- i've extended the scheduler context-switch mechanism with the following
  per-arch defines:

        prepare_arch_schedule(prev_task);
        finish_arch_schedule(prev_task);
        prepare_arch_switch(rq);
        finish_arch_switch(rq);

- plus switch_to() takes 3 parameters again:

        switch_to(prev, next, last);

- schedule_tail() has the 'prev' task parameter again, it must be passed
  over in switch_to() and passed in to the fork() startup path.

architectures that need to unlock the runqueue before doing the switch can
do the following:

 #define prepare_arch_schedule(prev)             task_lock(prev)
 #define finish_arch_schedule(prev)              task_unlock(prev)
 #define prepare_arch_switch(rq)                 spin_unlock(&(rq)->lock)
 #define finish_arch_switch(rq)                  __sti()

this way the task-lock makes sure that a task is not scheduled on some
other CPU before the switch-out finishes, but the runqueue lock is
dropped. (Local interrupts are kept disabled in this variant, just to
exclude things like TLB flushes - if that matters.)

architectures that can hold the runqueue lock during context-switch can do
the following simplification:

 #define prepare_arch_schedule(prev)             do { } while(0)
 #define finish_arch_schedule(prev)              do { } while(0)
 #define prepare_arch_switch(rq)                 do { } while(0)
 #define finish_arch_switch(rq)                  spin_unlock_irq(&(rq)->lock)

further optimizations possible in the 'simple' variant:

- an architecture does not have to handle the 'last' parameter in
  switch_to() if the 'prev' parameter is unused in finish_arch_schedule().
  This way the inlined return value of context_switch() too gets optimized
  away at compile-time.

- an architecture does not have to pass the 'prev' pointer to
  schedule_tail(), if the 'prev' parameter is unused in
  finish_arch_schedule().

the x86 architecture makes use of these optimizations.

Via this solution we have a reasonably flexible context-switch setup which
falls back to the current (faster) code on x86, but on other platforms the
runqueue lock can be dropped before doing the context-switch as well.

        Ingo

NOTE: i have coded and tested the 'complex' variant on x86 as well to make
      sure it works for you on Sparc64 - but since x86's switch_mm() is
      not too subtle it can use the simpler variant. [ The following
      things had to be done to make x86 arch use the complex variant: the
      4 complex macros have to be used in system.h, entry.S has to
      'pushl %ebx' and 'addl $4, %esp' around the call to schedule_tail(),
      and switch_to() had to be reverted to the 3-parameter variant
      present in the 2.4 kernels.

NOTE2: prepare_to_switch() functionality has been moved into the
       prepare_arch_switch() macro.

NOTE3: please use macros for prepare|finish_arch_switch() so that we can
       keep the scheduler data structures internal to sched.c.