Messages in this thread | | | From | Vitaly Kuznetsov <> | Subject | Re: [PATCH 3/5] Drivers: hv: vmbus: Fix a bug in hv_need_to_signal_on_read() | Date | Tue, 22 Mar 2016 10:55:56 +0100 |
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KY Srinivasan <kys@microsoft.com> writes:
>> -----Original Message----- >> From: Vitaly Kuznetsov [mailto:vkuznets@redhat.com] >> Sent: Monday, March 21, 2016 1:19 AM >> To: KY Srinivasan <kys@microsoft.com> >> Cc: gregkh@linuxfoundation.org; linux-kernel@vger.kernel.org; >> devel@linuxdriverproject.org; olaf@aepfle.de; apw@canonical.com; >> jasowang@redhat.com; stable@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@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 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 ...
-- Vitaly
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