x86/int3: Avoid atomic instrumentation

Use arch_atomic_*() and __READ_ONCE() to ensure nothing untoward
creeps in and ruins things.

That is; this is the INT3 text poke handler, strictly limit the code
that runs in it, lest it inadvertenly hits yet another INT3.
Reported-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Masami Hiramatsu <mhiramat@kernel.org>
Reviewed-by: Alexandre Chartre <alexandre.chartre@oracle.com>
Acked-by: Andy Lutomirski <luto@kernel.org>
Link: https://lkml.kernel.org/r/20200505135313.517429268@linutronix.de

arch/x86/kernel/alternative.c

@@ -1014,9 +1014,9 @@ static struct bp_patching_desc *bp_desc;
 static __always_inline
 struct bp_patching_desc *try_get_desc(struct bp_patching_desc **descp)
 {
-	struct bp_patching_desc *desc = READ_ONCE(*descp); /* rcu_dereference */
+	struct bp_patching_desc *desc = __READ_ONCE(*descp); /* rcu_dereference */
 
-	if (!desc || !atomic_inc_not_zero(&desc->refs))
+	if (!desc || !arch_atomic_inc_not_zero(&desc->refs))
 		return NULL;
 
 	return desc;
@@ -1025,7 +1025,7 @@ struct bp_patching_desc *try_get_desc(struct bp_patching_desc **descp)
 static __always_inline void put_desc(struct bp_patching_desc *desc)
 {
 	smp_mb__before_atomic();
-	atomic_dec(&desc->refs);
+	arch_atomic_dec(&desc->refs);
 }
 
 static __always_inline void *text_poke_addr(struct text_poke_loc *tp)