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

[Jens Axboe]

On 09/01/2016 04:21 AM, kernel test robot wrote:
> FYI, we noticed the following commit:
>
> https://git.kernel.org/pub/scm/linux/kernel/git/axboe/linux-block.git wb-buf-throttle
> commit 8bc4ad9498f81d6689da42457615db0989e720d3 ("writeback: throttle buffered writeback")
>
> in testcase: boot
>
> on test machine: qemu-system-x86_64 -enable-kvm -cpu Westmere -m 512M
>
> caused below changes:
>
>
> +--------------------------------------+------------+------------+
> |                                      | b2e4e4e34d | 8bc4ad9498 |
> +--------------------------------------+------------+------------+
> | boot_successes                       | 6          | 0          |
> | boot_failures                        | 0          | 8          |
> | INFO:suspicious_RCU_usage            | 0          | 8          |
> | message:INFO:suspicious_RCU_usage    | 0          | 0.0        |
> | pattern:INFO:suspicious_RCU_usage    | 0          | 0          |
> | calltrace:disk_events_workfn         | 0          | 8          |
> | message:calltrace:disk_events_workfn | 0          | 0.0        |
> | pattern:calltrace:disk_events_workfn | 0          | 0          |
> +--------------------------------------+------------+------------+
>
>
>
> [    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!
> [    7.355928]
> [    7.355928] other info that might help us debug this:
> [    7.355928]
> [    7.360548]
> [    7.360548] rcu_scheduler_active = 1, debug_locks = 0
> [    7.366185] 3 locks held by kworker/0:1/19:
> [    7.369225]  #0:  ("events_freezable_power_efficient"){.+.+..}, at: [<ffffffff810fc9d9>] process_one_work+0x151/0x4ee
> [    7.377225]  #1:  ((&(&ev->dwork)->work)){+.+...}, at: [<ffffffff810fc9d9>] process_one_work+0x151/0x4ee
> [    7.384175]  #2:  (&(&q->__queue_lock)->rlock){-.....}, at: [<ffffffff81789b1d>] cfq_set_request+0x5e/0x2a3
> [    7.389264]
> [    7.389264] stack backtrace:
> [    7.391300] CPU: 0 PID: 19 Comm: kworker/0:1 Not tainted 4.8.0-rc4-00008-g8bc4ad9 #1
> [    7.395279] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Debian-1.8.2-1 04/01/2014
> [    7.398012] Workqueue: events_freezable_power_ disk_events_workfn
> [    7.399933]  0000000000000000 ffff88003db1f8e8 ffffffff8178ed3b ffff88003db184c0
> [    7.404348]  0000000000000001 ffff88003db1f918 ffffffff81123472 ffff88003db184c0
> [    7.407158]  ffffffff84091180 0000000000000003 ffff880039d4c400 ffff88003db1f938
> [    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
> [    7.419027]  [<ffffffff81125fa7>] ? lock_acquire+0x125/0x1be
> [    7.421418]  [<ffffffff81125fa7>] ? lock_acquire+0x125/0x1be
> [    7.424049]  [<ffffffff811238d3>] ? lock_acquired+0xfc/0x2f2
> [    7.426893]  [<ffffffff81789b1d>] ? cfq_set_request+0x5e/0x2a3
> [    7.428722]  [<ffffffff81789b90>] cfq_set_request+0xd1/0x2a3
> [    7.430315]  [<ffffffff82efb493>] ? _raw_spin_unlock_irq+0x2c/0x3b
> [    7.432134]  [<ffffffff81124a52>] ? trace_hardirqs_on_caller+0x17c/0x198
> [    7.434083]  [<ffffffff81124a7b>] ? trace_hardirqs_on+0xd/0xf
> [    7.435703]  [<ffffffff82efb498>] ? _raw_spin_unlock_irq+0x31/0x3b
> [    7.437472]  [<ffffffff8176dd78>] ? ioc_create_icq+0x15e/0x175
> [    7.439690]  [<ffffffff81763fe3>] elv_set_request+0x1f/0x24
> [    7.442157]  [<ffffffff8176856d>] get_request+0x38f/0x77f
> [    7.444694]  [<ffffffff8111b59c>] ? add_wait_queue_exclusive+0x44/0x44
> [    7.447449]  [<ffffffff817689c2>] blk_get_request+0x65/0xa8
> [    7.449868]  [<ffffffff81c692df>] ide_cd_queue_pc+0x76/0x19d
> [    7.451986]  [<ffffffff8112539e>] ? __lock_acquire+0x679/0xed1
> [    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
> [    7.469137]  [<ffffffff810fd05c>] ? rescuer_thread+0x2b3/0x2b3
> [    7.471338]  [<ffffffff81101dd4>] kthread+0xc5/0xcd
> [    7.472978]  [<ffffffff82efbf2f>] ret_from_fork+0x1f/0x40
> [    7.474501]  [<ffffffff81101d0f>] ? kthread_create_on_node+0x1b0/0x1b0
> [    7.477061] Loading iSCSI transport class v2.0-870.
> [    7.479285] rdac: device handler registered

Pretty sure this has nothing to do with the writeback code, it's just 
triggering and now blaming one of those commits since they touch the 
same function. The RCU usage in there dates back to:

commit 598971bfbdfdc8701337dc1636c7919c44699914
Author: Tejun Heo <tj@xxxxxxxxxx>
Date:   Mon Mar 19 15:10:58 2012 -0700

    cfq: don't use icq_get_changed()

Tejun, see what's up there, if anything?

--
Jens Axboe