linux-kernel - Re: [PATCH] Fix tracing infrastructure to support multiple includes when defining CREATE_TRACE

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Date:	Tue, 15 Dec 2009 23:13:49 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Neil Horman <nhorman@...driver.com>
Cc:	linux-kernel@...r.kernel.org, fweisbec@...il.com, mingo@...hat.com,
	Mathieu Desnoyers <compudj@...stal.dyndns.org>,
	"David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH] Fix tracing infrastructure to support multiple
 includes when defining CREATE_TRACE_POINTS

On Tue, 2009-12-15 at 11:08 -0500, Neil Horman wrote:

Hi Neil,

I was playing with this, and I got really nasty errors in the trace
parsing tools. Then I noticed why:

> -DECLARE_TRACE(napi_poll,
> +TRACE_EVENT(napi_poll,
> +
>  	TP_PROTO(struct napi_struct *napi),
> -	TP_ARGS(napi));
> +
> +	TP_ARGS(napi),
> +
> +	TP_STRUCT__entry(
> +		__field(        struct napi_struct *,	napi)
> +        ),
> +
> +        TP_fast_assign(
> +                __entry->napi = napi;
> +        ),
> +
> +        TP_printk("napi poll on napi struct %p for device %s",
> +		__entry->napi, __entry->napi->dev->name)

You can't trust this! That "__entry" happens to reside on the ring
buffer. If for some reason the device goes away, this blows up when you
read the trace.

If you need to save the name of the device, then store it in the ring
buffer. You can do it with a dynamic array:

	TP_STRUCT__entry(
		__field(	struct napi_struct *, napi)
		__string(	dev_name, napi->dev->name)
	),

	TP_fast_assign(
		__entry->napi = napi;
		__assign_str(dev_name, napi->dev->name);
	),

	TP_printk("napi poll on napi struct %p for device %s",
		__entry->napi, __get_string(dev_name))

-- Steve

> +);
>  
>  #endif


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