Re: [PATCH 7/7] block/nbd: stop manipulating in_flight counter

[Roman Kagan]

On Mon, Mar 15, 2021 at 11:15:44PM +0300, Vladimir Sementsov-Ogievskiy wrote:
> 15.03.2021 09:06, Roman Kagan wrote:
> > As the reconnect logic no longer interferes with drained sections, it
> > appears unnecessary to explicitly manipulate the in_flight counter.
> > 
> > Fixes: 5ad81b4946 ("nbd: Restrict connection_co reentrance")
> 
> And here you actually allow qemu_aio_coroutine_enter() call in
> nbd_client_attach_aio_context_bh() to enter connection_co in any yield
> point which is possible during drained section. The analysis should be
> done to be sure that all these yield points are safe for reentering by
> external qemu_aio_coroutine_enter(). (By external I mean not by the
> actual enter() we are waiting for at the yield() point. For example
> qemu_channel_yield() supports reentering.. And therefore (as I
> understand after fast looking through) nbd_read() should support
> reentering too..

I'll do a more thorough analysis of how safe it is.

FWIW this hasn't triggered any test failures yet, but that assert in
patch 3 didn't ever go off either so I'm not sure I can trust the tests
on this.

Thanks,
Roman.