Commit 47295830 authored Feb 27, 2014 by Li Zefan Committed by Tejun Heo Feb 27, 2014

cpuset: fix a locking issue in cpuset_migrate_mm()



I can trigger a lockdep warning:

  # mount -t cgroup -o cpuset xxx /cgroup
  # mkdir /cgroup/cpuset
  # mkdir /cgroup/tmp
  # echo 0 > /cgroup/tmp/cpuset.cpus
  # echo 0 > /cgroup/tmp/cpuset.mems
  # echo 1 > /cgroup/tmp/cpuset.memory_migrate
  # echo $$ > /cgroup/tmp/tasks
  # echo 1 > /cgruop/tmp/cpuset.mems

  ===============================
  [ INFO: suspicious RCU usage. ]
  3.14.0-rc1-0.1-default+ #32 Not tainted
  -------------------------------
  include/linux/cgroup.h:682 suspicious rcu_dereference_check() usage!
  ...
    [<ffffffff81582174>] dump_stack+0x72/0x86
    [<ffffffff810b8f01>] lockdep_rcu_suspicious+0x101/0x140
    [<ffffffff81105ba1>] cpuset_migrate_mm+0xb1/0xe0
  ...

We used to hold cgroup_mutex when calling cpuset_migrate_mm(), but now
we hold cpuset_mutex, which causes task_css() to complain.

This is not a false-positive but a real issue.

Holding cpuset_mutex won't prevent a task from migrating to another
cpuset, and it won't prevent the original task->cgroup from destroying
during this change.

Fixes: 5d21cc2d (cpuset: replace cgroup_mutex locking with cpuset internal locking)
Cc: <stable@vger.kernel.org> # 3.9+
Signed-off-by: Li Zefan <lizefan@huawei.com>
Sigend-off-by: Tejun Heo <tj@kernel.org>

parent 532de3fc

kernel/cpuset.c

+2 −6

Original line number	Diff line number	Diff line
		@@ -974,12 +974,6 @@ static int update_cpumask(struct cpuset cs, struct cpuset trialcs,
		* Temporarilly set tasks mems_allowed to target nodes of migration,
		* so that the migration code can allocate pages on these nodes.
		*
		* Call holding cpuset_mutex, so current's cpuset won't change
		* during this call, as manage_mutex holds off any cpuset_attach()
		* calls. Therefore we don't need to take task_lock around the
		* call to guarantee_online_mems(), as we know no one is changing
		* our task's cpuset.
		*
		* While the mm_struct we are migrating is typically from some
		* other task, the task_struct mems_allowed that we are hacking
		* is for our current task, which must allocate new pages for that
		@@ -996,8 +990,10 @@ static void cpuset_migrate_mm(struct mm_struct mm, const nodemask_t from,

		do_migrate_pages(mm, from, to, MPOL_MF_MOVE_ALL);

		rcu_read_lock();
		mems_cs = effective_nodemask_cpuset(task_cs(tsk));
		guarantee_online_mems(mems_cs, &tsk->mems_allowed);
		rcu_read_unlock();
		}

		/*

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