ring-buffer: Add page_stamp to iterator for synchronization

Have the ring_buffer_iter structure contain a page_stamp, such that it can
be used to see if the writer entered the page the iterator is on. When going
to a new page, the iterator will record the time stamp of that page. When
reading events, it can copy the event to an internal buffer on the iterator
(to be implemented later), then check the page's time stamp with its own to
see if the writer entered the page. If so, it will need to try to read the
event again.

Link: http://lkml.kernel.org/r/20200317213416.163549674@goodmis.orgSigned-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

kernel/trace/ring_buffer.c

@@ -507,6 +507,7 @@ struct ring_buffer_iter {
 	struct buffer_page		*cache_reader_page;
 	unsigned long			cache_read;
 	u64				read_stamp;
+	u64				page_stamp;
 };
 
 /**
@@ -1959,7 +1960,7 @@ static void rb_inc_iter(struct ring_buffer_iter *iter)
 	else
 		rb_inc_page(cpu_buffer, &iter->head_page);
 
-	iter->read_stamp = iter->head_page->page->time_stamp;
+	iter->page_stamp = iter->read_stamp = iter->head_page->page->time_stamp;
 	iter->head = 0;
 }
 
@@ -3551,10 +3552,13 @@ static void rb_iter_reset(struct ring_buffer_iter *iter)
 	iter->cache_reader_page = iter->head_page;
 	iter->cache_read = cpu_buffer->read;
 
-	if (iter->head)
+	if (iter->head) {
 		iter->read_stamp = cpu_buffer->read_stamp;
-	else
+		iter->page_stamp = cpu_buffer->reader_page->page->time_stamp;
+	} else {
 		iter->read_stamp = iter->head_page->page->time_stamp;
+		iter->page_stamp = iter->read_stamp;
+	}
 }
 
 /**