Re: [PATCH 0/7] block/nbd: decouple reconnect from drain

[Roman Kagan]

On Mon, Mar 15, 2021 at 10:45:39PM +0300, Vladimir Sementsov-Ogievskiy wrote:
> 15.03.2021 09:06, Roman Kagan wrote:
> > The reconnection logic doesn't need to stop while in a drained section.
> > Moreover it has to be active during the drained section, as the requests
> > that were caught in-flight with the connection to the server broken can
> > only usefully get drained if the connection is restored.  Otherwise such
> > requests can only either stall resulting in a deadlock (before
> > 8c517de24a), or be aborted defeating the purpose of the reconnection
> > machinery (after 8c517de24a).
> > 
> > This series aims to just stop messing with the drained section in the
> > reconnection code.
> > 
> > While doing so it undoes the effect of 5ad81b4946 ("nbd: Restrict
> > connection_co reentrance"); as I've missed the point of that commit I'd
> > appreciate more scrutiny in this area.
> 
> 
> The actual point is:
> 
> connection_co (together with all functions called from it) has a lot of yield 
> points. And we can't just enter the coroutine in any of the when we want, as 
> it may break some BH which is actually waited for in this yield point..
> 
> Still, we should care only about yield points possible during drained 
> section, so we don't need to care about direct qemu_coroutine_yield() inside 
> nbd_connection_entry().
> 
> Many things changed since 5ad81b4946.. So probably, now all the (possible 
> during drained section) yield points in nbd_connection_entry support 
> reentering. But some analysis of possible yield points should be done.

Thanks for the explanation.  Will do this analysis.

Roman.