the attached patch is a new approach to get rid of Linux's Big Kernel
Lock as we know it today.

The trick is to turn the BKL spinlock + depth counter into a special
type of cpu-affine, recursive semaphore, which gets released by
schedule() but not by preempt_schedule().

this gives the following advantages:

- BKL critical sections are fully preemptable with this patch applied

- there is no time wasted spinning on the BKL if there's BKL contention

- no changes are needed for lock_kernel() users - the new
  semaphore-based approach is fully compatible with the BKL.

code using lock_kernel()/unlock_kernel() will see very similar semantics
as they got from the BKL, so correctness should be fully preserved. 
Per-CPU assumptions still work, locking exclusion and lock-recursion
still works the same way as it did with the BKL.

non-BKL code sees no overhead from this approach. (other than the
slighly smaller code due to the uninlining of the BKL APIs.)

(the patch is against vanilla 2.6.9-rc2. I have tested it on x86
UP+PREEMPT, UP+!PREEMPT, SMP+PREEMPT, SMP+!PREEMPT and x64 SMP+PREEMPT.)

	Ingo