linux-kernel - [PATCH 08/12 v2] ring-buffer: Optimize rb_iter_head

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Message-Id: <20200319232732.672722739@goodmis.org>
Date:   Thu, 19 Mar 2020 19:22:27 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     linux-kernel@...r.kernel.org
Cc:     Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Peter Wu <peter@...ensteyn.nl>,
        Jonathan Corbet <corbet@....net>,
        Tom Zanussi <zanussi@...nel.org>,
        Shuah Khan <shuahkhan@...il.com>, bpf <bpf@...r.kernel.org>
Subject: [PATCH 08/12 v2] ring-buffer: Optimize rb_iter_head_event()

From: "Steven Rostedt (VMware)" <rostedt@...dmis.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@...dmis.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