On Thu, 2010-09-16 at 09:12 -0700, Paul E. McKenney wrote:
> On Thu, Sep 16, 2010 at 05:50:31PM +0200, Peter Zijlstra wrote:
> > On Mon, 2010-08-09 at 09:12 -0700, Paul E. McKenney wrote:
> >
> > > > [ 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_hotplug_begin+0x2a/0x51
> > > > [ 0.054999] #2: (&rq->lock){-.-...}, at: [<ffffffff814be2f7>]
> > > > init_idle+0x2f/0x113
> > > > [ 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?
> >
> > I would hope so!, the idle tasks should be part of the root cgroup,
> > which is not removable.
> >
> > The problem is that while we do in-fact hold rq->lock, the newly spawned
> > idle thread's cpu is not yet set to the correct cpu so the lockdep check
> > in task_group():
> >
> > lockdep_is_held(&task_rq(p)->lock)
> >
> > will fail.
> >
> > But of a chicken and egg problem. Setting the cpu needs to have the cpu
> > set ;-)
>
> OK, makes sense to me.
>
> > Ingo, why do we have rq->lock there at all? The CPU isn't up and running
> > yet, nothing should be touching it.
>
> Acked-by: Paul E. McKenney <paul...@linux.vnet.ibm.com>
>
> > Signed-off-by: Peter Zijlstra <a.p.zijls...@chello.nl>
Thanks Peter for the fix.
Regards--
Subrata
> > ---
> > kernel/sched.c | 12 ++++++++++++
> > 1 files changed, 12 insertions(+), 0 deletions(-)
> >
> > diff --git a/kernel/sched.c b/kernel/sched.c
> > index bd8b487..6241049 100644
> > --- a/kernel/sched.c
> > +++ b/kernel/sched.c
> > @@ -5332,7 +5332,19 @@ void __cpuinit init_idle(struct task_struct *idle,
> > int cpu)
> > idle->se.exec_start = sched_clock();
> >
> > cpumask_copy(&idle->cpus_allowed, cpumask_of(cpu));
> > + /*
> > + * We're having a chicken and egg problem, even though we are
> > + * holding rq->lock, the cpu isn't yet set to this cpu so the
> > + * lockdep check in task_group() will fail.
> > + *
> > + * Similar case to sched_fork(). / Alternatively we could
> > + * use task_rq_lock() here and obtain the other rq->lock.
> > + *
> > + * Silence PROVE_RCU
> > + */
> > + rcu_read_lock();
> > __set_task_cpu(idle, cpu);
> > + rcu_read_unlock();
> >
> > rq->curr = rq->idle = idle;
> > #if defined(CONFIG_SMP) && defined(__ARCH_WANT_UNLOCKED_CTXSW)
> >
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