[IA64] disable irq's and check need_resched before safe_halt

While sending interrupts to a cpu to repeatedly wake a thread, on occasion
that thread will take a full timer tick cycle (4002 usec in my case)
to wakeup.

The problem concerns a race condition in the code around the safe_halt()
call in the default_idle() routine.  Setting 'nohalt' on the kernel
command line causes the long wakeups to disappear.

void
default_idle (void)
{
        local_irq_enable();
        while (!need_resched()) {
-->             if (can_do_pal_halt)
-->                     safe_halt();
                else

A timer tick could arrive between the check for !need_resched and the
actual call to safe_halt() (which does a pal call to PAL_HALT_LIGHT).
By the time the timer tick completes, a thread that might now need to run
could get held up for as long as a timer tick waiting for the halted cpu.

I'm proposing that we disable irq's and check need_resched again before
calling safe_halt().  Does anyone see any problem with this approach?

Signed-off-by: Dimitri Sivanich <sivanich@sgi.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>

arch/ia64/kernel/process.c

@@ -198,9 +198,13 @@ default_idle (void)
 {
 	local_irq_enable();
 	while (!need_resched()) {
-		if (can_do_pal_halt)
+		if (can_do_pal_halt) {
+			local_irq_disable();
+			if (!need_resched()) {
 				safe_halt();
-		else
+			}
+			local_irq_enable();
+		} else
 			cpu_relax();
 	}
 }