new helper: daemonize_descriptors()

descriptor-related parts of daemonize, done right.  As the
result we simplify the locking rules for ->files - we
hold task_lock in *all* cases when we modify ->files.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/file.c

@@ -519,6 +519,12 @@ struct files_struct init_files = {
 	.file_lock	= __SPIN_LOCK_UNLOCKED(init_task.file_lock),
 };
 
+void daemonize_descriptors(void)
+{
+	atomic_inc(&init_files.count);
+	reset_files_struct(&init_files);
+}
+
 /*
  * allocate a file descriptor, mark it busy.
  */

include/linux/fdtable.h

@@ -95,6 +95,7 @@ struct task_struct;
 struct files_struct *get_files_struct(struct task_struct *);
 void put_files_struct(struct files_struct *fs);
 void reset_files_struct(struct files_struct *);
+void daemonize_descriptors(void);
 int unshare_files(struct files_struct **);
 struct files_struct *dup_fd(struct files_struct *, int *);
 void do_close_on_exec(struct files_struct *);

kernel/exit.c

@@ -457,9 +457,7 @@ void daemonize(const char *name, ...)
 	/* Become as one with the init task */
 
 	daemonize_fs_struct();
-	exit_files(current);
-	current->files = init_task.files;
-	atomic_inc(&current->files->count);
+	daemonize_descriptors();
 
 	reparent_to_kthreadd();
 }