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

[Felipe Balbi]

On Tue, Sep 01, 2015 at 11:12:18AM +0200, Sebastian Ott wrote:
> On Mon, 31 Aug 2015, 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!
> > [   12.033576] other info that might help us debug this:
> > 
> > [   12.041942] rcu_scheduler_active = 1, debug_locks = 0
> > [   12.048796] 4 locks held by systemd/1:
> > [   12.052700]  #0:  (sb_writers#7){.+.+.+}, at: [<c017af84>] __sb_start_write+0x8c/0xb0
> > [   12.060954]  #1:  (&of->mutex){+.+.+.}, at: [<c01f1600>] kernfs_fop_write+0x50/0x1b8
> > [   12.069085]  #2:  (s_active#30){++++.+}, at: [<c01f1608>] kernfs_fop_write+0x58/0x1b8
> > [   12.077310]  #3:  (devcgroup_mutex){+.+...}, at: [<c0317bfc>] devcgroup_access_write+0x20/0x658
> > [   12.086575] stack backtrace:
> > [   12.091124] CPU: 0 PID: 1 Comm: systemd Not tainted 4.2.0-next-20150831 #1154
> > [   12.098609] Hardware name: Generic AM43 (Flattened Device Tree)
> > [   12.104807] [<c001770c>] (unwind_backtrace) from [<c0013a58>] (show_stack+0x10/0x14)
> > [   12.112924] [<c0013a58>] (show_stack) from [<c034f014>] (dump_stack+0x84/0x9c)
> > [   12.120491] [<c034f014>] (dump_stack) from [<c0317a04>] (verify_new_ex+0xc4/0xdc)
> > [   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)
> > 
> > Fix it by making sure rcu_read_lock() is held
> > around devcgroup_update_access().
> 
> With this patch applied I got the following:
> 
> [    4.079102] BUG: sleeping function called from invalid context at mm/slub.c:1266
> [    4.079105] in_atomic(): 0, irqs_disabled(): 0, pid: 1, name: systemd

that didn't trigger here. maybe I didn't have lock debugging enabled.
I'll have a look.

-- 
balbi

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