softirq: Avoid stack switch from ksoftirqd

ksoftirqd() calls do_softirq() which switches stacks on several
architectures. That makes no sense at all. ksoftirqd's stack is
sufficient.

Call __do_softirq() directly.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Acked-by: David Miller <davem@davemloft.net>
Cc: Paul Mundt <lethal@linux-sh.org>
Reviewed-by: Frank Rowand <frank.rowand@am.sony.com>
LKML-Reference: <alpine.LFD.2.00.1102021704530.31804@localhost6.localdomain6>

kernel/softirq.c

@@ -738,7 +738,10 @@ static int run_ksoftirqd(void * __bind_cpu)
 			   don't process */
 			if (cpu_is_offline((long)__bind_cpu))
 				goto wait_to_die;
-			do_softirq();
+			local_irq_disable();
+			if (local_softirq_pending())
+				__do_softirq();
+			local_irq_enable();
 			preempt_enable_no_resched();
 			cond_resched();
 			preempt_disable();