x86/traps: Get rid of unnecessary preempt_disable/preempt_enable_no_resched

Exception handlers which may run on IST stack call ist_enter() at the start
of execution and ist_exit() in the end. ist_enter() disables preemption
unconditionally and ist_exit() enables it.

So the extra preempt_disable/enable() pairs nested inside the
ist_enter/exit() regions are pointless and can be removed.

Signed-off-by: Alexander Kuleshov <kuleshovmail@gmail.com>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Jianyu Zhan <nasa4836@gmail.com>
Cc: Paul Gortmaker <paul.gortmaker@windriver.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@suse.de>
Link: http://lkml.kernel.org/r/20161128075057.7724-1-kuleshovmail@gmail.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

arch/x86/kernel/traps.c

@@ -563,11 +563,9 @@ dotraplinkage void notrace do_int3(struct pt_regs *regs, long error_code)
 	 * as we may switch to the interrupt stack.
 	 */
 	debug_stack_usage_inc();
-	preempt_disable();
 	cond_local_irq_enable(regs);
 	do_trap(X86_TRAP_BP, SIGTRAP, "int3", regs, error_code, NULL);
 	cond_local_irq_disable(regs);
-	preempt_enable_no_resched();
 	debug_stack_usage_dec();
 exit:
 	ist_exit(regs);
@@ -742,14 +740,12 @@ dotraplinkage void do_debug(struct pt_regs *regs, long error_code)
 	debug_stack_usage_inc();
 
 	/* It's safe to allow irq's after DR6 has been saved */
-	preempt_disable();
 	cond_local_irq_enable(regs);
 
 	if (v8086_mode(regs)) {
 		handle_vm86_trap((struct kernel_vm86_regs *) regs, error_code,
 					X86_TRAP_DB);
 		cond_local_irq_disable(regs);
-		preempt_enable_no_resched();
 		debug_stack_usage_dec();
 		goto exit;
 	}
@@ -769,7 +765,6 @@ dotraplinkage void do_debug(struct pt_regs *regs, long error_code)
 	if (tsk->thread.debugreg6 & (DR_STEP | DR_TRAP_BITS) || user_icebp)
 		send_sigtrap(tsk, regs, error_code, si_code);
 	cond_local_irq_disable(regs);
-	preempt_enable_no_resched();
 	debug_stack_usage_dec();
 
 exit: