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, 10 insertions, 1 deletions

diff --git a/src/mman/munmap.c b/src/mman/munmap.c
index ab7da9e2..91aefd40 100644
--- a/src/mman/munmap.c
+++ b/src/mman/munmap.c
@@ -3,9 +3,18 @@
 #include "syscall.h"
 #include "libc.h"
 
+static void dummy1(int x) { }
+static void dummy0(void) { }
+weak_alias(dummy1, __vm_lock);
+weak_alias(dummy0, __vm_unlock);
+
 int __munmap(void *start, size_t len)
 {
-	return syscall(SYS_munmap, start, len);
+	int ret;
+	__vm_lock(-1);
+	ret = syscall(SYS_munmap, start, len);
+	__vm_unlock();
+	return ret;
 }
 
 weak_alias(__munmap, munmap);