Author: sewardj
Date: 2007-10-26 11:44:38 +0100 (Fri, 26 Oct 2007)
New Revision: 7036
Log:
Change this test so it actually tests happens-before tracking through
the barrier functions. Also add references to discussions on thread
safeness (or lack of) in gcc's generated code. Finally a spelling fix
from Reuben Thomas.
Modified:
branches/THRCHECK/thrcheck/tests/tc17_sembar.c
Modified: branches/THRCHECK/thrcheck/tests/tc17_sembar.c
===================================================================
--- branches/THRCHECK/thrcheck/tests/tc17_sembar.c 2007-10-25 23:12:48 UTC
(rev 7035)
+++ branches/THRCHECK/thrcheck/tests/tc17_sembar.c 2007-10-26 10:44:38 UTC
(rev 7036)
@@ -103,7 +103,7 @@
sem_wait (&bar->sem2); // 2 down
}
pthread_mutex_unlock (&bar->mutex1);
- /* Resultat professionnel! First we made this thread have an
+ /* «Résultats 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.
@@ -133,46 +133,80 @@
are arriving at the barrier and by mutex2 whilst they are leaving,
but not consistently by either of them. Oh well. */
-gomp_barrier_t bar;
+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:
+ http://gcc.gnu.org/ml/gcc/2007-10/msg00266.html
+ http://lkml.org/lkml/2007/10/24/673
+ and this is interesting background:
+ www.hpl.hp.com/techreports/2004/HPL-2004-209.pdf
+*/
+volatile static 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 );
- gomp_barrier_wait( &bar );
- gomp_barrier_wait( &bar );
- gomp_barrier_wait( &bar );
- gomp_barrier_wait( &bar );
- gomp_barrier_wait( &bar );
return NULL;
}
-#define NNN 4
int main (int argc, char *argv[])
{
- int j;
long i; int res;
- pthread_t thr[NNN];
+ pthread_t thr[4];
fprintf(stderr, "starting\n");
- for (j = 0; j < 1; j++) {
- gomp_barrier_init( &bar, NNN );
+ gomp_barrier_init( &bar, 4 );
- for (i = 0; i < NNN; i++) {
- res = pthread_create( &thr[i], NULL, child, (void*)i );
+ for (i = 0; i < 4; i++) {
+ res = pthread_create( &thr[i], NULL, child, (void*)(i+2) );
assert(!res);
}
- for (i = 0; i < NNN; i++) {
+ for (i = 0; i < 4; i++) {
res = pthread_join( thr[i], NULL );
assert(!res);
}
gomp_barrier_destroy( &bar );
- }
- fprintf(stderr, "done\n");
+ /* 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;
}
-------------------------------------------------------------------------
This SF.net email is sponsored by: Splunk Inc.
Still grepping through log files to find problems? Stop.
Now Search log events and configuration files using AJAX and a browser.
Download your FREE copy of Splunk now >> http://get.splunk.com/
_______________________________________________
Valgrind-developers mailing list
[email protected]
https://lists.sourceforge.net/lists/listinfo/valgrind-developers
