Re: [PATCH v2 4/4] block: Close block exports in two steps

[Sergio Lopez <slp@redhat.com>]

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On Tue, Dec 15, 2020 at 04:34:05PM +0100, Kevin Wolf wrote:
> Am 14.12.2020 um 18:05 hat Sergio Lopez geschrieben:
> > There's a cross-dependency between closing the block exports and
> > draining the block layer. The latter needs that we close all export's
> > client connections to ensure they won't queue more requests, but the
> > exports may have coroutines yielding in the block layer, which implies
> > they can't be fully closed until we drain it.
> 
> A coroutine that yielded must have some way to be reentered. So I guess
> the quesiton becomes why they aren't reentered until drain. We do
> process events:
> 
>     AIO_WAIT_WHILE(NULL, blk_exp_has_type(type));
> 
> So in theory, anything that would finalise the block export closing
> should still execute.
> 
> What is the difference that drain makes compared to a simple
> AIO_WAIT_WHILE, so that coroutine are reentered during drain, but not
> during AIO_WAIT_WHILE?
> 
> This is an even more interesting question because the NBD server isn't a
> block node nor a BdrvChildClass implementation, so it shouldn't even
> notice a drain operation.

I agree in that this deserves a deeper analysis. I'm going to drop
this patch from the series, and will re-analyze the issue later.

Thanks,
Sergio.

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