lkml.org 
[lkml]   [2016]   [Mar]   [22]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
SubjectRE: [PATCH 3/5] Drivers: hv: vmbus: Fix a bug in hv_need_to_signal_on_read()
Date


> -----Original Message-----
> From: Vitaly Kuznetsov [mailto:vkuznets@redhat.com]
> Sent: Tuesday, March 22, 2016 2:56 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()
>
> 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 ...

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

\
 
 \ /
  Last update: 2016-03-22 16:21    [W:0.277 / U:0.016 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site