Re: [Qemu-devel] restart a coroutine?

[Avi Kivity <avi@redhat.com>]

On 03/04/2012 02:41 PM, Michael Tokarev wrote:
> Since all block (bdrv) layer is now implemented using
> coroutines, I thought I'd give it a try.  But immediately
> hit a question to which I don't know a good answer.
>
> Suppose we've some networking block device (like NBD) and
> want to be able to support reconnection - this is actually
> very useful feature, in order to be able to reboot/restart
> the NBD server without a need to restart all the clients.
>
> For this to work, we should have an ability to reconnect
> to the server and re-issue all requests which were waiting
> for reply.
>
> Traditionally, in asyncronous event-loop-based scheme, this
> is implemented as a queue of requests linked to the block
> driver state structure, and in case of reconnection we just
> walk over all requests and requeue these.
>
> But if the block driver is implemented as a set of coroutines
> (like nbd currently does), I see no sane/safe way to restart
> the requests.  Setjmp/longjmp can be uses with extra care
> there, but with these it is extremly fragile.
>
> Any hints on how to do that?
>

>From the block layer's point of view, the requests should still be
pending.  For example, if a read request sees a dropped connection, it
adds itself to a list of coroutines waiting for a reconnect, wakes up a
connection manager coroutine (or thread), and sleeps.  The connection
manager periodically tries to connect, and if it succeeds, it wakes up
the coroutines waiting for a reconnection.

It's important to implement request cancellation correctly here, or we
can end up with a device that cannot be unplugged or a guest that cannot
be shutdown.

-- 
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