Re: [GIT PULL rcu/next] RCU commits for 3.1

[Ingo Molnar <mingo@elte.hu>]

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* Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote:

> On Mon, Oct 03, 2011 at 07:53:02AM +0200, Ingo Molnar wrote:
> > 
> > Not sure whether you've seen this one already:
> 
> This is a new one for me.
> 
> > [   18.110320] Adding 3911820k swap on /dev/sda2.  Priority:-1 extents:1 across:3911820k 
> > [   31.803721] 
> > [   31.804597] ===============================
> > [   31.804597] [ INFO: suspicious RCU usage. ]
> > [   31.804597] -------------------------------
> > [   31.804597] include/linux/cgroup.h:548 suspicious rcu_dereference_check() usage!
> > [   31.804597] 
> > [   31.804597] other info that might help us debug this:
> > [   31.804597] 
> > [   31.804597] 
> > [   31.804597] rcu_scheduler_active = 1, debug_locks = 0
> > [   31.804597] 1 lock held by true/845:
> > [   31.804597]  #0:  (&sig->cred_guard_mutex){+.+.+.}, at: [<4109f06f>] prepare_bprm_creds+0x20/0x55
> > [   31.804597] 
> > [   31.804597] stack backtrace:
> > [   31.804597] Pid: 845, comm: true Not tainted 3.1.0-rc8-tip-01699-gde204a2-dirty #157471
> > [   31.804597] Call Trace:
> > [   31.804597]  [<412d78fa>] ? printk+0x18/0x1a
> > [   31.804597]  [<41044190>] lockdep_rcu_suspicious+0xb1/0xb9
> > [   31.804597]  [<4106d6de>] perf_event_comm+0xb1/0x357
> > [   31.804597]  [<4109f048>] set_task_comm+0x4d/0x54
> 
> 	This one does task_lock(), which acquires the task's
> 	->alloc lock.  In theory, this should prevent the
> 	lockdep-RCU splat.  It clearly does not, and here are
> 	some possible reasons why:
> 
> 	1.	Something redirects to some other task along the way.
> 
> 	2.	Something releases ->alloc_lock along the way.
> 
> 	The output above shows no locks held, which points to #2.
> 
> 	set_task_comm() calls perf_event_comm() shown above, which calls
> 	perf_event_comm_event(), which does rcu_read_lock(), which should
> 	also prevent the splat.  Then perf_event_comm_event() calls
> 	perf_event_comm_ctx(), which calls perf_event_comm_output()...
> 
> 	Holy inlining, Batman!!!
> 
> 	OK, I confess, I am a wuss...  Any chance of reproducing this
> 	with CONFIG_SCHED_OMIT_FRAME_POINTER=n?  Or would someone more
> 	familiar with these functions be willing to enlighten me?

Ok, i ran the tests some more and here's a similar splat with 
framepointers enabled:

[   50.402719] eth0: no IPv6 routers present
[   59.147572] 
[   59.149064] ===============================
[   59.151257] [ INFO: suspicious RCU usage. ]
[   59.156865] -------------------------------
[   59.156865] include/linux/cgroup.h:548 suspicious rcu_dereference_check() usage!
[   59.156865] 
[   59.156865] other info that might help us debug this:
[   59.156865] 
[   59.156865] 
[   59.156865] rcu_scheduler_active = 1, debug_locks = 0
[   59.156865] 1 lock held by true/667:
[   59.156865]  #0:  (&sig->cred_guard_mutex){+.+.+.}, at: [<c111d927>] prepare_bprm_creds+0x27/0x70
[   59.156865] 
[   59.156865] stack backtrace:
[   59.156865] Pid: 667, comm: true Not tainted 3.1.0-rc8-tip+ #157499
[   59.156865] Call Trace:
[   59.156865]  [<c1a41f7c>] ? printk+0x28/0x2a
[   59.156865]  [<c109d540>] lockdep_rcu_suspicious+0xc0/0xd0
[   59.156865]  [<c10d8a98>] perf_event_enable_on_exec+0x1c8/0x1d0
[   59.156865]  [<c109c364>] ? __lock_release+0x54/0xd0
[   59.156865]  [<c10daef8>] perf_event_comm+0x18/0x60
[   59.156865]  [<c111d80d>] ? set_task_comm+0x5d/0x80
[   59.156865]  [<c1a6ac6d>] ? _raw_spin_unlock+0x1d/0x30
[   59.156865]  [<c111d814>] set_task_comm+0x64/0x80
[   59.156865]  [<c111e355>] setup_new_exec+0xc5/0x1f0
[   59.156865]  [<c115313b>] load_elf_binary+0x28b/0xa00
[   59.156865]  [<c111de59>] ? search_binary_handler+0xd9/0x1d0
[   59.156865]  [<c109c364>] ? __lock_release+0x54/0xd0
[   59.156865]  [<c1152eb0>] ? do_mmap+0x60/0x60
[   59.156865]  [<c111de60>] search_binary_handler+0xe0/0x1d0
[   59.156865]  [<c111ddb0>] ? search_binary_handler+0x30/0x1d0
[   59.156865]  [<c111e17f>] do_execve_common+0x22f/0x2a0
[   59.156865]  [<c111e202>] do_execve+0x12/0x20
[   59.156865]  [<c1036462>] sys_execve+0x32/0x70
[   59.156865]  [<c1a6c052>] ptregs_execve+0x12/0x18
[   59.156865]  [<c1a6bfd7>] ? sysenter_do_call+0x12/0x36

Config and full bootlog attached.

Thanks,

	Ingo

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