linux-kernel - Re: [PATCH v2] tracing: Ignore mmiotrace from kernel commandline

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Date:   Fri, 8 Sep 2017 10:38:52 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     "Ziqian SUN (Zamir)" <zsun@...hat.com>
Cc:     linux-kernel@...r.kernel.org, mingo@...hat.com,
        karolherbst@...il.com, ppaalanen@...il.com,
        akpm@...ux-foundation.org
Subject: Re: [PATCH v2] tracing: Ignore mmiotrace from kernel commandline

On Wed, 30 Aug 2017 12:19:30 +0800
"Ziqian SUN (Zamir)" <zsun@...hat.com> wrote:

> From: "Ziqian SUN (Zamir)" <zsun@...hat.com>
> 
> The mmiotrace tracer cannot be enabled with ftrace=mmiotrace in
> kernel commandline. With this patch, a trace_noboot_tracer_list
> is implemented and will be checked during system boot. If the
> tracer declares itself as not for boot up, system will print out
> a message and continue booting.
> 
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=196557
> Signed-off-by: Ziqian SUN (Zamir) <zsun@...hat.com>
> ---
>  kernel/trace/trace.c           | 34 +++++++++++++++++++++++++++++++++-
>  kernel/trace/trace.h           | 11 +++++++++++
>  kernel/trace/trace_mmiotrace.c |  4 ++++
>  3 files changed, 48 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index 44004d8..0d25b1e 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -319,6 +319,33 @@ void trace_free_pid_list(struct trace_pid_list *pid_list)
>  	kfree(pid_list);
>  }
>  
> +struct mutex trace_noboot_tracer_lock;
> +
> +DEFINE_MUTEX(trace_noboot_tracer_lock);
> +LIST_HEAD(trace_noboot_tracer_list);
> +
> +void trace_noboot_tracer_put(struct tracer_noboot *noboot_tracer)
> +{
> +	mutex_lock(&trace_noboot_tracer_lock);
> +	list_add(&noboot_tracer->list, &trace_noboot_tracer_list);
> +	mutex_unlock(&trace_noboot_tracer_lock);
> +}
> +
> +bool trace_noboot_tracer_find(const char *str)
> +{
> +	mutex_lock(&trace_noboot_tracer_lock);
> +	struct tracer_noboot *ptr = NULL;
> +
> +	list_for_each_entry(ptr, &trace_noboot_tracer_list, list) {
> +		if (strcmp(ptr->name, str) == 0) {
> +			mutex_unlock(&trace_noboot_tracer_lock);
> +			return true;
> +		}
> +	}
> +	mutex_unlock(&trace_noboot_tracer_lock);
> +	return false;
> +}
> +
>  /**
>   * trace_find_filtered_pid - check if a pid exists in a filtered_pid list
>   * @filtered_pids: The list of pids to check
> @@ -1641,8 +1668,13 @@ int __init register_tracer(struct tracer *type)
>  	if (strncmp(default_bootup_tracer, type->name, MAX_TRACER_SIZE))
>  		goto out_unlock;
>  
> +	/* If the tracer should not be enabled by kernel parameter */
> +	if (trace_noboot_tracer_find(type->name)) {
> +		pr_warn("Tracer '%s' cannot be enabled during boot\n", type->name);
> +		goto out_unlock;
> +	}
> +
>  	printk(KERN_INFO "Starting tracer '%s'\n", type->name);
> -	/* Do we want this tracer to start on bootup? */
>  	tracing_set_tracer(&global_trace, type->name);
>  	default_bootup_tracer = NULL;
>  
> diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
> index 490ba22..23bc668 100644
> --- a/kernel/trace/trace.h
> +++ b/kernel/trace/trace.h
> @@ -502,6 +502,17 @@ enum {
>  	TRACE_IRQ_BIT,
>  };
>  
> +/*
> + * For tracer that cannot be enabled during boot time.
> + */
> +
> +struct tracer_noboot {
> +	const char		*name;
> +	struct list_head	list;
> +};
> +
> +void trace_noboot_tracer_put(struct tracer_noboot *noboot_tracer);
> +
>  #define trace_recursion_set(bit)	do { (current)->trace_recursion |= (1<<(bit)); } while (0)
>  #define trace_recursion_clear(bit)	do { (current)->trace_recursion &= ~(1<<(bit)); } while (0)
>  #define trace_recursion_test(bit)	((current)->trace_recursion & (1<<(bit)))
> diff --git a/kernel/trace/trace_mmiotrace.c b/kernel/trace/trace_mmiotrace.c
> index cd7480d..06aa0a5 100644
> --- a/kernel/trace/trace_mmiotrace.c
> +++ b/kernel/trace/trace_mmiotrace.c
> @@ -286,6 +286,10 @@ static enum print_line_t mmio_print_line(struct trace_iterator *iter)
>  
>  __init static int init_mmio_trace(void)
>  {
> +	struct tracer_noboot mmio_notrace;
> +
> +	mmio_notrace.name = "mmiotrace";
> +	trace_noboot_tracer_put(&mmio_notrace);
>  	return register_tracer(&mmio_tracer);
>  }
>  device_initcall(init_mmio_trace);

This is way overkill. Just add

	bool		noboot;

to struct tracer in trace.h and then to the mmiotracer:

struct tracer mmio_tracer = {
	[..]
	.noboot		= true,
};


And then in tracing_set_tracer() add after:

	if (t == tr->current_trace)
		goto out;

this:

	if (system_state < SYSTEM_RUNNING && t->noboot) {
		WARN(1, "Tracer %s not allowed on command line",
			t->name);
		goto out;
	}

That's it. No need for locking or anything else.

-- Steve