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Date: Mon, 24 Oct 2011 04:48:06 -0700
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To: Ingo Molnar <mingo@...e.hu>
Cc: eric.dumazet@...il.com, shaohua.li@...el.com, ak@...ux.intel.com,
mhocko@...e.cz, alex.shi@...el.com, efault@....de,
linux-kernel@...r.kernel.org
Subject: Re: [GIT PULL rcu/next] RCU commits for 3.1
On Mon, Oct 24, 2011 at 03:05:01AM -0700, Paul E. McKenney wrote:
> On Tue, Oct 04, 2011 at 09:46:37AM +0200, Ingo Molnar wrote:
> >
> > * Paul E. McKenney <paulmck@...ux.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.
>
> Hello, Ingo,
>
> It appears that inlining has defeated me, so I tried reproducing under
> KVM, using the closest bootable approximation to your .config (attached).
> I booted ten times without seeing this error. I have my changes against
> 3.1-rc8. I will try against 3.1, but in the meantime any enlightenment
> would be most welcome. ;-)
And I cannot reproduce after merging into 3.1. :-(
Thanx, Paul
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