Re: [PATCH v2] security: device_cgroup: fix RCU lockdep splat

Tejun Heo
2015-09-02T16:24:54Z

cc'ing Paul.

On Wed, Sep 02, 2015 at 08:12:28AM -0500, Felipe Balbi wrote:
> while booting AM437x device, the following splat
> triggered:
>
> [ 12.005238] ===============================
> [ 12.009749] [ INFO: suspicious RCU usage. ]
> [ 12.014116] 4.2.0-next-20150831 #1154 Not tainted
> [ 12.019050] -------------------------------
> [ 12.023408] security/device_cgroup.c:405 device_cgroup:verify_new_ex called without proper synchronization!
...
> [ 12.128326] [<c0317a04>] (verify_new_ex) from [<c0317f50>] (devcgroup_access_write+0x374/0x658)
> [ 12.137426] [<c0317f50>] (devcgroup_access_write) from [<c00d2800>] (cgroup_file_write+0x28/0x1bc)
> [ 12.146796] [<c00d2800>] (cgroup_file_write) from [<c01f1670>] (kernfs_fop_write+0xc0/0x1b8)
> [ 12.155620] [<c01f1670>] (kernfs_fop_write) from [<c0177c94>] (__vfs_write+0x1c/0xd8)
> [ 12.163783] [<c0177c94>] (__vfs_write) from [<c0178594>] (vfs_write+0x90/0x16c)
> [ 12.171426] [<c0178594>] (vfs_write) from [<c0178db4>] (SyS_write+0x44/0x9c)
> [ 12.178806] [<c0178db4>] (SyS_write) from [<c000f680>] (ret_fast_syscall+0x0/0x1c)

This shouldn't be happening because devcgroup_access_write() always
grabs devcgroup_mutex. Looking at the log, the culprit seems to be
f78f5b90c4ff ("rcu: Rename rcu_lockdep_assert() to
RCU_LOCKDEP_WARN()"). It missed the bang for the second test while
inverting it, so adding rcu_read_lock() isn't the right fix here.

Paul, can you please fix it?

Thanks.

--
tejun