Re: [Qemu-devel] migration: qemu-coroutine-lock.c:141: qemu_co_mutex_unlock: Assertion `mutex->locked == 1' failed

[Alexey Kardashevskiy <aik@ozlabs.ru>]

On 09/18/2014 07:56 PM, Paolo Bonzini wrote:
> Il 18/09/2014 05:26, Alexey Kardashevskiy ha scritto:
>> On 09/18/2014 01:07 AM, Stefan Hajnoczi wrote:
>>> On Wed, Sep 17, 2014 at 2:44 PM, Alexey Kardashevskiy <aik@ozlabs.ru> wrote:
>>>> On 09/17/2014 07:25 PM, Paolo Bonzini wrote:
>>>> btw any better idea of a hack to try? Testers are pushing me - they want to
>>>> upgrade the broken setup and I am blocking them :) Thanks!
>>>
>>> Paolo's qemu_co_mutex_lock(&s->lock) idea in qcow2_invalidate_cache()
>>> is good.  Have you tried that patch?
>>
>>
>> Yes, did not help.
>>
>>>
>>> I haven't checked the qcow2 code whether that works properly across
>>> bdrv_close() (is the lock freed?) but in principle that's how you
>>> protect against concurrent I/O.
>>
>> I thought we have to avoid qemu_coroutine_yield() in this particular case.
>> I fail to see how the locks may help if we still do yeild. But the whole
>> thing is already way behind of my understanding :) For example - how many
>> BlockDriverState things are layered here? NBD -> QCOW2 -> RAW?
> 
> No, this is an NBD server.  So we have three users of the same QCOW2
> image: migration, NBD server and virtio disk (not active while the bug
> happens, and thus not depicted):
> 
> 
>               NBD server   ->    QCOW2     <-     migration
>                                    |
>                                    v
>                                  File
> 
> The problem is that the NBD server accesses the QCOW2 image while
> migration does qcow2_invalidate_cache.


Ufff. Cool. Anyway, the qemu_co_mutex_lock(&s->lock) hack does not work as
after qcow2_close() the lock is cleared and qemu_co_mutex_unlock(&s->lock)
fails. Moving the lock to BlockDriverState caused weird side effects,
debugging...



-- 
Alexey