4KSTACKS + DEBUG_STACKOVERFLOW harmful

[Eric Sandeen <sandeen@redhat.com>]

Noticed today that the combination of 4KSTACKS and DEBUG_STACKOVERFLOW
config options is a bit deadly.

DEBUG_STACKOVERFLOW warns in do_IRQ if we're within THREAD_SIZE/8 of the
end of useable stack space, or 512 bytes on a 4k stack.

If we are, then it goes down the dump_stack path, which uses most, if
not all, of the remaining stack, thereby turning a well-intentioned
warning into a full-blown catastrophe.

The callchain from the warning looks something like this, with stack
usage shown as found on my x86 box:

4 dump_stack
4   show_trace
8     show_trace_log_lvl
4       dump_trace
          print_context_stack
12          print_trace_address
              print_symbol
232             __print_symbol
164               sprint_symbol
20                  printk
___
448

448 bytes to tell us that we're within 512 bytes (or less) of certain
doom... and I think there's call overhead on top of that?

The large stack usage in those 2 functions is due to big char arrays, of
size KSYM_NAME_LEN (128 bytes) and KSYM_SYMBOL_LEN (223 bytes).

IOW, the stack warning effectively reduces useful stack left in our itty
bitty 4k stacks by over 10%.

Any suggestions for ways around this?  The warning is somewhat helpful,
and I guess the obvious option is to lighten up the dump_stack path, but
it's still effectively reducing precious available stack space by some
amount.

With CONFIG_DEBUG_STACK_USAGE, we could print at oops time: "oh, and by
the way, you blew your stack" if there is no zeroed stack space left, as
a post-mortem.  Even without that option, I think we could still check
whether the *current* %esp at oops time has gone too far?  But if we
blew the stack, returned, and *then* oops, I think it'd be hard to know
without the DEBUG_STACK_USAGE option that we ran out of room.

-Eric