x86/entry: Get rid of ist_begin/end_non_atomic()

This is completely overengineered and definitely not an interface which
should be made available to anything else than this particular MCE case.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Alexandre Chartre <alexandre.chartre@oracle.com>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20200505134059.462640294@linutronix.de

arch/x86/include/asm/traps.h

@@ -120,8 +120,6 @@ asmlinkage void smp_irq_move_cleanup_interrupt(void);
 
 extern void ist_enter(struct pt_regs *regs);
 extern void ist_exit(struct pt_regs *regs);
-extern void ist_begin_non_atomic(struct pt_regs *regs);
-extern void ist_end_non_atomic(void);
 
 #ifdef CONFIG_VMAP_STACK
 void __noreturn handle_stack_overflow(const char *message,

arch/x86/kernel/cpu/mce/core.c

@@ -1352,13 +1352,15 @@ void notrace do_machine_check(struct pt_regs *regs, long error_code)
 
 	/* Fault was in user mode and we need to take some action */
 	if ((m.cs & 3) == 3) {
-		ist_begin_non_atomic(regs);
+		/* If this triggers there is no way to recover. Die hard. */
+		BUG_ON(!on_thread_stack() || !user_mode(regs));
 		local_irq_enable();
+		preempt_enable();
 
 		if (kill_it || do_memory_failure(&m))
 			force_sig(SIGBUS);
+		preempt_disable();
 		local_irq_disable();
-		ist_end_non_atomic();
 	} else {
 		if (!fixup_exception(regs, X86_TRAP_MC, error_code, 0))
 			mce_panic("Failed kernel mode recovery", &m, msg);

arch/x86/kernel/traps.c

@@ -117,43 +117,6 @@ void ist_exit(struct pt_regs *regs)
 		rcu_nmi_exit();
 }
 
-/**
- * ist_begin_non_atomic() - begin a non-atomic section in an IST exception
- * @regs:	regs passed to the IST exception handler
- *
- * IST exception handlers normally cannot schedule.  As a special
- * exception, if the exception interrupted userspace code (i.e.
- * user_mode(regs) would return true) and the exception was not
- * a double fault, it can be safe to schedule.  ist_begin_non_atomic()
- * begins a non-atomic section within an ist_enter()/ist_exit() region.
- * Callers are responsible for enabling interrupts themselves inside
- * the non-atomic section, and callers must call ist_end_non_atomic()
- * before ist_exit().
- */
-void ist_begin_non_atomic(struct pt_regs *regs)
-{
-	BUG_ON(!user_mode(regs));
-
-	/*
-	 * Sanity check: we need to be on the normal thread stack.  This
-	 * will catch asm bugs and any attempt to use ist_preempt_enable
-	 * from double_fault.
-	 */
-	BUG_ON(!on_thread_stack());
-
-	preempt_enable_no_resched();
-}
-
-/**
- * ist_end_non_atomic() - begin a non-atomic section in an IST exception
- *
- * Ends a non-atomic section started with ist_begin_non_atomic().
- */
-void ist_end_non_atomic(void)
-{
-	preempt_disable();
-}
-
 int is_valid_bugaddr(unsigned long addr)
 {
 	unsigned short ud;