4.x mbuf binary compatibility; can it be broken?

[Daniel C. Sobral]

Mike Silbersack wrote:
> In the process of hunting down reported panics in xl_newbuf, I've come to
> the conclusion that the panics are a result of mbuf cluster refcounts
> overflowing.  This is not too surprising, as we use an array of chars to
> store the refcounts.  (-current uses ints, and doesn't have this problem.)
> 
> It's easy enough to switch from a char to an int array in 4.x to fix the
> problem there, but there is a problem:  Our friendly mbuf macros (MCLALLOC
> and MCLFREE) manipulate the refcount.  This means that 3rd party modules
> which use the macros will no longer work properly.
> 
> Hence, the question posed on the subject line.  Aside from putting hacks
> in many of the mbuf functions so that they avoid reference counts growing
> into the danger zone, there's no solution to the problem that I can see.
> 
> So, what's our policy on ABI breakage for modules?  It'd be nice to ignore
> this problem, but the xl-related PRs filed which seem to describe this
> exact problem are too numerous to ignore.  (No, this isn't if_xl's fault;
> it's simply a victim because it properly uses its descriptor lists,
> thereby using mbuf cluster refcounts rather than packet copies as many
> cheaper NICs are required to do.)

I think that breaking the ABI at the winter of 4.x is a bad idea. It 
would be bad at it's spring, but given the seriousness of the matter, 
perhaps a necessity. At this point, though? Dubious proposition...

-- 
Daniel C. Sobral                   (8-DCS)
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