Re: [Qemu-devel] [PATCH v6 2/2] block: Support GlusterFS as a QEMU block backend

[Kevin Wolf]

Am 06.09.2012 12:18, schrieb Paolo Bonzini:
> Il 06/09/2012 12:07, Kevin Wolf ha scritto:
>>> The AIOCB is already invalid at the time the callback is entered, so we
>>> could release it before the call.  However, not all implementation of
>>> AIO are ready for that and I'm not really in the mood for large scale
>>> refactoring...
>>
>> But the way, what I'd really want to see in the end is to get rid of
>> qemu_aio_flush() and replace it by .bdrv_drain() callbacks in each
>> BlockDriver. The way we're doing it today is a layering violation.
> 
> That's quite difficult.  Completion of an I/O operation can trigger
> another I/O operation on another block device, and so on until we go
> back to the first device (think of a hypothetical RAID-5 device).

You always have a tree of BDSes, and children should only ever trigger
completion of I/O operations in their parents. Am I missing anything?

>> Doesn't change anything about this problem, though. So the options that
>> we have are:
>>
>> 1. Delay the callback using a BH. Doing this in each driver is ugly.
>>    But is there actually more than one possible callback in today's
>>    coroutine world? I only see bdrv_co_io_em_complete(), which could
>>    reenter the coroutine from a BH.
> 
> Easy and safe, but it feels a bit like a timebomb.  Also, I'm not
> entirely sure of _why_ the bottom half works. :)

Hm, safe and time bomb is contradictory in my book. :-)

The bottom half work because we're not reentering the qcow2_create
coroutine immediately, so the gluster AIO callback can complete all of
its cleanup work without being interrupted by code that might wait on
this particular request and create a deadlock this way.

>> 2. Delay the callback by just calling it later when the cleanup has
>>    been completed and .io_flush() can return 0. You say that it's hard
>>    to implement for some drivers, except if the AIOCB are leaked until
>>    the end of functions like qcow2_create().
> 
> ... which is what we do in posix-aio-compat.c; nobody screamed so far.

True. Would be easy to fix in posix-aio-compat, though, or can a
callback expect that the AIOCB is still valid?

> Not really hard, it just has to be assessed for each driver separately.
>  We can just do it in gluster and refactor it later.

Okay, so let's keep it as an option for now.

>> 3. Add a delay only later in functions like bdrv_drain_all() that assume
>>    that the request has completed. Are there more of this type? AIOCBs
>>    are leaked until a bdrv_drain_all() call. Does it work with draining
>>    specific BDSes instead of everything?
>>
>> Unless I forgot some important point, it almost looks like option 1 is
>> the easiest and safest.
> 
> I agree with your opinion, but I would feel better if I understood
> better why it works.  (2) can be done easily in each driver (no
> ugliness) and refactored later.

I think option 2 must be done in each driver by design, or do you see
even a theoretical way how to do it generically?

Kevin