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

* 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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