powerpc/kprobes: Disable preemption before invoking probe handler for optprobes

Per Documentation/kprobes.txt, probe handlers need to be invoked with
preemption disabled. Update optimized_callback() to do so. Also move
get_kprobe_ctlblk() invocation post preemption disable, since it
accesses pre-cpu data.

This was not an issue so far since optprobes wasn't selected if
CONFIG_PREEMPT was enabled. Commit a30b85df7d599f ("kprobes: Use
synchronize_rcu_tasks() for optprobe with CONFIG_PREEMPT=y") changes
this.

Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/optprobes.c

@@ -115,7 +115,6 @@ static unsigned long can_optimize(struct kprobe *p)
 static void optimized_callback(struct optimized_kprobe *op,
 			       struct pt_regs *regs)
 {
-	struct kprobe_ctlblk *kcb = get_kprobe_ctlblk();
 	unsigned long flags;
 
 	/* This is possible if op is under delayed unoptimizing */
@@ -124,13 +123,14 @@ static void optimized_callback(struct optimized_kprobe *op,
 
 	local_irq_save(flags);
 	hard_irq_disable();
+	preempt_disable();
 
 	if (kprobe_running()) {
 		kprobes_inc_nmissed_count(&op->kp);
 	} else {
 		__this_cpu_write(current_kprobe, &op->kp);
 		regs->nip = (unsigned long)op->kp.addr;
-		kcb->kprobe_status = KPROBE_HIT_ACTIVE;
+		get_kprobe_ctlblk()->kprobe_status = KPROBE_HIT_ACTIVE;
 		opt_pre_handler(&op->kp, regs);
 		__this_cpu_write(current_kprobe, NULL);
 	}
@@ -140,6 +140,7 @@ static void optimized_callback(struct optimized_kprobe *op,
 	 * local_irq_restore() will re-enable interrupts,
 	 * if they were hard disabled.
 	 */
+	preempt_enable_no_resched();
 	local_irq_restore(flags);
 }
 NOKPROBE_SYMBOL(optimized_callback);