Commit d8742e22 authored May 23, 2018 by Tejun Heo

cgroup: css_set_lock should nest inside tasklist_lock



cgroup_enable_task_cg_lists() incorrectly nests non-irq-safe
tasklist_lock inside irq-safe css_set_lock triggering the following
lockdep warning.

  WARNING: possible irq lock inversion dependency detected
  4.17.0-rc1-00027-gb37d049 #6 Not tainted
  --------------------------------------------------------
  systemd/1 just changed the state of lock:
  00000000fe57773b (css_set_lock){..-.}, at: cgroup_free+0xf2/0x12a
  but this lock took another, SOFTIRQ-unsafe lock in the past:
   (tasklist_lock){.+.+}

  and interrupts could create inverse lock ordering between them.

  other info that might help us debug this:
   Possible interrupt unsafe locking scenario:

	 CPU0                    CPU1
	 ----                    ----
    lock(tasklist_lock);
				 local_irq_disable();
				 lock(css_set_lock);
				 lock(tasklist_lock);
    <Interrupt>
      lock(css_set_lock);

   *** DEADLOCK ***

The condition is highly unlikely to actually happen especially given
that the path is executed only once per boot.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Boqun Feng <boqun.feng@gmail.com>

parent cc659e76

kernel/cgroup/cgroup.c

+8 −8

Original line number	Diff line number	Diff line
		@@ -1798,13 +1798,6 @@ static void cgroup_enable_task_cg_lists(void)
		{
		struct task_struct p, g;

		spin_lock_irq(&css_set_lock);

		if (use_task_css_set_links)
		goto out_unlock;

		use_task_css_set_links = true;

		/*
		* We need tasklist_lock because RCU is not safe against
		* while_each_thread(). Besides, a forking task that has passed
		@@ -1813,6 +1806,13 @@ static void cgroup_enable_task_cg_lists(void)
		* tasklist if we walk through it with RCU.
		*/
		read_lock(&tasklist_lock);
		spin_lock_irq(&css_set_lock);

		if (use_task_css_set_links)
		goto out_unlock;

		use_task_css_set_links = true;

		do_each_thread(g, p) {
		WARN_ON_ONCE(!list_empty(&p->cg_list) \|\|
		task_css_set(p) != &init_css_set);
		@@ -1840,9 +1840,9 @@ static void cgroup_enable_task_cg_lists(void)
		}
		spin_unlock(&p->sighand->siglock);
		} while_each_thread(g, p);
		read_unlock(&tasklist_lock);
		out_unlock:
		spin_unlock_irq(&css_set_lock);
		read_unlock(&tasklist_lock);
		}

		static void init_cgroup_housekeeping(struct cgroup *cgrp)

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