Re: deadlock when using iothread during backup_clean()

[Fiona Ebner <f.ebner@proxmox.com>]

Am 19.10.23 um 14:14 schrieb Kevin Wolf:
> Am 18.10.2023 um 11:42 hat Fiona Ebner geschrieben:
>> Am 17.10.23 um 16:20 schrieb Kevin Wolf:
>>> Am 17.10.2023 um 15:37 hat Fiona Ebner geschrieben:
>>>> Am 17.10.23 um 14:12 schrieb Kevin Wolf:
>>>>> Am 17.10.2023 um 12:18 hat Fiona Ebner geschrieben:
>>>>>> I ran into similar issues now with mirror, (both deadlocks and stuck
>>>>>> guest IO at other times), and interestingly, also during job start.
>>>>>>
>>>>>> Also had a backtrace similar to [0] once, so I took a closer look.
>>>>>> Probably was obvious to others already, but for the record:
>>>>>>
>>>>>> 1. the graph is locked by the main thread
>>>>>> 2. the iothread holds the AioContext lock
>>>>>> 3. the main thread waits on the AioContext lock
>>>>>> 4. the iothread waits for coroutine spawned by blk_is_available()
>>>>>
>>>>> Where does this blk_is_available() in the iothread come from? Having it
>>>>> wait without dropping the AioContext lock sounds like something that
>>>>> we'd want to avoid. Ideally, devices using iothreads shouldn't use
>>>>> synchronous requests at all, but I think scsi-disk might have some of
>>>>> them.
>>>>>
>>>>
>>>> It's part of the request handling in virtio-scsi:
>>>>
>>>>> #0  0x00007ff7f5f55136 in __ppoll (fds=0x7ff7e40030c0, nfds=8, timeout=<optimized out>, sigmask=0x0) at ../sysdeps/unix/sysv/linux/ppoll.c:42
>>>>> #1  0x00005587132615ab in qemu_poll_ns (fds=0x7ff7e40030c0, nfds=8, timeout=-1) at ../util/qemu-timer.c:339
>>>>> #2  0x000055871323e8b1 in fdmon_poll_wait (ctx=0x55871598d5e0, ready_list=0x7ff7f288ebe0, timeout=-1) at ../util/fdmon-poll.c:79
>>>>> #3  0x000055871323e1ed in aio_poll (ctx=0x55871598d5e0, blocking=true) at ../util/aio-posix.c:670
>>>>> #4  0x0000558713089efa in bdrv_poll_co (s=0x7ff7f288ec90) at /home/febner/repos/qemu/block/block-gen.h:43
>>>>> #5  0x000055871308c362 in blk_is_available (blk=0x55871599e2f0) at block/block-gen.c:1426
>>>>> #6  0x0000558712f6843b in virtio_scsi_ctx_check (s=0x558716c049c0, d=0x55871581cd30) at ../hw/scsi/virtio-scsi.c:290
>>>
>>> Oh... So essentially for an assertion.
>>>
>>> I wonder if the blk_is_available() check introduced in 2a2d69f490c is
>>> even necessary any more, because BlockBackend has its own AioContext
>>> now. And if blk_bs(blk) != NULL isn't what we actually want to check if
>>> the check is necessary, because calling bdrv_is_inserted() doesn't seem
>>> to have been intended. blk_bs() wouldn't have to poll.
>>>
>>
>> Could virtio_scsi_hotunplug() be an issue with removing or modifying
>> the check? There's a call there which sets the blk's AioContext to
>> qemu_get_aio_context(). Or are we sure that the assert in
>> virtio_scsi_ctx_check() can't be reached after that?
> 
> I think that would be the kind of bug that the assertion tries to
> catch, because then we would be sending requests to blk from a thread
> that doesn't match its AioContext (which will be allowed soon, but not
> quite yet).
> 
> Before resetting the AioContext, virtio_scsi_hotunplug() calls
> qdev_simple_device_unplug_cb(), which unrealizes the SCSI device. This
> calls scsi_qdev_unrealize() -> scsi_device_purge_requests(), which in
> turn drains blk and cancels all pending requests. So there should be
> nothing left that could call into virtio_scsi_ctx_check() any more.
> 
> The other argument is that after unrealize, virtio_scsi_device_get()
> would return NULL anyway, so even if a request were still pending, it
> would just fail instead of accessing the unplugged device.
> 

Okay, sounds like a way to get around that deadlock issue then :)

(...)

>>>
>>> What does the stuck I/O look like? Is it stuck in the backend, i.e. the
>>> device started requests that never complete? Or stuck from the guest
>>> perspective, i.e. the device never checks for new requests?
>>>
>>
>> AFAICT, from the guest perspective.
>>
>>> I don't really have an idea immediately, we'd have to find out where the
>>> stuck I/O stops being processed.
>>>
>>
>> I've described it in an earlier mail in this thread:
>> https://lists.nongnu.org/archive/html/qemu-devel/2023-10/msg01900.html
>>
>> Quoting from there:
>>
>>> After the IO was stuck in the guest, I used bdrv_next_all_states() to
>>> iterate over the states and there's only the bdrv_raw and the
>>> bdrv_host_device. For both, tracked_requests was empty.
> 
> And bs->in_flight and blk->in_flight are 0, too?
> 

Yes. And queued_requests in the BlockBackend is also empty.

> Is anything quiesced?

No. quiesce_counter is 0 for both BlockDriverState instances as well as
for the BlockBackend. quiesced_parent is false for both parents (i.e.
child_root for the bdrv_raw and child_of_bds for the bdrv_file (this
time I used VirtIO SCSI, in the quote it was VirtIO block)).

>>> What is also very interesting is that the IO isn't always dead
>>> immediately. It can be that the fio command still runs with lower speed
>>> for a while (sometimes even up to about a minute, but most often about
>>> 10-15 seconds or so). During that time, I still can see calls to
>>> virtio_scsi_handle_cmd() and blk_aio_write_entry(). Then they suddenly stop.
>>
>> Noting again that (at least for backup) it happens with both virtio-blk
>> and virtio-scsi and with both aio=io_uring and aio=threads. I also tried
>> different host kernels 5.15, 6.2 and 6.5 and guest kernels 5.10 and 6.1.
> 
> You say "at least for backup". Did you see the bug for other job types,
> too?
> 

Yes, sorry. I meant to say that I only tested it for the backup
canceling with all those configurations. I did run into the stuck guest
IO issue in three different scenarios: canceling backup, canceling
mirror and starting mirror:

>>>>> Am 17.10.2023 um 12:18 hat Fiona Ebner geschrieben:
>>>>>> I ran into similar issues now with mirror, (both deadlocks and stuck
>>>>>> guest IO at other times), and interestingly, also during job start.

But for mirror, I only used VirtIO SCSI and aio=io_uring and didn't test
other configurations.

Best Regards,
Fiona