Re: [PATCH v4 RFC 0/3] virtio: add 'device_lost' to virtio_device

[Heinz Graalfs <graalfs@linux.vnet.ibm.com>]

On 17/12/13 04:42, Rusty Russell wrote:
> Heinz Graalfs <graalfs@linux.vnet.ibm.com> writes:
>> Hi, here is my v4 patch-set update to the v3 RFC submitted on Nov 27th.
>>
>> When an active virtio block device is hot-unplugged from a KVM guest,
>> affected guest user applications are not aware of any errors that occur
>> due to the lost device. This patch-set adds code to avoid further request
>> queueing when a lost block device is detected, resulting in appropriate
>> error info. Additionally a potential hang situation can be avoided by not
>> waiting for requests (e.g. in-flight requests) in blk_cleanup_queue() that
>> will never complete.
>>
>> On System z there exists no handshake mechanism between host and guest
>> when a device is hot-unplugged. The device is removed and no further I/O
>> is possible.
>
>
> Hi Heinz,
>
>          If you simply mark every virtqueue as broken when this
> unexpected unplug happens, does that not Just Work?
>
> I think I've asked this before...
> Rusty.

Hi Rusty,

setting the (one) virtqueue, vblk is currently using, as broken doesn't 
solve the problems.

In that case virtblk_request()s still succeed  - like this one...

([<0000000000112b28>] show_trace+0xf8/0x154)
  [<0000000000112bde>] show_stack+0x5a/0xdc
  [<000000000045eb56>] virtblk_request+0x25a/0x2b8
  [<00000000003e749c>] __blk_run_queue+0x50/0x64
  [<00000000003edb54>] blk_queue_bio+0x358/0x3f0
  [<00000000003eb446>] generic_make_request+0xea/0x130
  [<00000000003eb536>] submit_bio+0xaa/0x1a8
  [<00000000002c95e8>] _submit_bh+0x1c4/0x2f4
  [<00000000003a25e4>] journal_write_superblock+0xa0/0x1fc
  [<00000000003a3ed4>] journal_update_sb_log_tail+0x48/0x7c
  [<000000000039e742>] journal_commit_transaction+0x1586/0x1aa0
  [<00000000003a2a0e>] kjournald+0xfe/0x2a0
  [<00000000001786fc>] kthread+0xd8/0xe0
  [<0000000000698fee>] kernel_thread_starter+0x6/0xc
2 locks held by kjournald/1984:

... and end up in hang situations ...

PID: 13     TASK: 1e3f8000          CPU: 0   COMMAND: "kworker/u128:1"
  #0 [1e2033e0] __schedule at 695ff2
  #1 [1e203530] log_wait_commit at 3a28a6
  #2 [1e2035a0] ext3_sync_fs at 328dea
  #3 [1e2035d8] sync_filesystem at 2c785c
  #4 [1e203600] fsync_bdev at 2d4650
  #5 [1e203628] invalidate_partition at 3f80c8
  #6 [1e203650] del_gendisk at 3f8f5c
  #7 [1e2036c8] virtblk_remove at 45e60e
  #8 [1e203700] virtio_dev_remove at 42d72e
  #9 [1e203738] __device_release_driver at 44f0b0
#10 [1e203760] device_release_driver at 44f16c
#11 [1e203788] bus_remove_device at 44ea92
#12 [1e2037b8] device_del at 44bb40
#13 [1e2037f0] device_unregister at 44bbfa
#14 [1e203810] unregister_virtio_device at 42d9e6
#15 [1e203830] virtio_ccw_remove at 53b834
#16 [1e203850] ccw_device_remove at 4c5bf6
#17 [1e2038d8] __device_release_driver at 44f0b0
#18 [1e203900] device_release_driver at 44f16c
#19 [1e203928] bus_remove_device at 44ea92
#20 [1e203958] device_del at 44bb40
#21 [1e203990] ccw_device_unregister at 4c645c
#22 [1e2039b0] io_subchannel_remove at 4c6b1a
#23 [1e2039e8] css_remove at 4c054e
#24 [1e203a08] __device_release_driver at 44f0b0
#25 [1e203a30] device_release_driver at 44f16c
#26 [1e203a58] bus_remove_device at 44ea92
#27 [1e203a88] device_del at 44bb40
#28 [1e203ac0] device_unregister at 44bbfa
#29 [1e203ae0] css_sch_device_unregister at 4c06cc
#30 [1e203b08] io_subchannel_sch_event at 4c8c3a
#31 [1e203b80] css_evaluate_known_subchannel at 4c09bc
#32 [1e203be0] slow_eval_known_fn at 4c19a6
#33 [1e203c10] bus_for_each_dev at 44d50e
#34 [1e203c50] for_each_subchannel_staged at 4c1066
#35 [1e203c98] css_slow_path_func at 4c1124
#36 [1e203cc0] process_one_work at 16c7f6
#37 [1e203d60] worker_thread at 16dce4
#38 [1e203da8] kthread at 1786fc
#39 [1e203eb0] kernel_thread_starter at 698fee

Heinz

>
>>
>> When an online channel device disappears on System z the kernel's CIO layer
>> informs the driver (virtio_ccw) about the lost device.
>>
>> Here are some more error details:
>>
>> For a particular block device virtio's request function virtblk_request()
>> is called by the block layer to queue requests to be handled by the host.
>> In case of a lost device requests can still be queued, but an appropriate
>> subsequent host kick usually fails. This leads to situations where no error
>> feedback is shown.
>>
>> In order to prevent request queueing for lost devices appropriate settings
>> in the block layer should be made. Exploiting System z's CIO notify handler
>> callback, and passing on device loss information via the surprize_removal
>> flag to the remove callback of the backend driver, can solve this task.
>>
>> v3->v4 changes:
>>   - patch 1: solves some vcdev pointer handling issues in the virtio_ccw driver
>>     (e.g. locked vcdev pointer reset/query; serialize remove()/set_offline()
>>     callback processing).
>>   - patch 2: introduces 'device_lost' atomic in virtio_device and use in
>>     backend driver virtio_blk accordingly (original 3 patches merged).
>>   - patch 3: the notify() callback is now serialized with remove()/set_offline()
>>     callbacks. The notification is ignored if the vcdev pointer has been cleared
>>     already (by remove() or set_offline()).
>>
>> v2->v3 changes:
>>   - remove virtio_driver's notify callback (and appropriate code) introduced
>>     in my v1 RFC
>>   - introduce 'surprize_removal' in struct virtio_device
>>   - change virtio_blk's remove callback to perform special actions when the
>>     surprize_removal flag is set
>>     - avoid final I/O by preventing further request queueing
>>     - avoid hangs in blk_cleanup_queue() due to waits on 'in-flight' requests
>>   - set surprize_removal in virtio_ccw's notify callback when a device is lost
>>
>> v1->v2 changes:
>>   - add include of linux/notifier.h (I also added it to the 3rd patch)
>>   - get queue lock in order to be able to use safe queue_flag_set() functions
>>     in virtblk_notify() handler
>>
>>
>> Heinz Graalfs (3):
>>    virtio_ccw: fix vcdev pointer handling issues
>>    virtio: introduce 'device_lost' flag in virtio_device
>>    virtio_ccw: set 'device_lost' on CIO_GONE notification
>>
>>   drivers/block/virtio_blk.c    | 14 ++++++++++-
>>   drivers/s390/kvm/virtio_ccw.c | 58 ++++++++++++++++++++++++++++++++++++-------
>>   include/linux/virtio.h        |  2 ++
>>   3 files changed, 64 insertions(+), 10 deletions(-)
>>
>> --
>> 1.8.3.1
>