RE: [PATCH 3/5] Drivers: hv: vmbus: Fix a bug in hv_need_to_signal_on_read()

[KY Srinivasan]

> -----Original Message-----
> From: Vitaly Kuznetsov [mailto:vkuznets@xxxxxxxxxx]
> Sent: Monday, March 21, 2016 1:19 AM
> To: KY Srinivasan <kys@xxxxxxxxxxxxx>
> Cc: gregkh@xxxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
> devel@xxxxxxxxxxxxxxxxxxxxxx; olaf@xxxxxxxxx; apw@xxxxxxxxxxxxx;
> jasowang@xxxxxxxxxx; stable@xxxxxxxxxxxxxxx
> Subject: Re: [PATCH 3/5] Drivers: hv: vmbus: Fix a bug in
> hv_need_to_signal_on_read()
> 
> "K. Y. Srinivasan" <kys@xxxxxxxxxxxxx> writes:
> 
> > We need to issue a full memory barrier prior making a signalling
> > decision.
> 
> Any reason this should be mb()? This is pretty strong and will probably
> lead to performace regression ... and, btw, we have another mb() in
> hv_ringbuffer_read().
> 
> Could you please describe the scenarion you're trying to protect against
> so we could search for a better solution?

If the reading of the pend_sz (in the function hv_need_to_signal_on_read)
were to be reordered and read before we commit the new read index we could 
have a problem. If the host were to set the pending_sz after we have sampled pending_sz
and go to sleep before we commit the read index, we could miss sending the interrupt.

Regards,

K. Y