Fix deadlock in multithreaded fork in OS X.

On OS X, we rely on the zone machinery to call our prefork and postfork
handlers.

In zone_force_unlock, we call jemalloc_postfork_child, reinitializing all our
mutexes regardless of state, since the mutex implementation will assert if the
tid of the unlocker is different from that of the locker.  This has the effect
of unlocking the mutexes, but also fails to wake any threads waiting on them in
the parent.

To fix this, we track whether or not we're the parent or child after the fork,
and unlock or reinit as appropriate.

This resolves #895.

1 files changed, 24 insertions, 6 deletions

diff --git a/src/zone.c b/src/zone.c
index 9d3b7b49..23dfdd04 100644
--- a/src/zone.c
+++ b/src/zone.c
@@ -89,6 +89,7 @@ JEMALLOC_ATTR(weak_import);
 static malloc_zone_t *default_zone, *purgeable_zone;
 static malloc_zone_t jemalloc_zone;
 static struct malloc_introspection_t jemalloc_zone_introspect;
+static pid_t zone_force_lock_pid = -1;
 
 /******************************************************************************/
 /* Function prototypes for non-inline static functions. */
@@ -270,6 +271,12 @@ zone_log(malloc_zone_t *zone, void *address) {
 static void
 zone_force_lock(malloc_zone_t *zone) {
 	if (isthreaded) {
+		/*
+		 * See the note in zone_force_unlock, below, to see why we need
+		 * this.
+		 */
+		assert(zone_force_lock_pid == -1);
+		zone_force_lock_pid = getpid();
 		jemalloc_prefork();
 	}
 }
@@ -277,14 +284,25 @@ zone_force_lock(malloc_zone_t *zone) {
 static void
 zone_force_unlock(malloc_zone_t *zone) {
 	/*
-	 * Call jemalloc_postfork_child() rather than
-	 * jemalloc_postfork_parent(), because this function is executed by both
-	 * parent and child.  The parent can tolerate having state
-	 * reinitialized, but the child cannot unlock mutexes that were locked
-	 * by the parent.
+	 * zone_force_lock and zone_force_unlock are the entry points to the
+	 * forking machinery on OS X.  The tricky thing is, the child is not
+	 * allowed to unlock mutexes locked in the parent, even if owned by the
+	 * forking thread (and the mutex type we use in OS X will fail an assert
+	 * if we try).  In the child, we can get away with reinitializing all
+	 * the mutexes, which has the effect of unlocking them.  In the parent,
+	 * doing this would mean we wouldn't wake any waiters blocked on the
+	 * mutexes we unlock.  So, we record the pid of the current thread in
+	 * zone_force_lock, and use that to detect if we're in the parent or
+	 * child here, to decide which unlock logic we need.
 	 */
 	if (isthreaded) {
-		jemalloc_postfork_child();
+		assert(zone_force_lock_pid != -1);
+		if (getpid() == zone_force_lock_pid) {
+			jemalloc_postfork_parent();
+		} else {
+			jemalloc_postfork_child();
+		}
+		zone_force_lock_pid = -1;
 	}
 }