netdev - Re: [PATCHv3 net-next-2.6 2/3] qlcnic: Take FW dump via ethtool

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Date:	Thu, 12 May 2011 13:54:19 -0700
From:	Anirban Chakraborty <anirban.chakraborty@...gic.com>
To:	Ben Hutchings <bhutchings@...arflare.com>
CC:	netdev <netdev@...r.kernel.org>, David Miller <davem@...emloft.net>
Subject: Re: [PATCHv3 net-next-2.6 2/3] qlcnic: Take FW dump via ethtool


On May 12, 2011, at 12:18 PM, Ben Hutchings wrote:

> On Thu, 2011-05-12 at 11:53 -0700, Anirban Chakraborty wrote:
>> On May 12, 2011, at 10:27 AM, Ben Hutchings wrote:
>> 
>>> On Wed, 2011-05-11 at 15:54 -0700, Anirban Chakraborty wrote:
>>>> Driver checks if the previous dump has been cleared before taking the dump.
>>>> It doesn't take the dump if it is not cleared.
>>>> 
>>>> Changes from v2:
>>>> Added lock to protect dump data structures from being mangled while
>>>> dumping or setting them via ethtool.
>>> 
>>> Unfortunately it still seems to be possible for the dump length to
>>> change between the ethtool core calling qlcnic_get_dump_flag() and
>>> qlcnic_get_dump_data().
>> 
>> dump length is serialized via the driver lock. dump length is a static
>> entity for a given capture mask and it can only be changed when there
>> is a different capture mask set in the driver (via calling set_dump()
>> from ethtool core).
> 
> OK.
> 
>> Actual dump size is determined during the initial steps of FW dump
>> which takes the driver lock to start with. So, I am not sure how the
>> dump length could be changed between the calls to get_dump_flag and
>> get_dump_data from within the ethtool core without a call to
>> set_dump() in between.
> [...]
> 
> What prevents this sequence:
> 
> 1. Driver detects firmware dump is required, and creates the dump
> (length L1).
> 2. User changes firmware dump flags through ethtool.
> 3. User starts to save firmware dump through ethtool:
>   a. ethtool utility reads dump length (= L1) and allocates user buffer
>   b. ethtool utility reads dump:
>   c. ethtool core reads dump length (L1) and allocates kernel buffer
> 4. Meanwhile, driver detects firmware dump is required again, and
> creates a new dump (length L2, since dump flags changed)

This step won't happen as driver ensures that if a dump is taken that is not
cleared yet by ethtool utility, it is not going to take any further dump. So, until
the get_dump_data() has been called to fetch the dump data, changing dump
flag (and hence dump size) will not have any effect.
In 3 above, ethtool core created a buffer of L1 size but hasn't yet called 
get_dump_data() of the driver, so driver is still holding onto its previously
dumped data even though capture mask has been changed and the driver
encountered a situation where it ought to take the dump.

-Anirban
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