linux-kernel - [RFC][PATCH 06/11] ring-buffer: Do not die if rb_iter

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Message-Id: <20200317213416.452888193@goodmis.org>
Date:   Tue, 17 Mar 2020 17:32:28 -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>
Subject: [RFC][PATCH 06/11] ring-buffer: Do not die if rb_iter_peek() fails more than thrice

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

As the iterator will be reading a live buffer, and if the event being read
is on a page that a writer crosses, it will fail and try again, the
condition in rb_iter_peek() that only allows a retry to happen three times
is no longer valid. Allow rb_iter_peek() to retry more than three times
without killing the ring buffer, but only if rb_iter_head_event() had failed
at least once.

Signed-off-by: Steven Rostedt (VMware) <rostedt@...dmis.org>
---
 kernel/trace/ring_buffer.c | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 3d718add73c1..475338fda969 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -4012,6 +4012,7 @@ rb_iter_peek(struct ring_buffer_iter *iter, u64 *ts)
 	struct ring_buffer_per_cpu *cpu_buffer;
 	struct ring_buffer_event *event;
 	int nr_loops = 0;
+	bool failed = false;
 
 	if (ts)
 		*ts = 0;
@@ -4038,10 +4039,14 @@ rb_iter_peek(struct ring_buffer_iter *iter, u64 *ts)
 	 * to a data event, we should never loop more than three times.
 	 * Once for going to next page, once on time extend, and
 	 * finally once to get the event.
-	 * (We never hit the following condition more than thrice).
+	 * We should never hit the following condition more than thrice,
+	 * unless the buffer is very small, and there's a writer
+	 * that is causing the reader to fail getting an event.
 	 */
-	if (RB_WARN_ON(cpu_buffer, ++nr_loops > 3))
+	if (++nr_loops > 3) {
+		RB_WARN_ON(cpu_buffer, !failed);
 		return NULL;
+	}
 
 	if (rb_per_cpu_empty(cpu_buffer))
 		return NULL;
@@ -4052,8 +4057,10 @@ rb_iter_peek(struct ring_buffer_iter *iter, u64 *ts)
 	}
 
 	event = rb_iter_head_event(iter);
-	if (!event)
+	if (!event) {
+		failed = true;
 		goto again;
+	}
 
 	switch (event->type_len) {
 	case RINGBUF_TYPE_PADDING:
-- 
2.25.1