[PATCH 1/2] cpuset: fix RCU lockdep splat

Tejun Heo
2013-01-15T16:40:53Z

>From 27e89ae5d6e94e30231ee89a7736f62d84ba4c6f Mon Sep 17 00:00:00 2001
From: Li Zefan <lizefan@huawei.com>
Date: Tue, 15 Jan 2013 14:10:57 +0800

5d21cc2db040d01f8c19b8602f6987813e1176b4 ("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>
---
I rewrote the patch description for clarity and commited it to
cgroup/for-3.9-cpuset.

Thanks.

kernel/cpuset.c | 8 ++++----
1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/kernel/cpuset.c b/kernel/cpuset.c
index 6aa5bbb..1a675e4 100644
--- a/kernel/cpuset.c
+++ b/kernel/cpuset.c
@@ -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);
--
1.8.0.2