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

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Message-ID: <56F2A482.1010302@suse.cz>
Date:	Wed, 23 Mar 2016 15:13:22 +0100
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Steven Rostedt <rostedt@...dmis.org>, linux-kernel@...r.kernel.org
Cc:	Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	stable@...r.kernel.org
Subject: Re: [for-next][PATCH 1/2] tracing: Fix trace_printk() to print when
 not using bprintk()

On 03/23/2016 01:24 PM, Steven Rostedt wrote:
> From: "Steven Rostedt (Red Hat)" <rostedt@...dmis.org>
>
> 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
> happened).
>
> 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@...e.cz>
> Fixes: 07d777fe8c398 "tracing: Add percpu buffers for trace_printk()"
> Cc: stable@...r.kernel.org # v3.5+
> Signed-off-by: Steven Rostedt <rostedt@...dmis.org>

Tested-by: Vlastimil Babka <vbabka@...e.cz>

Thanks!