Patch "ring-buffer: Update read stamp with first real commit on page" has been added to the 3.14-stable tree

[<gregkh@linuxfoundation.org>]

This is a note to let you know that I've just added the patch titled

    ring-buffer: Update read stamp with first real commit on page

to the 3.14-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     ring-buffer-update-read-stamp-with-first-real-commit-on-page.patch
and it can be found in the queue-3.14 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@vger.kernel.org> know about it.


>From b81f472a208d3e2b4392faa6d17037a89442f4ce Mon Sep 17 00:00:00 2001
From: "Steven Rostedt (Red Hat)" <rostedt@goodmis.org>
Date: Mon, 23 Nov 2015 10:35:36 -0500
Subject: ring-buffer: Update read stamp with first real commit on page

From: Steven Rostedt (Red Hat) <rostedt@goodmis.org>

commit b81f472a208d3e2b4392faa6d17037a89442f4ce upstream.

Do not update the read stamp after swapping out the reader page from the
write buffer. If the reader page is swapped out of the buffer before an
event is written to it, then the read_stamp may get an out of date
timestamp, as the page timestamp is updated on the first commit to that
page.

rb_get_reader_page() only returns a page if it has an event on it, otherwise
it will return NULL. At that point, check if the page being returned has
events and has not been read yet. Then at that point update the read_stamp
to match the time stamp of the reader page.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 kernel/trace/ring_buffer.c |   12 +++++-------
 1 file changed, 5 insertions(+), 7 deletions(-)

--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -1949,12 +1949,6 @@ rb_set_commit_to_write(struct ring_buffe
 		goto again;
 }
 
-static void rb_reset_reader_page(struct ring_buffer_per_cpu *cpu_buffer)
-{
-	cpu_buffer->read_stamp = cpu_buffer->reader_page->page->time_stamp;
-	cpu_buffer->reader_page->read = 0;
-}
-
 static void rb_inc_iter(struct ring_buffer_iter *iter)
 {
 	struct ring_buffer_per_cpu *cpu_buffer = iter->cpu_buffer;
@@ -3592,7 +3586,7 @@ rb_get_reader_page(struct ring_buffer_pe
 
 	/* Finally update the reader page to the new head */
 	cpu_buffer->reader_page = reader;
-	rb_reset_reader_page(cpu_buffer);
+	cpu_buffer->reader_page->read = 0;
 
 	if (overwrite != cpu_buffer->last_overrun) {
 		cpu_buffer->lost_events = overwrite - cpu_buffer->last_overrun;
@@ -3602,6 +3596,10 @@ rb_get_reader_page(struct ring_buffer_pe
 	goto again;
 
  out:
+	/* Update the read_stamp on the first event */
+	if (reader && reader->read == 0)
+		cpu_buffer->read_stamp = reader->page->time_stamp;
+
 	arch_spin_unlock(&cpu_buffer->lock);
 	local_irq_restore(flags);
 


Patches currently in stable-queue which might be from rostedt@goodmis.org are

queue-3.14/sched-rt-remove-return-value-from-pull_rt_task.patch
queue-3.14/sched-replace-post_schedule-with-a-balance-callback-list.patch
queue-3.14/sched-clean-up-idle-task-smp-logic.patch
queue-3.14/ring-buffer-update-read-stamp-with-first-real-commit-on-page.patch
queue-3.14/sched-dl-convert-switched_-from-to-_dl-prio_changed_dl-to-balance-callbacks.patch
queue-3.14/sched-dl-remove-return-value-from-pull_dl_task.patch
queue-3.14/sched-allow-balance-callbacks-for-check_class_changed.patch
queue-3.14/sched-rt-convert-switched_-from-to-_rt-prio_changed_rt-to-balance-callbacks.patch
queue-3.14/tracepoints-do-not-trace-when-cpu-is-offline.patch