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

[Avi Kivity]

On 01/27/2011 12:34 PM, Kevin Wolf wrote:
> 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.

The callbacks need to be wrapped in the same way.  Schedule a coroutine 
to run the true callback.

> 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.

I don't follow.  Please elaborate.

The idea behind this is, we keep exactly the same serialization 
characteristics we had before.  If a aio_X callback or its intermediate 
completion executed without blocking, it will continue to do so.  
Holding a co_mutex over a non-blocking code sequence does not serialize, 
except if a blocking sequence is executing concurrently (in which case 
the serialization was present before).

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