLE_LOCKING
      if (pthread_rwlock_unlock (&fire_signal[repeat]))
        abort ();
#else
      fire_signal_state = repeat + 1;
#endif
    }

  /* Wait for the threads to terminate.  */
  for (i = 0; i < THREAD_COUNT; i++)
    {
      void *retval;
      if (pthread_join (threads[i], &retval) != 0)
        abort ();
    }
}

int
main ()
{
  printf ("Starting test_once ..."); fflush (stdout);
  test_once ();
  printf (" OK\n"); fflush (stdout);

  return 0;
}

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