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* [PATCH 1/1] tracing: Fix event_trace_printk loss on printk_format.
@ 2019-02-12 11:41 Mars Cheng
  2019-02-13  1:41 ` Steven Rostedt
  0 siblings, 1 reply; 5+ messages in thread
From: Mars Cheng @ 2019-02-12 11:41 UTC (permalink / raw)
  To: Steven Rostedt
  Cc: Andress Kuo, CC Hwang, linux-kernel, Mars Cheng, linux-mediatek,

From: Andress Kuo <>

If fmt on event_trace_printk is not a constant, It means that something not
guaranteed, so the compiler optimizes the fmt out, and then the
__trace_printk_fmt section is not filled. if __trace_printk_fmt is not
filled, the kernel will not allocate the special buffers needed for the
event_trace_printk() and then not shown in the file output

Adding a "__used" to the variable in the __trace_printk_fmt section on
event_trace_printk() will keep it around, even though it is set to NULL.
This will keep the string from being printed in the
sys/kernel/debug/tracing/printk_formats section.

We can also refer to commit 3debb0a9ddb1 ("tracing: Fix trace_printk()
to print when not using bprintk()")that it had similar issue on path of

Signed-off-by: Andress Kuo <>
Signed-off-by: Mars Cheng <>
 include/linux/trace_events.h |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/trace_events.h b/include/linux/trace_events.h
index 8a62731..26bd7c4 100644
--- a/include/linux/trace_events.h
+++ b/include/linux/trace_events.h
@@ -551,7 +551,7 @@ extern int trace_define_field(struct trace_event_call *call, const char *type,
 	__trace_printk_check_format(fmt, ##args);			\
 	tracing_record_cmdline(current);				\
 	if (__builtin_constant_p(fmt)) {				\
-		static const char *trace_printk_fmt			\
+		static const char *trace_printk_fmt __used		\
 		  __attribute__((section("__trace_printk_fmt"))) =	\
 			__builtin_constant_p(fmt) ? fmt : NULL;		\

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2019-02-12 11:41 [PATCH 1/1] tracing: Fix event_trace_printk loss on printk_format Mars Cheng
2019-02-13  1:41 ` Steven Rostedt
2019-02-13  4:54   ` Mars Cheng
2019-02-13  6:49     ` Mars Cheng
2019-02-13 13:45       ` Steven Rostedt

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