svn commit: r186187 - head/sys/net

[Max Laier]

As the upper half of this thread has turned into a style(9) bikeshed, let me 
replay the lower half and add more locking folks.

The change in question is here: 
http://svn.freebsd.org/viewvc/base/head/sys/net/pfil.c?r1=173904&r2=186187 

On Tuesday 16 December 2008 19:20:40 Robert Watson wrote:
> On Tue, 16 Dec 2008, Max Laier wrote:
...
> >>   - Don't lock the hook during registration, since it's not yet in use.
> >
> > Shouldn't the WLOCK be obtained regardless in order to obtain a memory
> > barrier for the reading threads?  pfil_run_hooks doesn't interact with
> > the LIST_LOCK so it's unclear in what state the hook head will be when
> > the hook head is first read.
>
> Hmm.  Interesting observation.  However, that approach is not consistent
> with lots of other code, so possibly that other code needs to change as
> well.  For example, ip_init() initializes lots of other global data
> structures, including the fragment reassembly queue and protocol switch,
> without acquiring locks in such a way as to force visibility on other CPUs.
>
> I've CC'd John Baldwin, who might be able to comment on this issue more
> generally.  Should we be, for example, calling { IPQ_LOCK(); IPQ_UNLOCK();
> } after initializing the IP reassembly queues to make sure that
> initialization is visible to other CPUs that will be calling IPQ_LOCK()
> before using it?
>
...
> >>   - Don't write-lock hooks during removal because they are assumed
> >>     quiesced.
> >
> > Again, this lock also ensures that we see all the changes done in other
> > threads to the pfil head recently - otherwise we might leak a hook
> > function pointer or even fault due to list inconsistencies.  Again,
> > add/del_hook don't interact with the LIST_LOCK.
>
> We may run into similar problems with VIMAGE destructors now that we are
> interested in tearing down network stacks.  Again, perhaps jhb can opine
> some.

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