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

[Farhan Ali <alifm@linux.ibm.com>]

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.

Thanks
Farhan