Description of problem:
IBM and TI have reported to us that under low stack pressure, the
x86_64 platform can encounter a stack overflow when handling a
softirq.  This is due to the fact that local_bh_enable, as defined for
the x86_64 platform, calls do_softirq_thunk, which in turn enters
do_softirq using the process stack, rather than the normal per-irq
stacks which the softirq task normally uses.

Version-Release number of selected component (if applicable):


How reproducible:
sometimes

Steps to Reproduce:
1.force process stack usage down to a point where > 1k of free stack
remains
2.lock a spinlock with spin_lock_irqsave
3.trigger a softirq (I believe scheduling a tasklet will do this)
4.unlock the spinlock with spin_unlock_irqrestore
  
Actual results:
system will oops on stack overflow

Expected results:
system should not oops


Additional info:
The above reproducer instructions are generic.  The problem was
initially reported in IT numbers 39062 and 46982 as problems with
clearcase, as clearcase makes significant stack usage and can trigger
the problem.  However, any method of eating most of a process stack
can trigger this issue.