x86: Clarify/fix no-op barriers for text_poke_bp()

So I was looking at text_poke_bp() today and I couldn't make sense of
the barriers there.

How's for something like so?

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Acked-by: Jiri Kosina <jkosina@suse.cz>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: masami.hiramatsu.pt@hitachi.com
Link: http://lkml.kernel.org/r/20170731102154.f57cvkjtnbmtctk6@hirez.programming.kicks-ass.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/kernel/alternative.c

@@ -742,7 +742,16 @@ static void *bp_int3_handler, *bp_int3_addr;
 
 int poke_int3_handler(struct pt_regs *regs)
 {
-	/* bp_patching_in_progress */
+	/*
+	 * Having observed our INT3 instruction, we now must observe
+	 * bp_patching_in_progress.
+	 *
+	 * 	in_progress = TRUE		INT3
+	 * 	WMB				RMB
+	 * 	write INT3			if (in_progress)
+	 *
+	 * Idem for bp_int3_handler.
+	 */
 	smp_rmb();
 
 	if (likely(!bp_patching_in_progress))
@@ -788,9 +797,8 @@ void *text_poke_bp(void *addr, const void *opcode, size_t len, void *handler)
 	bp_int3_addr = (u8 *)addr + sizeof(int3);
 	bp_patching_in_progress = true;
 	/*
-	 * Corresponding read barrier in int3 notifier for
-	 * making sure the in_progress flags is correctly ordered wrt.
-	 * patching
+	 * Corresponding read barrier in int3 notifier for making sure the
+	 * in_progress and handler are correctly ordered wrt. patching.
 	 */
 	smp_wmb();
 
@@ -815,9 +823,11 @@ void *text_poke_bp(void *addr, const void *opcode, size_t len, void *handler)
 	text_poke(addr, opcode, sizeof(int3));
 
 	on_each_cpu(do_sync_core, NULL, 1);
-
+	/*
+	 * sync_core() implies an smp_mb() and orders this store against
+	 * the writing of the new instruction.
+	 */
 	bp_patching_in_progress = false;
-	smp_wmb();
 
 	return addr;
 }