Commit 27e89ae5 authored Jan 15, 2013 by Li Zefan Committed by Tejun Heo Jan 15, 2013

cpuset: fix RCU lockdep splat



5d21cc2d ("cpuset: replace
cgroup_mutex locking with cpuset internal locking") incorrectly
converted proc_cpuset_show() from cgroup_lock() to cpuset_mutex.
proc_cpuset_show() is accessing cgroup hierarchy proper to determine
cgroup path which can't be protected by cpuset_mutex.  This triggered
the following RCU warning.

 ===============================
 [ INFO: suspicious RCU usage. ]
 3.8.0-rc3-next-20130114-sasha-00016-ga107525-dirty #262 Tainted: G        W
 -------------------------------
 include/linux/cgroup.h:534 suspicious rcu_dereference_check() usage!

 other info that might help us debug this:

 rcu_scheduler_active = 1, debug_locks = 1
 2 locks held by trinity/7514:
  #0:  (&p->lock){+.+.+.}, at: [<ffffffff812b06aa>] seq_read+0x3a/0x3e0
  #1:  (cpuset_mutex){+.+...}, at: [<ffffffff811abae4>] proc_cpuset_show+0x84/0x190

 stack backtrace:
 Pid: 7514, comm: trinity Tainted: G        W
+3.8.0-rc3-next-20130114-sasha-00016-ga107525-dirty #262
 Call Trace:
  [<ffffffff81182cab>] lockdep_rcu_suspicious+0x10b/0x120
  [<ffffffff811abb71>] proc_cpuset_show+0x111/0x190
  [<ffffffff812b0827>] seq_read+0x1b7/0x3e0
  [<ffffffff812b0670>] ? seq_lseek+0x110/0x110
  [<ffffffff8128b4fb>] do_loop_readv_writev+0x4b/0x90
  [<ffffffff8128b776>] do_readv_writev+0xf6/0x1d0
  [<ffffffff8128b8ee>] vfs_readv+0x3e/0x60
  [<ffffffff8128b960>] sys_readv+0x50/0xd0
  [<ffffffff83d33d18>] tracesys+0xe1/0xe6

The operation can be performed under RCU read lock.  Replace
cpuset_mutex locking with RCU read locking.

tj: Rewrote patch description.

Reported-by: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: Li Zefan <lizefan@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

parent f47b89c7

kernel/cpuset.c

+4 −4

Original line number	Diff line number	Diff line
		@@ -2678,15 +2678,15 @@ static int proc_cpuset_show(struct seq_file m, void unused_v)
		goto out_free;

		retval = -EINVAL;
		mutex_lock(&cpuset_mutex);
		rcu_read_lock();
		css = task_subsys_state(tsk, cpuset_subsys_id);
		retval = cgroup_path(css->cgroup, buf, PAGE_SIZE);
		rcu_read_unlock();
		if (retval < 0)
		goto out_unlock;
		goto out_put_task;
		seq_puts(m, buf);
		seq_putc(m, '\n');
		out_unlock:
		mutex_unlock(&cpuset_mutex);
		out_put_task:
		put_task_struct(tsk);
		out_free:
		kfree(buf);

Admin message