Re: [writeback] 8bc4ad9498: INFO: suspicious RCU usage. ]

[Jens Axboe <axboe@fb.com>]

On 09/02/2016 09:36 AM, Paul E. McKenney wrote:
> On Fri, Sep 02, 2016 at 10:56:22AM -0400, Tejun Heo wrote:
>> (cc'ing Paul, hi!)
>>
>> Hello,
>>
>> On Thu, Sep 01, 2016 at 02:13:34PM -0600, Jens Axboe wrote:
>>> On 09/01/2016 04:21 AM, kernel test robot wrote:
>>>> [    7.323356] cdrom: Uniform CD-ROM driver Revision: 3.20
>>>> [    7.334239]
>>>> [    7.337256] ===============================
>>>> [    7.340532] [ INFO: suspicious RCU usage. ]
>>>> [    7.342419] 4.8.0-rc4-00008-g8bc4ad9 #1 Not tainted
>>>> [    7.347065] -------------------------------
>>>> [    7.350132] include/linux/cgroup.h:435 suspicious rcu_dereference_check() usage!
>
> Lockdep does not believe that any locks are held, correct?

Which is correct, the queue lock has been dropped at this point.

>>>> [    7.410074] Call Trace:
>>>> [    7.411328]  [<ffffffff8178ed3b>] dump_stack+0x82/0xb8
>>>> [    7.413982]  [<ffffffff81123472>] lockdep_rcu_suspicious+0xf7/0x100
>>>> [    7.415828]  [<ffffffff817873f4>] bio_blkcg+0x89/0x93
>>>> [    7.417336]  [<ffffffff817891f0>] check_blkcg_changed+0x58/0x1b8
>>>> [    7.428722]  [<ffffffff81789b90>] cfq_set_request+0xd1/0x2a3
>>>> [    7.439690]  [<ffffffff81763fe3>] elv_set_request+0x1f/0x24
>>>> [    7.442157]  [<ffffffff8176856d>] get_request+0x38f/0x77f
>>>> [    7.447449]  [<ffffffff817689c2>] blk_get_request+0x65/0xa8
>>>> [    7.449868]  [<ffffffff81c692df>] ide_cd_queue_pc+0x76/0x19d
>>>> [    7.453757]  [<ffffffff81c695ee>] cdrom_check_status+0x51/0x53
>>>> [    7.455372]  [<ffffffff81c6a27e>] ide_cdrom_check_events_real+0x20/0x3f
>>>> [    7.457294]  [<ffffffff82579f94>] cdrom_update_events+0x18/0x21
>>>> [    7.458987]  [<ffffffff82579faf>] cdrom_check_events+0x12/0x1f
>>>> [    7.460713]  [<ffffffff81c68317>] idecd_check_events+0x1c/0x1e
>>>> [    7.462393]  [<ffffffff81778d12>] disk_check_events+0x47/0x103
>>>> [    7.464129]  [<ffffffff81778dea>] disk_events_workfn+0x1c/0x1e
>>>> [    7.465844]  [<ffffffff810fcafa>] process_one_work+0x272/0x4ee
>>>> [    7.467462]  [<ffffffff810fd247>] worker_thread+0x1eb/0x2c9
>>
>> The warning is from
>>
>> #define task_css_set_check(task, __c)					\
>> 	rcu_dereference_check((task)->cgroups,				\
>> 		lockdep_is_held(&cgroup_mutex) ||			\
>> 		lockdep_is_held(&css_set_lock) ||			\
>> 		((task)->flags & PF_EXITING) || (__c))
>>
>> which is used by bio_blkcg() which is called by the following code in
>> check_blkcg_changed().
>>
>> 	rcu_read_lock();
>> 	serial_nr = bio_blkcg(bio)->css.serial_nr;
>> 	rcu_read_unlock();
>>
>> So, I have no idea.  It looks like rcu_dereference_check() is being
>> called with rcu read locked but still triggering suspicious RCU usage
>> warning.
>
> Perhaps there is an rcu_read_unlock() somewhere on the code path?
>
>> The code hasn't changed for quite a while now, so it's also really
>> weird that it's triggering now.  Paul, does anything ring a bell?
>
> I have not see something like this recently.

The question is if it's really new, or just re-triggering because the
writeback branch changes that function? It's further down though, so
can't impact the RCU section. And the writeback changes don't have any
RCU code in them...


-- 
Jens Axboe