x86: cpu_init(): fix memory leak when using CPU hotplug

Exception stacks are allocated each time a CPU is set online.
But the allocated space is never freed. Thus with one CPU hotplug
offline/online cycle there is a memory leak of 24K (6 pages) for
a CPU.

Fix is to allocate exception stacks only once -- when the CPU is
set online for the first time.

Signed-off-by: Andreas Herrmann <andreas.herrmann3@amd.com>
Cc: akpm@linux-foundation.org
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/kernel/cpu/common_64.c

@@ -640,19 +640,22 @@ void __cpuinit cpu_init(void)
 	/*
 	 * set up and load the per-CPU TSS
 	 */
-	for (v = 0; v < N_EXCEPTION_STACKS; v++) {
+	if (!orig_ist->ist[0]) {
 		static const unsigned int order[N_EXCEPTION_STACKS] = {
-			[0 ... N_EXCEPTION_STACKS - 1] = EXCEPTION_STACK_ORDER,
-			[DEBUG_STACK - 1] = DEBUG_STACK_ORDER
+		  [0 ... N_EXCEPTION_STACKS - 1] = EXCEPTION_STACK_ORDER,
+		  [DEBUG_STACK - 1] = DEBUG_STACK_ORDER
 		};
-		if (cpu) {
-			estacks = (char *)__get_free_pages(GFP_ATOMIC, order[v]);
-			if (!estacks)
-				panic("Cannot allocate exception stack %ld %d\n",
-				      v, cpu);
+		for (v = 0; v < N_EXCEPTION_STACKS; v++) {
+			if (cpu) {
+				estacks = (char *)__get_free_pages(GFP_ATOMIC, order[v]);
+				if (!estacks)
+					panic("Cannot allocate exception "
+					      "stack %ld %d\n", v, cpu);
+			}
+			estacks += PAGE_SIZE << order[v];
+			orig_ist->ist[v] = t->x86_tss.ist[v] =
+					(unsigned long)estacks;
 		}
-		estacks += PAGE_SIZE << order[v];
-		orig_ist->ist[v] = t->x86_tss.ist[v] = (unsigned long)estacks;
 	}
 
 	t->x86_tss.io_bitmap_base = offsetof(struct tss_struct, io_bitmap);