netdev - Re: [IPV4] ROUTE: Avoid sparse warnings

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Date:	Tue, 8 Jan 2008 13:59:02 +0100
From:	Jarek Poplawski <jarkao2@...il.com>
To:	Eric Dumazet <dada1@...mosbay.com>
Cc:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [IPV4] ROUTE: Avoid sparse warnings

On 08-01-2008 08:53, Eric Dumazet wrote:
> David Miller a écrit :
...
>> Furthermore, these:
>>
>>     rcu_read_unlock_bh()
>>     rcu_read_lock_bh()
>>
>> sequences are at best funny looking.  For other lock types we would
>> look at this and ask "Does this even accomplish anything reliably?"
> 
> Well, original code exactly does the same thing.
> 
>>
>> The answer here is that it wants the preempt_enable() to run to get
>> any potential kernel preemptions executed.  It also allows any
>> pending software interrupts to run.
>>
>> So this does something reliably only because rcu_read_unlock_bh() has
>> specific and explicit side effects.
>>
> 
> I will post a patch to introduce a helper function, so that this is 
> clearly documented and not relying on side effects. Actual 
> implementation has latency
> problems on empty hash tables if CONFIG_PREEMPT=n
> 
> 
>> I don't know, to me it just looks awful :-)  I better understood the
>> original code.

It seems this patch only made it more visible how it currently works.
I don't know what changes do you plan for this helper function, but
my proposal is to add some counter and break this rcu only after
looping for some time. Alternatively cpu_relax() could be probably
used between these "locks". Without this probably some cache problems
are possible, but you know this better, I guess.

Regards,
Jarek P.
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