[GIT PULL] tracing: Some fixes for 4.4 and before!

[Steven Rostedt]

Linus,

Found two minor bugs while doing development on the ring buffer code.

The first is something that's been there since its creation. If a reader
reads a page out of the ring buffer before there's any events on it, it
can get an out of date timestamp for that event. It may be off by a few
microseconds, more if the first event gets discarded. The fix was to
only update the reader time stamp when it actually sees an event on
the page, instead of just reading the timestamp from the page even if
it has no events on it. That timestamp is still volatile until an event
is present.

The second bug is more recent. Instead of passing around parameters
a descriptor was made and the parameters are passed via a single
descriptor. This simplified the code a bit. But there was one place that
expected the parameter to be passed by value not reference (which a
descriptor now does). And it added to the length of the event, which
may be ignored later, but the length should not have been increased.
The only real problem with this bug is that it may allocate more than
was needed for the event.


Please pull the latest trace-v4.4-rc2 tree, which can be found at:


  git://git.kernel.org/pub/scm/linux/kernel/git/rostedt/linux-trace.git
trace-v4.4-rc2

Tag SHA1: 7973626472174534bcb8f942df776cd18252bf05
Head SHA1: bd1b7cd360f529394936f28746eb4aaa12d6770a


Steven Rostedt (Red Hat) (2):
      ring-buffer: Update read stamp with first real commit on page
      ring-buffer: Put back the length if crossed page with add_timestamp

----
 kernel/trace/ring_buffer.c | 17 +++++++++--------
 1 file changed, 9 insertions(+), 8 deletions(-)
---------------------------
diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 75f1d05ea82d..9c6045a27ba3 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -1887,12 +1887,6 @@ rb_event_index(struct ring_buffer_event *event)
 	return (addr & ~PAGE_MASK) - BUF_PAGE_HDR_SIZE;
 }
 
-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;
@@ -2803,8 +2797,11 @@ rb_reserve_next_event(struct ring_buffer *buffer,
 
 	event = __rb_reserve_next(cpu_buffer, &info);
 
-	if (unlikely(PTR_ERR(event) == -EAGAIN))
+	if (unlikely(PTR_ERR(event) == -EAGAIN)) {
+		if (info.add_timestamp)
+			info.length -= RB_LEN_TIME_EXTEND;
 		goto again;
+	}
 
 	if (!event)
 		goto out_fail;
@@ -3626,7 +3623,7 @@ rb_get_reader_page(struct ring_buffer_per_cpu *cpu_buffer)
 
 	/* 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;
@@ -3636,6 +3633,10 @@ rb_get_reader_page(struct ring_buffer_per_cpu *cpu_buffer)
 	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);
 
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