cgroup: refactor the cgroup_attach_lock code to make it clearer

Dynamic cgroup migration involving threadgroup locks can be in one of
two states: no lock held, or holding the global lock. Explicitly
declaring the different lock modes to make the code easier to
understand and facilitates future extensions of the lock modes.

Signed-off-by: Yi Tao <[e-mail]>
Signed-off-by: Tejun Heo <[e-mail]>

include/linux/cgroup-defs.h

···
 	__CFTYPE_ADDED		= (1 << 18),
 };
 
+enum cgroup_attach_lock_mode {
+	/* Default */
+	CGRP_ATTACH_LOCK_GLOBAL,
+
+	/* When pid=0 && threadgroup=false, see comments in cgroup_procs_write_start */
+	CGRP_ATTACH_LOCK_NONE,
+};
+
 /*
  * cgroup_file is the handle for a file instance created in a cgroup which
  * is used, for example, to generate file changed notifications.  This can

kernel/cgroup/cgroup-internal.h

···
 
 int cgroup_attach_task(struct cgroup *dst_cgrp, struct task_struct *leader,
 		       bool threadgroup);
-void cgroup_attach_lock(bool lock_threadgroup);
-void cgroup_attach_unlock(bool lock_threadgroup);
+void cgroup_attach_lock(enum cgroup_attach_lock_mode lock_mode);
+void cgroup_attach_unlock(enum cgroup_attach_lock_mode lock_mode);
 struct task_struct *cgroup_procs_write_start(char *buf, bool threadgroup,
-					     bool *locked)
+					     enum cgroup_attach_lock_mode *lock_mode)
 	__acquires(&cgroup_threadgroup_rwsem);
-void cgroup_procs_write_finish(struct task_struct *task, bool locked)
+void cgroup_procs_write_finish(struct task_struct *task,
+			       enum cgroup_attach_lock_mode lock_mode)
 	__releases(&cgroup_threadgroup_rwsem);
 
 void cgroup_lock_and_drain_offline(struct cgroup *cgrp);

kernel/cgroup/cgroup-v1.c

···
 	int retval = 0;
 
 	cgroup_lock();
-	cgroup_attach_lock(true);
+	cgroup_attach_lock(CGRP_ATTACH_LOCK_GLOBAL);
 	for_each_root(root) {
 		struct cgroup *from_cgrp;
 
···
 		if (retval)
 			break;
 	}
-	cgroup_attach_unlock(true);
+	cgroup_attach_unlock(CGRP_ATTACH_LOCK_GLOBAL);
 	cgroup_unlock();
 
 	return retval;
···
 
 	cgroup_lock();
 
-	cgroup_attach_lock(true);
+	cgroup_attach_lock(CGRP_ATTACH_LOCK_GLOBAL);
 
 	/* all tasks in @from are being moved, all csets are source */
 	spin_lock_irq(&css_set_lock);
···
 	} while (task && !ret);
 out_err:
 	cgroup_migrate_finish(&mgctx);
-	cgroup_attach_unlock(true);
+	cgroup_attach_unlock(CGRP_ATTACH_LOCK_GLOBAL);
 	cgroup_unlock();
 	return ret;
 }
···
 	struct task_struct *task;
 	const struct cred *cred, *tcred;
 	ssize_t ret;
-	bool locked;
+	enum cgroup_attach_lock_mode lock_mode;
 
 	cgrp = cgroup_kn_lock_live(of->kn, false);
 	if (!cgrp)
 		return -ENODEV;
 
-	task = cgroup_procs_write_start(buf, threadgroup, &locked);
+	task = cgroup_procs_write_start(buf, threadgroup, &lock_mode);
 	ret = PTR_ERR_OR_ZERO(task);
 	if (ret)
 		goto out_unlock;
···
 	ret = cgroup_attach_task(cgrp, task, threadgroup);
 
 out_finish:
-	cgroup_procs_write_finish(task, locked);
+	cgroup_procs_write_finish(task, lock_mode);
 out_unlock:
 	cgroup_kn_unlock(of->kn);
 

kernel/cgroup/cgroup.c

···
 
 /**
  * cgroup_attach_lock - Lock for ->attach()
- * @lock_threadgroup: whether to down_write cgroup_threadgroup_rwsem
+ * @lock_mode: whether to down_write cgroup_threadgroup_rwsem
  *
  * cgroup migration sometimes needs to stabilize threadgroups against forks and
  * exits by write-locking cgroup_threadgroup_rwsem. However, some ->attach()
···
  * write-locking cgroup_threadgroup_rwsem. This allows ->attach() to assume that
  * CPU hotplug is disabled on entry.
  */
-void cgroup_attach_lock(bool lock_threadgroup)
+void cgroup_attach_lock(enum cgroup_attach_lock_mode lock_mode)
 {
 	cpus_read_lock();
-	if (lock_threadgroup)
+
+	switch (lock_mode) {
+	case CGRP_ATTACH_LOCK_NONE:
+		break;
+	case CGRP_ATTACH_LOCK_GLOBAL:
 		percpu_down_write(&cgroup_threadgroup_rwsem);
+		break;
+	default:
+		pr_warn("cgroup: Unexpected attach lock mode.");
+		break;
+	}
 }
 
 /**
  * cgroup_attach_unlock - Undo cgroup_attach_lock()
- * @lock_threadgroup: whether to up_write cgroup_threadgroup_rwsem
+ * @lock_mode: whether to up_write cgroup_threadgroup_rwsem
  */
-void cgroup_attach_unlock(bool lock_threadgroup)
+void cgroup_attach_unlock(enum cgroup_attach_lock_mode lock_mode)
 {
-	if (lock_threadgroup)
+	switch (lock_mode) {
+	case CGRP_ATTACH_LOCK_NONE:
+		break;
+	case CGRP_ATTACH_LOCK_GLOBAL:
 		percpu_up_write(&cgroup_threadgroup_rwsem);
+		break;
+	default:
+		pr_warn("cgroup: Unexpected attach lock mode.");
+		break;
+	}
+
 	cpus_read_unlock();
 }
 
···
 }
 
 struct task_struct *cgroup_procs_write_start(char *buf, bool threadgroup,
-					     bool *threadgroup_locked)
+					     enum cgroup_attach_lock_mode *lock_mode)
 {
 	struct task_struct *tsk;
 	pid_t pid;
···
 	 * Therefore, we can skip the global lock.
 	 */
 	lockdep_assert_held(&cgroup_mutex);
-	*threadgroup_locked = pid || threadgroup;
-	cgroup_attach_lock(*threadgroup_locked);
+
+	if (pid || threadgroup)
+		*lock_mode = CGRP_ATTACH_LOCK_GLOBAL;
+	else
+		*lock_mode = CGRP_ATTACH_LOCK_NONE;
+
+	cgroup_attach_lock(*lock_mode);
 
 	rcu_read_lock();
 	if (pid) {
···
 	goto out_unlock_rcu;
 
 out_unlock_threadgroup:
-	cgroup_attach_unlock(*threadgroup_locked);
-	*threadgroup_locked = false;
+	cgroup_attach_unlock(*lock_mode);
+	*lock_mode = CGRP_ATTACH_LOCK_NONE;
 out_unlock_rcu:
 	rcu_read_unlock();
 	return tsk;
 }
 
-void cgroup_procs_write_finish(struct task_struct *task, bool threadgroup_locked)
+void cgroup_procs_write_finish(struct task_struct *task,
+			       enum cgroup_attach_lock_mode lock_mode)
 {
 	/* release reference from cgroup_procs_write_start() */
 	put_task_struct(task);
 
-	cgroup_attach_unlock(threadgroup_locked);
+	cgroup_attach_unlock(lock_mode);
 }
 
 static void cgroup_print_ss_mask(struct seq_file *seq, u16 ss_mask)
···
 	struct cgroup_subsys_state *d_css;
 	struct cgroup *dsct;
 	struct css_set *src_cset;
+	enum cgroup_attach_lock_mode lock_mode;
 	bool has_tasks;
 	int ret;
 
···
 	 * write-locking can be skipped safely.
 	 */
 	has_tasks = !list_empty(&mgctx.preloaded_src_csets);
-	cgroup_attach_lock(has_tasks);
+
+	if (has_tasks)
+		lock_mode = CGRP_ATTACH_LOCK_GLOBAL;
+	else
+		lock_mode = CGRP_ATTACH_LOCK_NONE;
+
+	cgroup_attach_lock(lock_mode);
 
 	/* NULL dst indicates self on default hierarchy */
 	ret = cgroup_migrate_prepare_dst(&mgctx);
···
 	ret = cgroup_migrate_execute(&mgctx);
 out_finish:
 	cgroup_migrate_finish(&mgctx);
-	cgroup_attach_unlock(has_tasks);
+	cgroup_attach_unlock(lock_mode);
 	return ret;
 }
 
···
 	struct task_struct *task;
 	const struct cred *saved_cred;
 	ssize_t ret;
-	bool threadgroup_locked;
+	enum cgroup_attach_lock_mode lock_mode;
 
 	dst_cgrp = cgroup_kn_lock_live(of->kn, false);
 	if (!dst_cgrp)
 		return -ENODEV;
 
-	task = cgroup_procs_write_start(buf, threadgroup, &threadgroup_locked);
+	task = cgroup_procs_write_start(buf, threadgroup, &lock_mode);
 	ret = PTR_ERR_OR_ZERO(task);
 	if (ret)
 		goto out_unlock;
···
 	ret = cgroup_attach_task(dst_cgrp, task, threadgroup);
 
 out_finish:
-	cgroup_procs_write_finish(task, threadgroup_locked);
+	cgroup_procs_write_finish(task, lock_mode);
 out_unlock:
 	cgroup_kn_unlock(of->kn);