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Subject[RFC][PATCH 07/11] ring-buffer: Optimize rb_iter_head_event()
From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>

As it is fine to perform several "peeks" of event data in the ring buffer
via the iterator before moving it forward, do not re-read the event, just
return what was read before. Otherwise, it can cause inconsistent results,
especially when testing multiple CPU buffers to interleave them.

Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
---
kernel/trace/ring_buffer.c | 3 +++
1 file changed, 3 insertions(+)

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 475338fda969..5979327254f9 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -1929,6 +1929,9 @@ rb_iter_head_event(struct ring_buffer_iter *iter)
unsigned long commit;
unsigned length;

+ if (iter->head != iter->next_event)
+ return iter->event;
+
/*
* When the writer goes across pages, it issues a cmpxchg which
* is a mb(), which will synchronize with the rmb here.
--
2.25.1

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