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[08/12,v2] ring-buffer: Optimize rb_iter_head_event()

Message ID 20200319232732.672722739@goodmis.org
State Not Applicable
Delegated to: BPF Maintainers
Headers show
Series ring-buffer/tracing: Remove disabling of ring buffer while reading trace file | expand

Commit Message

Steven Rostedt March 19, 2020, 11:22 p.m. UTC 
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.

Link: http://lkml.kernel.org/r/20200317213416.592032170@goodmis.org

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

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.