blob: 7b2281649ed1bde385ede324903ef843aaadb854 [file] [log] [blame]
#include <stdio.h>
#include <stdlib.h>
#include <assert.h>
#include <pthread.h>
#include <semaphore.h>
#include <unistd.h>
/* This is really a test of semaphore handling
(sem_{init,destroy,post,wait}). Using semaphores a barrier
function is created. Helgrind-3.3 (p.k.a Thrcheck) does understand
the barrier semantics implied by the barrier, as pieced together
from happens-before relationships obtained from the component
semaphores. However, it does falsely report one race. Ah well.
Helgrind-3.4 is pure h-b and so reports no races (yay!). */
/* This code is derived from
gcc-4.3-20071012/libgomp/config/posix/bar.c, which is
Copyright (C) 2005 Free Software Foundation, Inc.
Contributed by Richard Henderson <>.
and available under version 2.1 or later of the GNU Lesser General
Public License.
Relative to the libgomp sources, the gomp_barrier_t type here has
an extra semaphore field, xxx. This is not functionally useful,
but it is used to create enough extra inter-thread dependencies
that the barrier-like behaviour of gomp_barrier_t is evident to
Thrcheck. There is no other purpose for the .xxx field. */
static sem_t* my_sem_init(char*, int, unsigned);
static int my_sem_destroy(sem_t*);
static int my_sem_wait(sem_t*); static int my_sem_post(sem_t*);
typedef struct
pthread_mutex_t mutex1;
pthread_mutex_t mutex2;
sem_t* sem1;
sem_t* sem2;
unsigned total;
unsigned arrived;
sem_t* xxx;
} gomp_barrier_t;
typedef long bool;
gomp_barrier_init (gomp_barrier_t *bar, unsigned count)
pthread_mutex_init (&bar->mutex1, NULL);
pthread_mutex_init (&bar->mutex2, NULL);
bar->sem1 = my_sem_init ("sem1", 0, 0);
bar->sem2 = my_sem_init ("sem2", 0, 0);
bar->xxx = my_sem_init ("xxx", 0, 0);
bar->total = count;
bar->arrived = 0;
gomp_barrier_destroy (gomp_barrier_t *bar)
/* Before destroying, make sure all threads have left the barrier. */
pthread_mutex_lock (&bar->mutex1);
pthread_mutex_unlock (&bar->mutex1);
pthread_mutex_destroy (&bar->mutex1);
pthread_mutex_destroy (&bar->mutex2);
gomp_barrier_reinit (gomp_barrier_t *bar, unsigned count)
pthread_mutex_lock (&bar->mutex1);
bar->total = count;
pthread_mutex_unlock (&bar->mutex1);
gomp_barrier_wait (gomp_barrier_t *bar)
unsigned int n;
pthread_mutex_lock (&bar->mutex1);
if (bar->arrived == bar->total)
n = bar->arrived;
if (n > 0)
{ unsigned int i;
for (i = 0; i < n; i++)
my_sem_wait(bar->xxx); // acquire an obvious dependency from
// all other threads arriving at the barrier
// 1 up n times, 2 down once
// now let all the other threads past the barrier, giving them
// an obvious dependency with this thread.
my_sem_post (bar->sem1); // 1 up
while (--n != 0);
// and wait till the last thread has left
my_sem_wait (bar->sem2); // 2 down
pthread_mutex_unlock (&bar->mutex1);
/* «Resultats professionnels!» First we made this thread have an
obvious (Thrcheck-visible) dependency on all other threads
calling gomp_barrier_wait. Then, we released them all again,
so they all have a (visible) dependency on this thread.
Transitively, the result is that all threads leaving the
barrier have a a Thrcheck-visible dependency on all threads
arriving at the barrier. As required. */
pthread_mutex_unlock (&bar->mutex1);
// first N-1 threads wind up waiting here
my_sem_wait (bar->sem1); // 1 down
pthread_mutex_lock (&bar->mutex2);
n = --bar->arrived; /* XXX see below */
pthread_mutex_unlock (&bar->mutex2);
if (n == 0)
my_sem_post (bar->sem2); // 2 up
/* re XXX, thrcheck reports a race at this point. It doesn't
understand that bar->arrived is protected by mutex1 whilst threads
are arriving at the barrier and by mutex2 whilst they are leaving,
but not consistently by either of them. Oh well. */
static gomp_barrier_t bar;
/* What's with the volatile here? It stops gcc compiling
"if (myid == 4) { unprotected = 99; }" and
"if (myid == 3) { unprotected = 88; }" into a conditional
load followed by a store. The cmov/store sequence reads and
writes memory in all threads and cause Thrcheck to (correctly)
report a race, the underlying cause of which is that gcc is
generating non threadsafe code.
(The lack of) thread safe code generation by gcc is currently a
hot topic. See the following discussions:
and this is interesting background:
static volatile long unprotected = 0;
void* child ( void* argV )
long myid = (long)argV;
// assert(myid >= 2 && myid <= 5);
/* First, we all wait to get to this point. */
gomp_barrier_wait( &bar );
/* Now, thread #4 writes to 'unprotected' and so becomes its
owner. */
if (myid == 4) {
unprotected = 99;
/* Now we all wait again. */
gomp_barrier_wait( &bar );
/* This time, thread #3 writes to 'unprotected'. If all goes well,
Thrcheck sees the dependency through the barrier back to thread
#4 before it, and so thread #3 becomes the exclusive owner of
'unprotected'. */
if (myid == 3) {
unprotected = 88;
/* And just to be on the safe side ... */
gomp_barrier_wait( &bar );
return NULL;
int main (int argc, char *argv[])
long i; int res;
pthread_t thr[4];
fprintf(stderr, "starting\n");
gomp_barrier_init( &bar, 4 );
for (i = 0; i < 4; i++) {
res = pthread_create( &thr[i], NULL, child, (void*)(i+2) );
for (i = 0; i < 4; i++) {
res = pthread_join( thr[i], NULL );
gomp_barrier_destroy( &bar );
/* And finally here, the root thread can get exclusive ownership
back from thread #4, because #4 has exited by this point and so
we have a dependency edge back to the write it did. */
fprintf(stderr, "done, result is %ld, should be 88\n", unprotected);
return 0;
static sem_t* my_sem_init (char* identity, int pshared, unsigned count)
sem_t* s;
#if defined(VGO_linux)
s = malloc(sizeof(*s));
if (s) {
if (sem_init(s, pshared, count) < 0) {
s = NULL;
#elif defined(VGO_darwin)
char name[100];
sprintf(name, "anonsem_%s_pid%d", identity, (int)getpid());
name[ sizeof(name)-1 ] = 0;
if (0) printf("name = %s\n", name);
s = sem_open(name, O_CREAT | O_EXCL, 0600, count);
if (s == SEM_FAILED) {
s = NULL;
# error "Unsupported OS"
return s;
static int my_sem_destroy ( sem_t* s )
return sem_destroy(s);
static int my_sem_wait(sem_t* s)
return sem_wait(s);
static int my_sem_post(sem_t* s)
return sem_post(s);