If I compile a recent 2.5.x kernel without CONFIG_ISA defined, I get
an oops in validate_mem().  Stack trace contains 0x6b6b6b6 - a clear
sign that freed memory is being accessed.

It's the second validate_mem() in drivers/pcmcia/rsrc_mgr.c - the one
used when CONFIG_PCMCIA_PROBE is not defined.  It turns out the memory
is freed in do_mem_probe() when it's called from validate_mem().

The solution is to use the same trick as in the first validate_mem().
This problem is quite serious and it's not specific to the plx9052
driver. I see it with yenta_socket as well.