Re: [PATCH 3/5] Drivers: hv: vmbus: Fix a bug in hv_need_to_signal_on_read()
From: Vitaly Kuznetsov
Date: Tue Mar 22 2016 - 05:56:13 EST
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
>> "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
>> 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 ...