ring-buffer: fix dangling commit race

Impact: fix stuck trace-buffers

If an interrupt comes in during the rb_set_commit_to_write and
pushes the tail page forward just at the right time, the commit
updates will miss the adding of the interrupt data. This will
cause the commit pointer to cease from moving forward.

Thanks to Jiaying Zhang for finding this race.
Reported-by: NJiaying Zhang <jiayingz@google.com>
Signed-off-by: NSteven Rostedt <srostedt@redhat.com>
Cc: <stable@kernel.org>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

kernel/trace/ring_buffer.c

@@ -838,6 +838,7 @@ rb_set_commit_to_write(struct ring_buffer_per_cpu *cpu_buffer)
 	 * back to us). This allows us to do a simple loop to
 	 * assign the commit to the tail.
 	 */
+ again:
 	while (cpu_buffer->commit_page != cpu_buffer->tail_page) {
 		cpu_buffer->commit_page->page->commit =
 			cpu_buffer->commit_page->write;
@@ -853,6 +854,17 @@ rb_set_commit_to_write(struct ring_buffer_per_cpu *cpu_buffer)
 			cpu_buffer->commit_page->write;
 		barrier();
 	}
+
+	/* again, keep gcc from optimizing */
+	barrier();
+
+	/*
+	 * If an interrupt came in just after the first while loop
+	 * and pushed the tail page forward, we will be left with
+	 * a dangling commit that will never go forward.
+	 */
+	if (unlikely(cpu_buffer->commit_page != cpu_buffer->tail_page))
+		goto again;
 }
 
 static void rb_reset_reader_page(struct ring_buffer_per_cpu *cpu_buffer)