Re: flow control in vmbus ring buffers

[Roman Kagan <rkagan@virtuozzo.com>]

On Tue, Dec 24, 2019 at 10:56:08AM +0000, Roman Kagan wrote:
> I'm trying to get my head around how the flow control in vmbus ring
> buffers works.
> 
> In particular, I'm failing to see how the following can be prevented:
> 
> 
>      producer                     |       consumer
> ==================================================================
> read read_index                   |
> (enough room for packet)          |
> write pending_send_sz = 0         |
> write packet                      |
> update write_index                |
>                                   | read write_index
> read read_index                   | read packet
> (not enough room for packet)      | update read_index (= write_index)
>                                   | read pending_send_sz = 0
> write pending_send_sz = X         | skip notification
> go to sleep                       | go to sleep
>                                 stall
> 
> Could anybody please shed some light on how it's supposed to work?

Sorry to reply to myself, but looks like the answer is to re-read
read_index on the producer side after setting pending_send_sz.

Thanks and sorry for the noise,
Roman.