sh: Replace DEBUG_STACKOVERFLOW with STACK_DEBUG.

STACK_DEBUG ties in to mcount in order to do function-granular stack
overflow checks as opposed to lazily checking from IRQ context. As the
default is nohz, the frequency of overflow checking is too irregular to
catch much useful information, and so the mcount approach employed by
sparc64 is adopted instead.

This kills off the old check entirely from the do_IRQ() path and now
adopts CONFIG_MCOUNT instead.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>

arch/sh/Kconfig.debug

@@ -61,12 +61,14 @@ config EARLY_PRINTK
 	  select both the EARLY_SCIF_CONSOLE and SH_STANDARD_BIOS, using
 	  the kernel command line option to toggle back and forth.
 
-config DEBUG_STACKOVERFLOW
+config STACK_DEBUG
 	bool "Check for stack overflows"
 	depends on DEBUG_KERNEL && SUPERH32
 	help
 	  This option will cause messages to be printed if free stack space
-	  drops below a certain limit.
+	  drops below a certain limit. Saying Y here will add overhead to
+	  every function call and will therefore incur a major
+	  performance hit. Most users should say N.
 
 config DEBUG_STACK_USAGE
 	bool "Stack utilization instrumentation"
@@ -123,17 +125,6 @@ config SH64_SR_WATCH
 	bool "Debug: set SR.WATCH to enable hardware watchpoints and trace"
 	depends on SUPERH64
 
-config STACK_DEBUG
-	bool "Enable diagnostic checks of the kernel stack"
-	depends on FUNCTION_TRACER
-	select DEBUG_STACKOVERFLOW
-	default n
-	help
-	  This option allows checks to be performed on the kernel stack
-	  at runtime. Saying Y here will add overhead to every function
-	  call and will therefore incur a major performance hit. Most
-	  users should say N.
-
 config MCOUNT
 	def_bool y
 	depends on SUPERH32

arch/sh/kernel/irq.c

@@ -114,23 +114,6 @@ asmlinkage int do_IRQ(unsigned int irq, struct pt_regs *regs)
 #endif
 
 	irq_enter();
-
-#ifdef CONFIG_DEBUG_STACKOVERFLOW
-	/* Debugging check for stack overflow: is there less than 1KB free? */
-	{
-		long sp;
-
-		__asm__ __volatile__ ("and r15, %0" :
-					"=r" (sp) : "0" (THREAD_SIZE - 1));
-
-		if (unlikely(sp < (sizeof(struct thread_info) + STACK_WARN))) {
-			printk("do_IRQ: stack overflow: %ld\n",
-			       sp - sizeof(struct thread_info));
-			dump_stack();
-		}
-	}
-#endif
-
 	irq = irq_demux(intc_evt2irq(irq));
 
 #ifdef CONFIG_IRQSTACKS