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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