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

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Message-ID: <107856333.20140618163447@eikelenboom.it>
Date:	Wed, 18 Jun 2014 16:34:47 +0200
From:	Sander Eikelenboom <linux@...elenboom.it>
To:	Wei Liu <wei.liu2@...rix.com>
CC:	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	netdev@...r.kernel.org, David Vrabel <david.vrabel@...rix.com>,
	Ian Campbell <ian.campbell@...rix.com>,
	<xen-devel@...ts.xen.org>
Subject: Re: [Xen-devel] [PATCH net] xen-netback: bookkeep number of queues in our own module


Wednesday, June 18, 2014, 4:21:53 PM, you 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:

Hmm i guess that was the bug_on i hit some time ago with testing the multiqueue 
patches, from what i remember i indeed had guest that crashed earlier on.

>> >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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