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

From: Steven Rostedt
Date: Tue Feb 12 2019 - 20:41:18 EST

On Tue, 12 Feb 2019 19:41:57 +0800
Mars Cheng <mars.cheng@xxxxxxxxxxxx> wrote:

> From: Andress Kuo <andress.kuo@xxxxxxxxxxxx>
> 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().

Honestly, I don't even remember why that macro was created. I think
it's a remnant from the creation of the trace events. I think the best
solution is just to nuke it. It shouldn't be used anymore.

I'll dig a bit deeper into the history of that macro, but I'm thinking
it shouldn't exist anymore.

[ /me returns from walking down memory lane ]

Wow, that brings back some wild memories. Yes, that macro must die, and
I wish the history of it could die along with it ;-) The horror of the
old ways I tell you. The event_trace_printk() was the original way we
displayed events! No real formatting, no parsing my userspace tools. It
was just a glamorized printk. It was called TRACE_FORMAT() which was
deprecated by today's TRACE_EVENT().

That macro should have been removed by commit b8e65554d80b4.

Please just send a patch to delete that macro. Let's not be maintaining
it. It gives me nightmares.

-- Steve

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