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: Tuesday, March 22, 2016 2:56 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()
> 
> KY Srinivasan <kys@xxxxxxxxxxxxx> writes:
> 
> >> -----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 these are two reads we can add a lightweight barrier just preventing
> compiler from reordering (e.g. smp_rmb()), right?
> 
> > 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.
> 
> so write and then we read and we need to prevent reordering... not sure
> how to get rid on mb() then ...

The other memory barrier in the function (prior to writing the read index)
has been there forever and I am not sure why that needs to be a full barrier.
I feel a read barrier should suffice.

Regards,

K. Y
> 
> --
>   Vitaly