On Fri, Sep 02, 2016 at 09:49:25AM -0600, Jens Axboe wrote:
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!)
>>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
>>>>[ 7.337256] ===============================
>>>>[ 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
>Lockdep does not believe that any locks are held, correct?
Which is correct, the queue lock has been dropped at this point.
Note that lockdep believes that the rcu_read_lock() has also been
dropped, otherwise it would have listed it.
>>>>[ 7.410074] Call Trace:
>>>>[ 7.411328] [<ffffffff8178ed3b>] dump_stack+0x82/0xb8
>>>>[ 7.413982] [<ffffffff81123472>]
>>>>[ 7.415828] [<ffffffff817873f4>] bio_blkcg+0x89/0x93
>>>>[ 7.417336] [<ffffffff817891f0>]
>>>>[ 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>]
>>>>[ 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
>> serial_nr = bio_blkcg(bio)->css.serial_nr;
>>So, I have no idea. It looks like rcu_dereference_check() is being
>>called with rcu read locked but still triggering suspicious RCU usage
>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...
At this point, I have to suggest "git bisect"...