ring-buffer: fix timestamp in partial ring_buffer_page_read

If a partial ring_buffer_page_read happens, then some of the
incremental timestamps may be lost. This patch writes the
recent timestamp into the page that is passed back to the caller.

A partial ring_buffer_page_read is where the full page would not
be written back to the user, and instead, just part of the page
is copied to the user. A full page would be a page swap with the
ring buffer and the timestamps would be correct.

Signed-off-by: Steven Rostedt <srostedt@redhat.com>

1 files changed, 5 insertions, 1 deletions

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index f2a163db52f9..f7473645b9c6 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -2461,6 +2461,7 @@ int ring_buffer_read_page(struct ring_buffer *buffer,
         unsigned long flags;
         unsigned int commit;
         unsigned int read;
+        u64 save_timestamp;
         int ret = -1;
 
         /*
@@ -2515,6 +2516,9 @@ int ring_buffer_read_page(struct ring_buffer *buffer,
                 if (len < size)
                         goto out;
 
+                /* save the current timestamp, since the user will need it */
+                save_timestamp = cpu_buffer->read_stamp;
+
                 /* Need to copy one event at a time */
                 do {
                         memcpy(bpage->data + pos, rpage->data + rpos, size);
@@ -2531,7 +2535,7 @@ int ring_buffer_read_page(struct ring_buffer *buffer,
 
                 /* update bpage */
                 local_set(&bpage->commit, pos);
-                bpage->time_stamp = rpage->time_stamp;
+                bpage->time_stamp = save_timestamp;
 
                 /* we copied everything to the beginning */
                 read = 0;