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

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 <[email protected]> know about it.

From: "Steven Rostedt (Red Hat)" <[email protected]>
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) <[email protected]>

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 If the reader page is swapped out of the buffer before an event is written to it, then the read_stamp may get an out of date timestamp, as the page timestamp is updated on the first commit to that page.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.

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

@@ -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 [email protected] 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