Re: deadlock when using iothread during backup_clean()

[Fiona Ebner]

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?

>>> #7  0x0000558712f697e4 in virtio_scsi_handle_cmd_req_prepare 
>>> (s=0x558716c049c0, req=0x7ff7e400b650) at ../hw/scsi/virtio-scsi.c:788
>>> #8  0x0000558712f699b0 in virtio_scsi_handle_cmd_vq (s=0x558716c049c0, 
>>> vq=0x558716c0d2a8) at ../hw/scsi/virtio-scsi.c:831
>>> #9  0x0000558712f69bcb in virtio_scsi_handle_cmd (vdev=0x558716c049c0, 
>>> vq=0x558716c0d2a8) at ../hw/scsi/virtio-scsi.c:867
>>> #10 0x0000558712f96812 in virtio_queue_notify_vq (vq=0x558716c0d2a8) at 
>>> ../hw/virtio/virtio.c:2263
>>> #11 0x0000558712f99b75 in virtio_queue_host_notifier_read 
>>> (n=0x558716c0d31c) at ../hw/virtio/virtio.c:3575
>>> #12 0x000055871323d8b5 in aio_dispatch_handler (ctx=0x55871598d5e0, 
>>> node=0x558716771000) at ../util/aio-posix.c:372
>>> #13 0x000055871323d988 in aio_dispatch_ready_handlers (ctx=0x55871598d5e0, 
>>> ready_list=0x7ff7f288eeb0) at ../util/aio-posix.c:401
>>
>>
>>>> As for why it doesn't progress, blk_co_is_available_entry() uses
>>>> bdrv_graph_co_rdlock() and can't get it, because the main thread has the
>>>> write lock. Should be fixed once the AioContext locks are gone, but not
>>>> sure what should be done to avoid it until then.
>>>
>>> Then the nested event loop in blk_is_available() would probably be
>>> enough to make progress, yes.
>>>
>>> Maybe we could actually drop the lock (and immediately reacquire it) in
>>> AIO_WAIT_WHILE() even if we're in the home thread? That should give the
>>> main thread a chance to make progress.
>>
>> Seems to work :) I haven't run into the issue with the following change
>> anymore, but I have to say, running into that specific deadlock only
>> happened every 10-15 tries or so before. Did 30 tests now. But
>> unfortunately, the stuck IO issue is still there.
>>
>>> diff --git a/include/block/aio-wait.h b/include/block/aio-wait.h
>>> index 5449b6d742..da159501ca 100644
>>> --- a/include/block/aio-wait.h
>>> +++ b/include/block/aio-wait.h
>>> @@ -88,7 +88,13 @@ extern AioWait global_aio_wait;
>>>      smp_mb__after_rmw();                                           \
>>>      if (ctx_ && in_aio_context_home_thread(ctx_)) {                \
>>>          while ((cond)) {                                           \
>>> +            if (unlock && ctx_) {                                  \
>>> +                aio_context_release(ctx_);                         \
>>> +            }                                                      \
>>>              aio_poll(ctx_, true);                                  \
>>> +            if (unlock && ctx_) {                                  \
>>> +                aio_context_acquire(ctx_);                         \
>>> +            }                                                      \
>>>              waited_ = true;                                        \
>>>          }                                                          \
>>>      } else {                                                       \
> 
> For reacquiring the lock, I really meant "immediately". Calling
> aio_poll() without the lock is wrong.

Unfortunately, then it's not enough, because the call to aio_poll() is
blocking and because the lock is held during that call, the very same
deadlock can still happen.

> 
> 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.
> 
> 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.

>>> But I think we're actually not very far from removing the AioContext
>>> lock, so if we can't find an easy fix in the meantime, waiting for that
>>> could be a realistic option.
>>>
>>
>> Glad to hear :) Do you think it will be in time for QEMU 8.2? Otherwise,
>> I'll go ahead and send what I have for fixing the deadlocks from this
>> mail thread in the following days. The stuck guest IO can happen even
>> without any of those changes (on current master, i.e.
>> ebca80bbdb5c1650e4b753a3d13b43634e7dfe05, at least when starting a
>> mirror job).
> 
> Having it in 8.2 is certainly the plan, but plans don't always work out.
> If you have fixes that aren't too ugly, we can still apply them.
> 

Okay, I'll give the ones for the deadlocks via
block_job_remove_all_bdrv() and bdrv_close() another look/spin and try
to send them in the next few days.

Best Regards,
Fiona