Re: [RFC PATCH] kernfs: create raw version kernfs_path_len and kernfs_path

["Shi, Yang" <yang.shi@linaro.org>]

On 2/26/2016 11:37 AM, Shi, Yang wrote:
> On 2/26/2016 10:56 AM, Steven Rostedt wrote:
>> On Fri, 26 Feb 2016 10:15:05 -0800
>> Yang Shi <yang.shi@linaro.org> wrote:
>>
>>> commit 5634cc2aa9aebc77bc862992e7805469dcf83dac ("writeback: update
>>> writeback
>>> tracepoints to report cgroup") made writeback tracepoints report cgroup
>>> writeback, but it may trigger the below bug on -rt kernel since
>>> kernfs_path
>>> and kernfs_path_len are called by tracepoints, which acquire sleeping
>>> lock.
>>>
>>> BUG: sleeping function called from invalid context at
>>> kernel/locking/rtmutex.c:930
>>> in_atomic(): 1, irqs_disabled(): 0, pid: 625, name: kworker/u16:3
>>> INFO: lockdep is turned off.
>>> Preemption disabled at:[<ffffffc000374a5c>] wb_writeback+0xec/0x830
>>>
>>> CPU: 7 PID: 625 Comm: kworker/u16:3 Not tainted 4.4.1-rt5 #20
>>> Hardware name: Freescale Layerscape 2085a RDB Board (DT)
>>> Workqueue: writeback wb_workfn (flush-7:0)
>>> Call trace:
>>> [<ffffffc00008d708>] dump_backtrace+0x0/0x200
>>> [<ffffffc00008d92c>] show_stack+0x24/0x30
>>> [<ffffffc0007b0f40>] dump_stack+0x88/0xa8
>>> [<ffffffc000127d74>] ___might_sleep+0x2ec/0x300
>>> [<ffffffc000d5d550>] rt_spin_lock+0x38/0xb8
>>> [<ffffffc0003e0548>] kernfs_path_len+0x30/0x90
>>> [<ffffffc00036b360>]
>>> trace_event_raw_event_writeback_work_class+0xe8/0x2e8
>>> [<ffffffc000374f90>] wb_writeback+0x620/0x830
>>> [<ffffffc000376224>] wb_workfn+0x61c/0x950
>>> [<ffffffc000110adc>] process_one_work+0x3ac/0xb30
>>> [<ffffffc0001112fc>] worker_thread+0x9c/0x7a8
>>> [<ffffffc00011a9e8>] kthread+0x190/0x1b0
>>> [<ffffffc000086ca0>] ret_from_fork+0x10/0x30
>>>
>>> Since kernfs_* functions are heavily used by cgroup, so it sounds not
>>> reasonable to convert kernfs_rename_lock to raw lock.
>>>
>>> Create raw version kernfs_path, kernfs_path_len and cgroup_path,
>>> which don't
>>> acquire lock and are used by tracepoints only.
>>>
>>
>> And what prevents name from being freed while the tracepoint is reading
>> it?
>>
>> Perhaps we need this change as well:
>
> Thanks for pointing this out. Yes, it looks we need this since the
> tracepoints are protected by rcu_read_lock_sched.
>
> Will add this in V2.

BTW, it sounds this is not the only point where kernfs_node could be 
updated, __kernfs_remove should need synchronize_sched too.

Thanks,
Yang

>
> Yang
>
>>
>> -- Steve
>>
>> diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
>> index 996b7742c90b..d2ef153145c0 100644
>> --- a/fs/kernfs/dir.c
>> +++ b/fs/kernfs/dir.c
>> @@ -1397,6 +1397,12 @@ int kernfs_rename_ns(struct kernfs_node *kn,
>> struct kernfs_node *new_parent,
>>       kn->hash = kernfs_name_hash(kn->name, kn->ns);
>>       kernfs_link_sibling(kn);
>>
>> +    /*
>> +     * Tracepoints may be reading the old name. They are protected
>> +     * by rcu_read_lock_sched().
>> +     */
>> +    synchronize_sched();
>> +
>>       kernfs_put(old_parent);
>>       kfree_const(old_name);
>>
>>
>