Re: [Qemu-devel] [PATCH] qcow2: Bring synchronous read/write back to life

[Kevin Wolf <kwolf@redhat.com>]

Am 08.10.2009 17:28, schrieb Jamie Lokier:
> Kevin Wolf wrote:
>> The original bdrv_read would mean that we read some data
>> synchronously and that we won't be interrupted during this read. The
>> latter assumption is no longer true with the emulation function
>> which needs to use qemu_aio_poll and therefore allows the callback
>> of any other concurrent AIO request to be run during the read. Which
>> in turn means that (meta)data read earlier could have changed and be
>> invalid now.
> 
> I'm not sure if I understand your description, specifically
> "(meta)data read earlier could have changed and be invalid now".
> 
> Do you mean:

No, it's not exactly what I meant. But you're right, your scenario could
happen, too. If global state is changed in an AIO callback called during
bdrv_read/write (e.g. a new L2 table is loaded into the cache), bad
things are almost guaranteed to happen.

What I was thinking of is:

> 
>                                   Async call into qcow2 #2
>                                   ------------------------
> 
>                                   issues a request with bdrv_read/write
> 
> Async call into qcow2 #1
> ------------------------
> reads some metadata from memory (**)

#1 reads some metadata from disk (from the image file)

> does some calculations
> issues a request with bdrv_read/write
> 
>                                   the request completes
>                                   updates some metadata in memory

#2 updates the metadata in the file

>                                   async call finished with result
> 
> the request completes
> updates some metadata in memory
> .... ERROR, memory isn't what it was at point (**)

#1 still uses the old metadata which it had loaded into memory
(specifically those parts on the stack).

Also, I was thinking of #2 as being a regular AIO write (no bdrv_write
involved), but again your version could work as well. As I said I don't
know yet which of them really happens.

Kevin