ring-buffer: Reset "real_end" when page is filled

The code to store the "lost events" requires knowing the real end
of the page. Since the 'commit' includes the padding at the end of
a page a "real_end" variable was used to keep track of the end not
including the padding.

If events were lost, the reader can place the count of events in
the padded area if there is enough room.

The bug this patch fixes is that when we fill the page we do not
reset the real_end variable, and if the writer had wrapped a few
times, the real_end would be incorrect.

This patch simply resets the real_end if the page was filled.
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

kernel/trace/ring_buffer.c

@@ -1768,6 +1768,14 @@ rb_reset_tail(struct ring_buffer_per_cpu *cpu_buffer,
 	 * must fill the old tail_page with padding.
 	 */
 	if (tail >= BUF_PAGE_SIZE) {
+		/*
+		 * If the page was filled, then we still need
+		 * to update the real_end. Reset it to zero
+		 * and the reader will ignore it.
+		 */
+		if (tail == BUF_PAGE_SIZE)
+			tail_page->real_end = 0;
+
 		local_sub(length, &tail_page->write);
 		return;
 	}