process-shared barrier support, based on discussion with bdonlan

this implementation is rather heavy-weight, but it's the first
solution i've found that's actually correct. all waiters actually wait
twice at the barrier so that they can synchronize exit, and they hold
a "vm lock" that prevents changes to virtual memory mappings (and
blocks pthread_barrier_destroy) until all waiters are finished
inspecting the barrier.

thus, it is safe for any thread to destroy and/or unmap the barrier's
memory as soon as pthread_barrier_wait returns, without further
synchronization.

1 files changed, 2 insertions, 2 deletions

diff --git a/src/thread/pthread_barrier_init.c b/src/thread/pthread_barrier_init.c
index 736d1014..ccaab4eb 100644
--- a/src/thread/pthread_barrier_init.c
+++ b/src/thread/pthread_barrier_init.c
@@ -2,7 +2,7 @@
 
 int pthread_barrier_init(pthread_barrier_t *b, const pthread_barrierattr_t *a, unsigned count)
 {
-	if (!count) return EINVAL;
-	*b = (pthread_barrier_t){ ._b_limit = count-1 };
+	if (count-1 > INT_MAX-1) return EINVAL;
+	*b = (pthread_barrier_t){ ._b_limit = count-1 | (a?*a:0) };
 	return 0;
 }