Re: [Qemu-devel] [PATCH v2 1/3] block: block: introduce bdrv_io_plug() and bdrv_io_unplug()

[Paolo Bonzini]

Il 01/07/2014 17:21, Kevin Wolf ha scritto:
> > > > Does this bs->file forwarding work for more than the raw driver? For
> > > > example, if drv is an image format driver that needs to read some
> > > > metadata from the image before it can submit the payload, does this
> > > > still do what you were intending?
> >
> > Sorry for not understanding the problem, and you are right, these
> > patches can't support other formats, and for solving the dependency,
> > changes to image format driver should be needed.
>
> Then let's drop the bs->file recursion here and add an explicit
> .bdrv_io_plug/unplug callback to the raw driver.

Actually I thought about this in my review, and there's no reason for 
this not to work for image formats.

While bs->file is plugged, image formats will start executing their 
bdrv_co_readv/bdrv_co_writev callbacks, and issue reads or writes as 
necessary.  The reads and writes will accumulate in bs->file until it is 
unplugged, which is exactly the effect we want.

The change in bdrv_drain_all is ugly though.  I don't have a better 
idea, but I would like to understand better why it is needed.  Ming Lei, 
did you see a deadlock without it?

Paolo