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cgroup: Eliminate the need for cgroup_mutex in proc_cgroup_show()

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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: Yafang Shao <laoar.shao@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
This commit is contained in:
Yafang Shao 2023-10-29 06:14:30 +00:00 committed by Tejun Heo
parent d23b5c5777
commit 9067d90006
1 changed files with 7 additions and 5 deletions
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12
kernel/cgroup/cgroup.c
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@ -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;