Re: [PATCH v5 4/4] blockdev: honor bdrv_try_set_aio_context() context requirements

[Eric Blake <eblake@redhat.com>]

On 12/9/19 10:06 AM, Kevin Wolf wrote:
> Am 28.11.2019 um 11:41 hat Sergio Lopez geschrieben:
>> bdrv_try_set_aio_context() requires that the old context is held, and
>> the new context is not held. Fix all the occurrences where it's not
>> done this way.
>>
>> Suggested-by: Max Reitz <mreitz@redhat.com>
>> Signed-off-by: Sergio Lopez <slp@redhat.com>
>> ---

> Or in fact, I think you need to hold the AioContext of a bs to
> bdrv_unref() it, so maybe 'goto out' is right, but you need to unref
> target_bs while you still hold old_context.

I suspect https://bugzilla.redhat.com/show_bug.cgi?id=1779036 is also a 
symptom of this.  The v5 patch did not fix this simple test case:


$ qemu-img create -f qcow2 f1 100m
$ qemu-img create -f qcow2 f2 100m
$ ./qemu-kvm -nodefaults -nographic -qmp stdio -object iothread,id=io0 \
  -drive driver=qcow2,id=drive1,file=f1,if=none -device 
virtio-scsi-pci,id=scsi0,iothread=io0 -device 
scsi-hd,id=image1,drive=drive1 \
  -drive driver=qcow2,id=drive2,file=f2,if=none -device 
virtio-blk-pci,id=image2,drive=drive2,iothread=io0

{'execute':'qmp_capabilities'}

{'execute':'transaction','arguments':{'actions':[ 
{'type':'blockdev-snapshot-sync','data':{'device':'drive1', 
'snapshot-file':'sn1','mode':'absolute-paths','format':'qcow2'}}, 
{'type':'blockdev-snapshot-sync','data':{'device':'drive2', 
'snapshot-file':'/aa/sn1','mode':'absolute-paths','format':'qcow2'}}]}}

which is an aio context bug somewhere on the error path of 
blockdev-snapshot-sync (the first one has to be rolled back because the 
second part of the transaction fails early on a nonexistent directory)

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org