Re: [Xen-devel] [PATCH net] xen-netback: bookkeep number of queues in our own module

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 06/18/2014 10:21 AM, Wei Liu wrote:
> On Wed, Jun 18, 2014 at 10:18:50AM -0400, Boris Ostrovsky wrote:
> > On 06/18/2014 10:09 AM, Wei Liu wrote:
> > > The original code uses netdev->real_num_tx_queues to bookkeep number of
> > > queues and invokes netif_set_real_num_tx_queues to set the number of
> > > queues. However, netif_set_real_num_tx_queues doesn't allow
> > > real_num_tx_queues to be smaller than 1, which means setting the number
> > > to 0 will not work and real_num_tx_queues is untouched.
> > >
> > > This is bogus when xenvif_free is invoked before any number of queues is
> > > allocated. That function needs to iterate through all queues to free
> > > resources. Using the wrong number of queues results in NULL pointer
> > > dereference.
> > >
> > > So we bookkeep the number of queues in xen-netback to solve this
> > > problem. The usage of real_num_tx_queues in core driver is to cap queue
> > > index to a valid value. In start_xmit we've already guarded against out
> > > of range queue index so we should be fine.
> > >
> > > This fixes a regression introduced by multiqueue patchset in 3.16-rc1.
> > David sent a couple of patches earlier today that I have been testing and
> > they appear to fix both netfront and netback. (I am waiting for 32-bit to
> > finish)
> >
> > http://lists.xenproject.org/archives/html/xen-devel/2014-06/msg02308.html
> I saw that, but they don't fix this backend bug. Try crashing the guest
> before it connects to backend. As I said in commit message:
Apparently it doesn't indeed since 32-bit just crashed on me in 
xenvif_free() (the moment I hit Send on my response to you). But 64-bit 
run completed without failures. And even 32-bit test ran fine for a while.

-boris


> > > This is bogus when xenvif_free is invoked before any number of queues is
> > > allocated. That function needs to iterate through all queues to free
> netif_set_real_num_tx_queues will need to be removed anyway.
>
> Wei.

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