Re: [Qemu-devel] qcow2 performance plan

[Avi Kivity]

 On 09/14/2010 05:25 PM, Anthony Liguori wrote:
> The incremental version of this is hard for me to understand.  
> bdrv_read() may be implemented in terms of bdrv_aio_read() + 
> qemu_io_wait() which dispatches bottom halves.  This is done through a 
> shared resource so if you allow bdrv_read() to be called in parallel, 
> there's a very real possibility that you'll get corruption of a shared 
> resource.
>
> You'd have to first instrument bdrv_read() to be re-entrant by 
> acquiring bs.mutex() in every bdrv_read() caller.  You would then need 
> to modify the file protocol so that it could safely be called in 
> parallel.
>
> IOW, you've got to make the whole block layer thread safe before you 
> can begin to make qcow2 thread safe.

It does rely on CONFIG_IO_THREAD and qemu_mutex.  However, we don't need 
to make the whole block layer safe, just the syncronous parts (drop 
qemu_mutex after I/O is issued, rely on the image's mutex for internal 
synchronization).

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