Re: [Qemu-devel] [PATCH v2 1/3] block: add bdrv_co_drain_end callback

[Fam Zheng <famz@redhat.com>]

On Fri, 09/22 13:03, Kevin Wolf wrote:
> Am 22.09.2017 um 04:30 hat Fam Zheng geschrieben:
> > On Thu, 09/21 18:39, Manos Pitsidianakis wrote:
> > > On Thu, Sep 21, 2017 at 09:29:43PM +0800, Fam Zheng wrote:
> > > > On Thu, 09/21 16:17, Manos Pitsidianakis wrote:
> > > It might imply to someone that there's an assert(drv->bdrv_co_drain_begin &&
> > > drv->bdrv_co_drain_end) somewhere unless you state they don't have to be
> > > implemented at the same time. How about we be completely explicit:
> > > 
> > >  bdrv_co_drain_begin is called if implemented in the beggining of a
> > >  drain operation to drain and stop any internal sources of requests in
> > >  the driver.
> > >  bdrv_co_drain_end is called if implemented at the end of the drain.
> > > 
> > >  They should be used by the driver to e.g. manage scheduled I/O
> > >  requests, or toggle an internal state. After the end of the drain new
> > >  requests will continue normally.
> > > 
> > > I hope this is easier for a reader to understand!
> > 
> > I don't like the inconsistent semantics of when the drained section
> > ends, if we allow drivers to implement bdrv_co_drain_begin but omit
> > bdrv_co_drained_end.  Currently the point where the section ends is,
> > as said in the comment, when next I/O callback is invoked. Now we are
> > adding the explicit ".bdrv_co_drain_end" into the fomular, if we still
> > keep the previous convention, the interface contract is just mixed of
> > two things for no good reason. I don't think it's technically
> > necessary.
> 
> We don't keep the convention with the next I/O callback. We just allow
> drivers to omit an empty implementation of either callback, which seems
> to be a very sensible default to me.
> 

OK, I'm fine with this.

Fam