Re: Worst recursion in the kernel

From: David Hinds
Date: Wed Dec 03 2003 - 13:07:55 EST

On Wed, Dec 03, 2003 at 03:31:22PM +0100, Jörn Engel wrote:
> Really bad code demand really rude words, sorry.
> After playing with stack checking again, I've found this little beauty
> in 2.6.0-test3: [1]
> WARNING: recursion detected:
> 20 read_cis_cache
> 36 pcmcia_get_tuple_data
> 308 read_tuple
> 448 pcmcia_validate_cis
> 12 readable
> 24 cis_readable
> 28 do_mem_probe
> 24 inv_probe
> 16 validate_mem
> 32 set_cis_map
> 28 read_cis_mem
> 284 verify_cis_cache
> Explanation:
> verify_cis_cache calls read_cis_mem, which calls set_cis_map, which
> call ..., which calls read_cis_cache, which finally calls
> verify_cis_cache again.

Err... no it doesn't. verify_cis_cache() is called from exactly one
place which is not in the list of functions here. I do not understand
how this recursion checking is being done but there's something weird
going on. set_cis_map() does not call any function on this list. I
think set_cis_map() should be setup_cis_mem().

> Most likely this recursion will never occur, as one of those calls can
> depends on circumstances that prohibit recursion, but semantic
> checking is a bitch for software and in this case even for humans.

Isn't that a bit strong a statement?

The semantics of the code goes like this. read_cis_mem() checks to
see if something has been done. If it hasn't been done, it leads to
validate_mem() which first does that thing, and then does some stuff
that leads to read_cis_mem() being called again. When read_cis_mem()
is reentered, it is guaranteed that the condition for recursion does
not exist.

Is that so complex as to be incomprehensible by a mere human? To
remove the apparent recursion seems to me to require duplicating a
fairly long code path, which is why I did it this way in the first
place. The stack usage of this code path is definitely something that
should be (and can be easily) fixed.

-- Dave
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