Re: [for-next][PATCH 1/2] tracing: Fix trace_printk() to print when not using bprintk()

From: Vlastimil Babka
Date: Wed Mar 23 2016 - 10:13:29 EST

On 03/23/2016 01:24 PM, Steven Rostedt wrote:
From: "Steven Rostedt (Red Hat)" <rostedt@xxxxxxxxxxx>

The trace_printk() code will allocate extra buffers if the compile detects
that a trace_printk() is used. To do this, the format of the trace_printk()
is saved to the __trace_printk_fmt section, and if that section is bigger
than zero, the buffers are allocated (along with a message that this has

If trace_printk() uses a format that is not a constant, and thus something
not guaranteed to be around when the print happens, the compiler optimizes
the fmt out, as it is not used, and the __trace_printk_fmt section is not
filled. This means the kernel will not allocate the special buffers needed
for the trace_printk() and the trace_printk() will not write anything to the
tracing buffer.

Adding a "__used" to the variable in the __trace_printk_fmt section will
keep it around, even though it is set to NULL. This will keep the string
from being printed in the debugfs/tracing/printk_formats section as it is
not needed.

Reported-by: Vlastimil Babka <vbabka@xxxxxxx>
Fixes: 07d777fe8c398 "tracing: Add percpu buffers for trace_printk()"
Cc: stable@xxxxxxxxxxxxxxx # v3.5+
Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx>

Tested-by: Vlastimil Babka <vbabka@xxxxxxx>