Re: [qemu-s390x] [Qemu-devel] [BUG?] aio_get_linux_aio: Assertion `ctx->linux_aio' failed

[Christian Borntraeger]

On 07/18/2018 08:52 PM, Nishanth Aravamudan wrote:
> On 18.07.2018 [11:10:27 -0400], Farhan Ali wrote:
>>
>>
>> On 07/18/2018 09:42 AM, Farhan Ali wrote:
>>>
>>>
>>> On 07/17/2018 04:52 PM, Nishanth Aravamudan wrote:
>>>> iiuc, this possibly implies AIO was not actually used previously on this
>>>> guest (it might have silently been falling back to threaded IO?). I
>>>> don't have access to s390x, but would it be possible to run qemu under
>>>> gdb and see if aio_setup_linux_aio is being called at all (I think it
>>>> might not be, but I'm not sure why), and if so, if it's for the context
>>>> in question?
>>>>
>>>> If it's not being called first, could you see what callpath is calling
>>>> aio_get_linux_aio when this assertion trips?
>>>>
>>>> Thanks!
>>>> -Nish
>>>
>>>
>>> Hi Nishant,
>>>
>>>  From the coredump of the guest this is the call trace that calls
>>> aio_get_linux_aio:
>>>
>>>
>>> Stack trace of thread 145158:
>>> #0  0x000003ff94dbe274 raise (libc.so.6)
>>> #1  0x000003ff94da39a8 abort (libc.so.6)
>>> #2  0x000003ff94db62ce __assert_fail_base (libc.so.6)
>>> #3  0x000003ff94db634c __assert_fail (libc.so.6)
>>> #4  0x000002aa20db067a aio_get_linux_aio (qemu-system-s390x)
>>> #5  0x000002aa20d229a8 raw_aio_plug (qemu-system-s390x)
>>> #6  0x000002aa20d309ee bdrv_io_plug (qemu-system-s390x)
>>> #7  0x000002aa20b5a8ea virtio_blk_handle_vq (qemu-system-s390x)
>>> #8  0x000002aa20db2f6e aio_dispatch_handlers (qemu-system-s390x)
>>> #9  0x000002aa20db3c34 aio_poll (qemu-system-s390x)
>>> #10 0x000002aa20be32a2 iothread_run (qemu-system-s390x)
>>> #11 0x000003ff94f879a8 start_thread (libpthread.so.0)
>>> #12 0x000003ff94e797ee thread_start (libc.so.6)
>>>
>>>
>>> Thanks for taking a look and responding.
>>>
>>> Thanks
>>> Farhan
>>>
>>>
>>>
>>
>> Trying to debug a little further, the block device in this case is a "host
>> device". And looking at your commit carefully you use the
>> bdrv_attach_aio_context callback to setup a Linux AioContext.
>>
>> For some reason the "host device" struct (BlockDriver bdrv_host_device in
>> block/file-posix.c) does not have a bdrv_attach_aio_context defined.
>> So a simple change of adding the callback to the struct solves the issue and
>> the guest starts fine.
>>
>>
>> diff --git a/block/file-posix.c b/block/file-posix.c
>> index 28824aa..b8d59fb 100644
>> --- a/block/file-posix.c
>> +++ b/block/file-posix.c
>> @@ -3135,6 +3135,7 @@ static BlockDriver bdrv_host_device = {
>>      .bdrv_refresh_limits = raw_refresh_limits,
>>      .bdrv_io_plug = raw_aio_plug,
>>      .bdrv_io_unplug = raw_aio_unplug,
>> +    .bdrv_attach_aio_context = raw_aio_attach_aio_context,
>>
>>      .bdrv_co_truncate       = raw_co_truncate,
>>      .bdrv_getlength    = raw_getlength,
>>
>>
>>
>> I am not too familiar with block device code in QEMU, so not sure if
>> this is the right fix or if there are some underlying problems.
> 
> Oh this is quite embarassing! I only added the bdrv_attach_aio_context
> callback for the file-backed device. Your fix is definitely corect for
> host device. Let me make sure there weren't any others missed and I will
> send out a properly formatted patch. Thank you for the quick testing and
> turnaround!

Farhan, can you respin your patch with proper sign-off and patch description?
Adding qemu-block.