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

From: Wei Liu <redacted>
Date: Wed, 18 Jun 2014 15:09:18 +0100

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.

Reported-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Wei Liu <redacted>

I say you should have a select queue method at all.

You're essentially providing a half-assed version of __netdev_pick_tx()
except that:

1) You're _completely_ ignoring the socket hash, if any.

2) You're not allowing XPS to work, _at all_.

I think you need to serious reevaluate providing any select queue
method at all, just let netdev_pick_tx() do all the work.

If you have some issue maintaining the release of queue resources,
maintain that privately and keep those details in the queue resource
allocation and freeing code _only_.  Don't make it an issue that
interferes at all with the normal mechanisms for SKB tx queue
selection.

Thanks.