Commit 9067d900 authored by Yafang Shao's avatar Yafang Shao Committed by Tejun Heo
Browse files

cgroup: Eliminate the need for cgroup_mutex in proc_cgroup_show() 



The cgroup root_list is already RCU-safe. Therefore, we can replace the
cgroup_mutex with the RCU read lock in some particular paths. This change
will be particularly beneficial for frequent operations, such as
`cat /proc/self/cgroup`, in a cgroup1-based container environment.

I did stress tests with this change, as outlined below
(with CONFIG_PROVE_RCU_LIST enabled):

- Continuously mounting and unmounting named cgroups in some tasks,
  for example:

  cgrp_name=$1
  while true
  do
      mount -t cgroup -o none,name=$cgrp_name none /$cgrp_name
      umount /$cgrp_name
  done

- Continuously triggering proc_cgroup_show() in some tasks concurrently,
  for example:
  while true; do cat /proc/self/cgroup > /dev/null; done

They can ran successfully after implementing this change, with no RCU
warnings in dmesg.

Signed-off-by: default avatarYafang Shao <laoar.shao@gmail.com>
Signed-off-by: default avatarTejun Heo <tj@kernel.org>
parent d23b5c57

kernel/cgroup/cgroup.c

+7 −5
Original line number Diff line number Diff line
@@ -6285,7 +6285,7 @@ int proc_cgroup_show(struct seq_file *m, struct pid_namespace *ns, 
	if (!buf)

		goto out;



	cgroup_lock();

	rcu_read_lock();

	spin_lock_irq(&css_set_lock);



	for_each_root(root) {
@@ -6296,6 +6296,11 @@ int proc_cgroup_show(struct seq_file *m, struct pid_namespace *ns, 
		if (root == &cgrp_dfl_root && !READ_ONCE(cgrp_dfl_visible))

			continue;



		cgrp = task_cgroup_from_root(tsk, root);

		/* The root has already been unmounted. */

		if (!cgrp)

			continue;



		seq_printf(m, "%d:", root->hierarchy_id);

		if (root != &cgrp_dfl_root)

			for_each_subsys(ss, ssid)
@@ -6306,9 +6311,6 @@ int proc_cgroup_show(struct seq_file *m, struct pid_namespace *ns, 
			seq_printf(m, "%sname=%s", count ? "," : "",

				   root->name);

		seq_putc(m, ':');



		cgrp = task_cgroup_from_root(tsk, root);



		/*

		 * On traditional hierarchies, all zombie tasks show up as

		 * belonging to the root cgroup.  On the default hierarchy,
@@ -6340,7 +6342,7 @@ int proc_cgroup_show(struct seq_file *m, struct pid_namespace *ns, 
	retval = 0;

out_unlock:

	spin_unlock_irq(&css_set_lock);

	cgroup_unlock();

	rcu_read_unlock();

	kfree(buf);

out:

	return retval;