I fixed the problems that I was having.  thread_info->preempt_count is 
now used to store softirq state, unlike in 2.5.20.  Preempt count was 
not preserved once the switch to the interrupt stack occurred.  This 
caused two nested softirqs and a deadlock.  It is fixed now.

Any comments?  Is this right for -mm, or should I try to push right to 
Linus?

part of Ben LaHaise's original message:
 > Below is a patch against 2.5.20 that implements 4KB stacks for
 > tasks, plus a seperate 4KB irq stack for use by interrupts.  There
 > are a couple of reasons for doing this: 4KB stacks put less pressure
 > on the VM subsystem, reduces the overall memory usage for systems
 > with large numbers of tasks, and increases the reliability of the
 > system when under heavy irq load by provide a fixed stack size for
 > interrupt handlers that other kernel code will not eat into.
 >
 > The interrupt stacks are stackable, so we could use multiple
 > 4KB irq stacks.  The thread_info structure is included in each
 > interrupt stack, and has the current pointer copied into it upon
 > entry.

-- 
Dave Hansen
haveblue@us.ibm.com