[PATCH 1/1] tracing: Fix event_trace_printk loss on printk_format.

[Mars Cheng <mars.cheng@mediatek.com>]

From: Andress Kuo <andress.kuo@mediatek.com>

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
sys/kernel/debug/tracing/print_formats.

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
trace_printk().

Signed-off-by: Andress Kuo <andress.kuo@mediatek.com>
Signed-off-by: Mars Cheng <mars.cheng@mediatek.com>
---
 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;		\
 									\
-- 
1.7.9.5