On Fri, Aug 13, 2010 at 8:55 AM, Subrata Modak
<subrata@xxxxxxxxxxxxxxxxxx> wrote:
> Hi Paul,
>
> Is there any specific person(s) whom we whom we should direct this mail
> to ? We have not received any response from CGROUP developers on this.
> Kindly let me know whom to contact for this. I am adding few more people
> i know :-)
>
It would help if you would add in the cgroup maintainers and the
containers mailing list to the cc as well.
> Regards--
> Subrata
>
> On Mon, 2010-08-09 at 09:12 -0700, Paul E. McKenney wrote:
>> On Mon, Aug 02, 2010 at 02:22:12PM +0530, Subrata Modak wrote:
>> > Hi,
>> >
>> > The following suspicious rcu_dereference_check() usage is detected
>> > during 2.6.35-stable boot on my ppc64/p7 machine:
>> >
>> > ==================================================
>> > [ INFO: suspicious rcu_dereference_check() usage. ]
>> > ---------------------------------------------------
>> > kernel/sched.c:616 invoked rcu_dereference_check() without protection!
>> > other info that might help us debug this:
>> >
>> > rcu_scheduler_active = 1, debug_locks = 0
>> > 1 lock held by swapper/1:
>> > #0: (&rq->lock){-.....}, at: [<c0000000007ca2f8>] .init_idle+0x78/0x4a8
>> > stack backtrace:
>> > Call Trace:
>> > [c000000f392bf990] [c000000000014f04] .show_stack+0xb0/0x1a0 (unreliable)
>> > [c000000f392bfa50] [c0000000007c87b4] .dump_stack+0x28/0x3c
>> > [c000000f392bfad0] [c000000000103e1c] .lockdep_rcu_dereference+0xbc/0xe4
>> > [c000000f392bfb70] [c0000000007ca434] .init_idle+0x1b4/0x4a8
>> > [c000000f392bfc30] [c0000000007cad04] .fork_idle+0xa4/0xd0
>> > [c000000f392bfe30] [c000000000aefaac] .smp_prepare_cpus+0x23c/0x2f4
>> > [c000000f392bfed0] [c000000000ae1424] .kernel_init+0xec/0x32c
>> > [c000000f392bff90] [c000000000033f40] .kernel_thread+0x54/0x70
>> > ==================================================
>> >
>> > Please note that this was reported earlier on 2.6.34-rc6:
>> > http://marc.info/?l=linux-kernel&m=127313031922395&w=2,
>> > The issue was fixed with:
>> > commit 1ce7e4ff24fe338438bc7837e02780f202bf202b
>> > Author: Li Zefan <lizf@xxxxxxxxxxxxxx>
>> > Date: Fri Apr 23 10:35:52 2010 +0800
>> > cgroup: Check task_lock in task_subsys_state()
>> >
>> > According to:
>> > http://lkml.org/lkml/2010/7/1/883,
>> > commit dc61b1d65e353d638b2445f71fb8e5b5630f2415
>> > Author: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
>> > Date: Tue Jun 8 11:40:42 2010 +0200
>> > sched: Fix PROVE_RCU vs cpu_cgroup
>> > should have fixed this. But this is reproducible on 2.6.35-stable.
>> >
>> > Please also see the config file attached.
>>
>> Hello, Subrata,
>>
>> Thank you for locating this one! This looks like the same issue that
>> Ilia Mirkin located. Please see below for my analysis -- no fix yet,
>> as I need confirmation from cgroups experts. I can easily create a
>> patch that suppresses the warning, but I don't yet know whether this is
>> the right thing to do.
>>
>> Thanx, Paul
>>
>> ------------------------------------------------------------------------
>>
>> On Thu, Aug 05, 2010 at 01:31:10PM -0400, Ilia Mirkin wrote:
>> > On Thu, Jul 1, 2010 at 6:18 PM, Paul E. McKenney
>> > <paulmck@xxxxxxxxxxxxxxxxxx> wrote:
>> > > On Thu, Jul 01, 2010 at 08:21:43AM -0400, Miles Lane wrote:
>> > >> [ INFO: suspicious rcu_dereference_check() usage. ]
>> > >> ---------------------------------------------------
>> > >> kernel/sched.c:616 invoked rcu_dereference_check() without protection!
>> > >>
>> > >> other info that might help us debug this:
>> > >>
>> > >> rcu_scheduler_active = 1, debug_locks = 1
>> > >> 3 locks held by swapper/1:
>> > >> #0: (cpu_add_remove_lock){+.+.+.}, at: [<ffffffff81042914>]
>> > >> cpu_maps_update_begin+0x12/0x14
>> > >> #1: (cpu_hotplug.lock){+.+.+.}, at: [<ffffffff8104294f>]
>> > >> cpu_hotplug_begin+0x27/0x4e
>> > >> #2: (&rq->lock){-.-...}, at: [<ffffffff812f8502>] init_idle+0x2b/0x114
>> > >
>> > > Hello, Miles!
>> > >
>> > > I believe that this one is fixed by commit dc61b1d6 in -tip.
>> >
>> > Hi Paul,
>> >
>> > Looks like that commit made it into 2.6.35:
>> >
>> > git tag -l --contains dc61b1d65e353d638b2445f71fb8e5b5630f2415 v2.6.35*
>> > v2.6.35
>> > v2.6.35-rc4
>> > v2.6.35-rc5
>> > v2.6.35-rc6
>> >
>> > However I still get:
>> >
>> > [ 0.051203] CPU0: AMD QEMU Virtual CPU version 0.12.4 stepping 03
>> > [ 0.052999] lockdep: fixing up alternatives.
>> > [ 0.054105]
>> > [ 0.054106] ===================================================
>> > [ 0.054999] [ INFO: suspicious rcu_dereference_check() usage. ]
>> > [ 0.054999] ---------------------------------------------------
>> > [ 0.054999] kernel/sched.c:616 invoked rcu_dereference_check()
>> > without protection
>> > !
>> > [ 0.054999]
>> > [ 0.054999] other info that might help us debug this:
>> > [ 0.054999]
>> > [ 0.054999]
>> > [ 0.054999] rcu_scheduler_active = 1, debug_locks = 1
>> > [ 0.054999] 3 locks held by swapper/1:
>> > [ 0.054999] #0: (cpu_add_remove_lock){+.+.+.}, at:
>> > [<ffffffff814be933>] cpu_up+
>> > 0x42/0x6a
>> > [ 0.054999] #1: (cpu_hotplug.lock){+.+.+.}, at:
>> > [<ffffffff810400d8>] cpu_hotplu
>> > g_begin+0x2a/0x51
>> > [ 0.054999] #2: (&rq->lock){-.-...}, at: [<ffffffff814be2f7>]
>> > init_idle+0x2f/0x
>> > 113
>> > [ 0.054999]
>> > [ 0.054999] stack backtrace:
>> > [ 0.054999] Pid: 1, comm: swapper Not tainted 2.6.35 #1
>> > [ 0.054999] Call Trace:
>> > [ 0.054999] [<ffffffff81068054>] lockdep_rcu_dereference+0x9b/0xa3
>> > [ 0.054999] [<ffffffff810325c3>] task_group+0x7b/0x8a
>> > [ 0.054999] [<ffffffff810325e5>] set_task_rq+0x13/0x40
>> > [ 0.054999] [<ffffffff814be39a>] init_idle+0xd2/0x113
>> > [ 0.054999] [<ffffffff814be78a>] fork_idle+0xb8/0xc7
>> > [ 0.054999] [<ffffffff81068717>] ? mark_held_locks+0x4d/0x6b
>> > [ 0.054999] [<ffffffff814bcebd>] do_fork_idle+0x17/0x2b
>> > [ 0.054999] [<ffffffff814bc89b>] native_cpu_up+0x1c1/0x724
>> > [ 0.054999] [<ffffffff814bcea6>] ? do_fork_idle+0x0/0x2b
>> > [ 0.054999] [<ffffffff814be876>] _cpu_up+0xac/0x127
>> > [ 0.054999] [<ffffffff814be946>] cpu_up+0x55/0x6a
>> > [ 0.054999] [<ffffffff81ab562a>] kernel_init+0xe1/0x1ff
>> > [ 0.054999] [<ffffffff81003854>] kernel_thread_helper+0x4/0x10
>> > [ 0.054999] [<ffffffff814c353c>] ? restore_args+0x0/0x30
>> > [ 0.054999] [<ffffffff81ab5549>] ? kernel_init+0x0/0x1ff
>> > [ 0.054999] [<ffffffff81003850>] ? kernel_thread_helper+0x0/0x10
>> > [ 0.056074] Booting Node 0, Processors #1lockdep: fixing up alternatives.
>> > [ 0.130045] #2lockdep: fixing up alternatives.
>> > [ 0.203089] #3 Ok.
>> > [ 0.275286] Brought up 4 CPUs
>> > [ 0.276005] Total of 4 processors activated (16017.17 BogoMIPS).
>>
>> This does look like a new one, thank you for reporting it!
>>
>> Here is my analysis, which should at least provide some humor value to
>> those who understand the code better than I do. ;-)
>>
>> So the corresponding rcu_dereference_check() is in
>> task_subsys_state_check(), and is fetching the cpu_cgroup_subsys_id
>> element of the newly created task's task->cgroups->subsys[] array.
>> The "git grep" command finds only three uses of cpu_cgroup_subsys_id,
>> but no definition.
>>
>> Now, fork_idle() invokes copy_process(), which invokes cgroup_fork(),
>> which sets the child process's ->cgroups pointer to that of the parent,
>> also invoking get_css_set(), which increments the corresponding reference
>> count, doing both operations under task_lock() protection (->alloc_lock).
>> Because fork_idle() does not specify any of CLONE_NEWNS, CLONE_NEWUTS,
>> CLONE_NEWIPC, CLONE_NEWPID, or CLONE_NEWNET, copy_namespaces() should
>> not create a new namespace, and so there should be no ns_cgroup_clone().
>> We should thus retain the parent's ->cgroups pointer. And copy_process()
>> installs the new task in the various lists, so that the task is externally
>> accessible upon return.
>>
>> After a non-error return from copy_process(), fork_init() invokes
>> init_idle_pid(), which does not appear to affect the task's cgroup
>> state. Next fork_init() invokes init_idle(), which in turn invokes
>> __set_task_cpu(), which invokes set_task_rq(), which calls task_group()
>> several times, which calls task_subsys_state_check(), which calls the
>> rcu_dereference_check() that complained above.
>>
>> However, the result returns by rcu_dereference_check() is stored into
>> the task structure:
>>
>> p->se.cfs_rq = task_group(p)->cfs_rq[cpu];
>> p->se.parent = task_group(p)->se[cpu];
>>
>> This means that the corresponding structure must have been tied down with
>> a reference count or some such. If such a reference has been taken, then
>> this complaint is a false positive, and could be suppressed by putting
>> rcu_read_lock() and rcu_read_unlock() around the call to init_idle()
>> from fork_idle(). However, although, reference to the enclosing ->cgroups
>> struct css_set is held, it is not clear to me that this reference applies
>> to the structures pointed to by the ->subsys[] array, especially given
>> that the cgroup_subsys_state structures referenced by this array have
>> their own reference count, which does not appear to me to be acquired
>> by this code path.
>>
>> Or are the cgroup_subsys_state structures referenced by idle tasks
>> never freed or some such?
>>
>> Thanx, Paul
>
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