linux-kernel - Re: [PATCH 2/3] trace: fix default boot up tracer

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Date:	Mon, 2 Feb 2009 19:50:21 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Arjan van de Ven <arjan@...radead.org>,
	Steven Rostedt <srostedt@...hat.com>
Subject: Re: [PATCH 2/3] trace: fix default boot up tracer

On Mon, 02 Feb 2009 21:38:32 -0500 Steven Rostedt <rostedt@...dmis.org> wrote:

> @@ -510,8 +520,25 @@ int register_tracer(struct tracer *type)
>   out:
>  	tracing_selftest_running = false;
>  	mutex_unlock(&trace_types_lock);
> -	lock_kernel();
>  
> +	if (!ret && default_bootup_tracer) {
> +		if (!strncmp(default_bootup_tracer, type->name,
> +			     BOOTUP_TRACER_SIZE)) {
> +			printk(KERN_INFO "Starting tracer '%s'\n",
> +			       type->name);
> +			/* Do we want this tracer to start on bootup? */
> +			tracing_set_tracer(type->name);
> +			default_bootup_tracer = NULL;
> +			/* disable other selftests, since this will break it. */
> +			tracing_selftest_disabled = 1;
> +#ifdef CONFIG_FTRACE_STARTUP_TEST
> +			printk(KERN_INFO "Disabling FTRACE selftests due"
> +			       " to running tracer '%s'\n", type->name);
> +#endif
> +		}
> +	}
> +
> +	lock_kernel();
>  	return ret;
>  }

The fun and games which register_tracer() plays with lock_kernel() tell
us that this function is only called at bootup time and hence could be
__init.  A quick whizz through callers confirms that.

And if register_tracer() is also only callable at bootup, one suspects
that unregister_tracer() isn't useful.  And lo, it has no callers.

This leads one to further surmise that trace_types_lock a) could be
__initdata and b) could be removed (the list is only altered when we're
running single-threaded).  This appears to be the case.

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