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

KY Srinivasan kys at microsoft.com
Tue Mar 22 00:04:03 UTC 2016



> -----Original Message-----
> From: Vitaly Kuznetsov [mailto:vkuznets at redhat.com]
> Sent: Monday, March 21, 2016 1:19 AM
> To: KY Srinivasan <kys at microsoft.com>
> Cc: gregkh at linuxfoundation.org; linux-kernel at vger.kernel.org;
> devel at linuxdriverproject.org; olaf at aepfle.de; apw at canonical.com;
> jasowang at redhat.com; stable at vger.kernel.org
> Subject: Re: [PATCH 3/5] Drivers: hv: vmbus: Fix a bug in
> hv_need_to_signal_on_read()
> 
> "K. Y. Srinivasan" <kys at microsoft.com> 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



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