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

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Message-ID: <20140622111657.GA7528@zion.uk.xensource.com>
Date:	Sun, 22 Jun 2014 12:16:57 +0100
From:	Wei Liu <wei.liu2@...rix.com>
To:	David Miller <davem@...emloft.net>
CC:	<wei.liu2@...rix.com>, <xen-devel@...ts.xen.org>,
	<netdev@...r.kernel.org>, <boris.ostrovsky@...cle.com>,
	<ian.campbell@...rix.com>
Subject: Re: [PATCH net] xen-netback: bookkeep number of queues in our own
 module

On Sun, Jun 22, 2014 at 12:13:55AM -0700, David Miller wrote:
> From: Wei Liu <wei.liu2@...rix.com>
> 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@...cle.com>
> > Signed-off-by: Wei Liu <wei.liu2@...rix.com>
> 
> 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.
> 

Looking at the core driver code in more details I think you're right. I
will remove the select queue method.

> 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.
> 

Sure. This is exactly the main idea of this patch, just that it
interfered the queue selection logic. :-(

Will send an updated version soon.

Wei.

> Thanks.
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