Re: execution context in netfilter hooks.

[Giacomo <delleceste@gmail.com>]

A last question:

 you said LOCAL_OUT is on behalf of process, so it is in process
context -> can be interrupted by timer -> the reason why code crashed!
If so, the question would be finally clear!

LOCAL_INPUT instead should be softirq, shouldn't it? Just like
FORWARD... so timers don't cause interruption..

Thanks a lot very much!

Giacomo.



2005/12/12, Harald Welte <laforge@netfilter.org>:
> On Sun, Dec 11, 2005 at 09:39:16PM +0100, Giacomo wrote:
> > Good morning, i would like to know exactly the context of execution
> > of code registered with netfilter hooks.
>
> >  As far as  I understood, the context is that of a softirq, inside a
> > bottom half. Correct me if i'm wrong..
>
> 'bottom half' context doesn't really exist anymore in 2.6.x, so it's
> softirq.  But that's only true for forwarded packets.  locally-generated
> packets at LOCAL_OUT are in 'kernel on behalf of process' context.
>
> > But is it concerned with tasklets?
>
> what do you mean by "is it concerned" ?
>
> > Is execution serialized? In other words, if packet A fr instance is
> > received from the net, and then packet B immediately, is packet A
> > processed entirely before packet B? Or code can be executed in
> > parallel for packet A and B?
>
> on multiple cpu's it's fully parallel, so yes, packet A will execute the
> same netfilter hook (plus attached code) on CPU1 where packet B will run
> on CPU2.
>
> On a single CPU, Packet B will be put onto a queue (by the hardirq
> handler of the network driver) until packet A's softirq processing has
> finished.  Then packet B will enter softirq context and pulled from that
> queue.
>
> > Moreover: interrupts are enabled in such context, aren't they? And that
> > means that also a software timer can interrupt a routine being
> > executed in bottom half.
>
> software timers dont interrupt softirq context. (neither did the
> interrupt bottom half context, when it still existed). you seem to be
> confused about the kernel contexts and their interaction.
>
> > But software timers run with sw interrupts disabled, so a timeout
> > handler for instance cannot be interrupted by a routine managing
> > reception of a packet arrived on the wire.
>
> I think you should read some general information on how softirq and
> hardirq processing work.  This is not really netfilter related.
>
> --
> - Harald Welte <laforge@netfilter.org>                 http://netfilter.org/
> ============================================================================
>   "Fragmentation is like classful addressing -- an interesting early
>    architectural error that shows how much experimentation was going
>    on while IP was being designed."                    -- Paul Vixie
>
>
>



--

Giacomo S.
http://www.giacomos.it

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