forking: Introduce last_pid_mutex and helpers

Introduce mutex for synchronization ns_last_pid file
on restore.
Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

criu/cr-restore.c

@@ -2328,6 +2328,7 @@ int prepare_task_entries(void)
 	task_entries->nr_helpers = 0;
 	futex_set(&task_entries->start, CR_STATE_FAIL);
 	mutex_init(&task_entries->userns_sync_lock);
+	mutex_init(&task_entries->last_pid_mutex);
 
 	return 0;
 }

criu/include/rst_info.h

@@ -11,6 +11,7 @@ struct task_entries {
 	futex_t start;
 	atomic_t cr_err;
 	mutex_t userns_sync_lock;
+	mutex_t last_pid_mutex;
 };
 
 struct fdt {
@@ -71,4 +72,16 @@ struct rst_info {
 	void			*breakpoint;
 };
 
+extern struct task_entries *task_entries;
+
+static inline void lock_last_pid(void)
+{
+	mutex_lock(&task_entries->last_pid_mutex);
+}
+
+static inline void unlock_last_pid(void)
+{
+	mutex_unlock(&task_entries->last_pid_mutex);
+}
+
 #endif /* __CR_RST_INFO_H__ */