wait4(): do not return ECHILD when child exists but not in PS_ZOMBIE state yet

kernel/syscall.c

@@ -326,10 +326,8 @@ rescan:
 
 		ihk_mc_spinlock_lock_noirq(&child_iter->lock);
 		
-		if (child_iter->status == PS_ZOMBIE
-			&& (pid == -1 || pid == child_iter->pid)) {
+		if (pid == -1 || pid == child_iter->pid) {
 			child = child_iter;
-			ihk_mc_spinlock_unlock_noirq(&child_iter->lock);
 			break;
 		}
 		
@@ -341,9 +339,12 @@ rescan:
 		return -ECHILD;
 	}
 
+	/* If child is valid we are still holding its ftn->lock */
 	if (child) {
+		if (child->status == PS_ZOMBIE) {
 			struct syscall_request request IHK_DMA_ALIGN;
 
+			ihk_mc_spinlock_unlock_noirq(&child->lock);
 			dkprintf("wait: found PS_ZOMBIE process: %d\n", child->pid);
 
 			list_del(&child->siblings_list);	
@@ -368,6 +369,9 @@ rescan:
 			goto exit;
 		}
 		
+		ihk_mc_spinlock_unlock_noirq(&child->lock);
+	}
+	
 	/* Don't sleep if WNOHANG requested */
 	if (options & WNOHANG) {
 		ihk_mc_spinlock_unlock_noirq(&proc->ftn->lock);