restore: fix race in calculation of a number of zombies

Currently each task subtracts number of zombies from
task_entries->nr_threads without locks, so if two tasks will do this
operation concurrently, the result may be unpredictable.

https://github.com/xemul/criu/issues/13

Cc: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Andrew Vagin <avagin@openvz.org>
Acked-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

cr-restore.c

@@ -1838,6 +1838,12 @@ static int restore_root_task(struct pstree_item *init)
 	if (ret < 0)
 		goto out_kill;
 
+	/*
+	 * The task_entries->nr_zombies is updated in the
+	 * CR_STATE_RESTORE_SIGCHLD in pie code.
+	 */
+	task_entries->nr_threads -= atomic_read(&task_entries->nr_zombies);
+
 	ret = stop_usernsd();
 	if (ret < 0)
 		goto out_kill;
@@ -1940,6 +1946,7 @@ static int prepare_task_entries(void)
 	task_entries->nr_threads = 0;
 	task_entries->nr_tasks = 0;
 	task_entries->nr_helpers = 0;
+	atomic_set(&task_entries->nr_zombies, 0);
 	futex_set(&task_entries->start, CR_STATE_RESTORE_NS);
 	mutex_init(&task_entries->userns_sync_lock);
 

include/rst_info.h

@@ -7,6 +7,7 @@
 
 struct task_entries {
 	int nr_threads, nr_tasks, nr_helpers;
+	atomic_t nr_zombies;
 	futex_t nr_in_progress;
 	futex_t start;
 	atomic_t cr_err;

pie/restorer.c

@@ -803,8 +803,7 @@ static int wait_zombies(struct task_restore_args *task_args)
 {
 	int i;
 
-	task_entries->nr_threads -= task_args->zombies_n;
-	task_entries->nr_tasks -= task_args->zombies_n;
+	atomic_add(task_args->zombies_n, &task_entries->nr_zombies);
 
 	for (i = 0; i < task_args->zombies_n; i++) {
 		if (sys_waitid(P_PID, task_args->zombies[i], NULL, WNOWAIT | WEXITED, NULL) < 0) {