From: Peter Wu <peter@lekensteyn.nl>
Date: Wed, 21 Aug 2019 00:08:59 +0100
Subject: bpf: clarify when bpf_trace_printk discards lines
Patch-mainline: v5.4-rc1
Git-commit: 55c33dfbeb831eb3ab7cc1a3e295b0d4d57f23a3
References: bsc#1155518

I opened /sys/kernel/tracing/trace once and kept reading from it.
bpf_trace_printk somehow did not seem to work, no entries were appended
to that trace file. It turns out that tracing is disabled when that file
is open. Save the next person some time and document this.

The trace file is described in Documentation/trace/ftrace.rst, however
the implication "tracing is disabled" did not immediate translate to
"bpf_trace_printk silently discards entries".

Signed-off-by: Peter Wu <peter@lekensteyn.nl>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Gary Lin <glin@suse.com>
---
 include/uapi/linux/bpf.h |    2 ++
 1 file changed, 2 insertions(+)

--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -580,6 +580,8 @@ union bpf_attr {
  * 		limited to five).
  *
  * 		Each time the helper is called, it appends a line to the trace.
+ * 		Lines are discarded while *\/sys/kernel/debug/tracing/trace* is
+ * 		open, use *\/sys/kernel/debug/tracing/trace_pipe* to avoid this.
  * 		The format of the trace is customizable, and the exact output
  * 		one will get depends on the options set in
  * 		*\/sys/kernel/debug/tracing/trace_options* (see also the