Re: [Qemu-devel] [RFC][PATCH 11/12] qcow2: Convert qcow2 to use coroutines for async I/O

[Kevin Wolf]

Am 27.01.2011 10:49, schrieb Avi Kivity:
> On 01/27/2011 11:27 AM, Kevin Wolf wrote:
>> Well, but in the case of qcow2, you don't want to have a big mutex
>> around everything. We perfectly know which parts are asynchronous and
>> which are synchronous, so we'd want to do it finer grained from the
>> beginning.
> 
> Yes we do.  And the way I proposed it, the new mutex does not introduce 
> any new serialization.
> 
> To repeat, for every qcow2 callback or completion X (not qcow2 read or 
> write operation), we transform it in the following manner:
> [...]

This works fine for code that is completely synchronous today (and you
can't serialize it more than it already is anyway).

It doesn't work for qemu_aio_readv/writev because these use AIO for
reading/writing the data. So you definitely need to rewrite that part,
or the AIO callback will cause the code to run outside its coroutine.
And during this rewrite you'll want to pay attention that you don't hold
the mutex for the bdrv_co_readv that was an AIO request before, or
you'll introduce additional serialization.

Kevin