linux-kernel - Re: [PATCH v4] tracing/uprobes: Support ftrace_event

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Date:	Mon, 1 Jul 2013 22:27:22 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	"zhangwei(Jovi)" <jovi.zhangwei@...wei.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4] tracing/uprobes: Support ftrace_event_file base
	multibuffer

On 06/29, zhangwei(Jovi) wrote:
>
> [v3->v4]:

I am wondering how much you will hate me if I suggest to make v5 ;)

But look, imho probe_event_enable() looks a bit more confusing than
it needs.

> -probe_event_enable(struct trace_uprobe *tu, int flag, filter_func_t filter)
> +probe_event_enable(struct trace_uprobe *tu, struct ftrace_event_file *file,
> +		   filter_func_t filter)
>  {
> +	bool enabled = is_trace_uprobe_enabled(tu);
> +	struct event_file_link *link;
>  	int ret = 0;

Unnecessary initialization.

> -	if (is_trace_uprobe_enabled(tu))
> -		return -EINTR;
> +	if (file) {
> +		if (tu->flags & TP_FLAG_PROFILE)
> +			return -EINTR;
> +
> +		link = kmalloc(sizeof(*link), GFP_KERNEL);
> +		if (!link)
> +			return -ENOMEM;
> +
> +		link->file = file;
> +		list_add_tail_rcu(&link->list, &tu->files);
> +
> +		tu->flags |= TP_FLAG_TRACE;
> +	} else {
> +		if (tu->flags & TP_FLAG_TRACE)
> +			return -EINTR;
> +
> +		tu->flags |= TP_FLAG_PROFILE;
> +	}
> 
>  	WARN_ON(!uprobe_filter_is_empty(&tu->filter));
> 
> -	tu->flags |= flag;
> -	tu->consumer.filter = filter;
> -	ret = uprobe_register(tu->inode, tu->offset, &tu->consumer);
> -	if (ret)
> -		tu->flags &= ~flag;
> +	/* we cannot call uprobe_register twice for same tu */

The comment is confusing, I'd suggest to simply remove it.

Yes, we can't do uprobe_register() twice as we already discussed.
But it is not that we "can't", we simply do not need this if uprobe
was already created.

> +	if (!enabled) {
> +		tu->consumer.filter = filter;
> +		ret = uprobe_register(tu->inode, tu->offset, &tu->consumer);
> +	}
> +
> +	if (ret) {
> +		if (file) {
> +			list_del_rcu(&link->list);

I won't insist, but _rcu is not needed in this case. Again, this looks
a bit confusing, as if we expect that some rcu reader can ever see this
entry. But this is not true and we are going to just kfree it without
synchronize_rcu().

> +			kfree(link);
> +			tu->flags &= ~TP_FLAG_TRACE;
> +		} else
> +			tu->flags &= ~TP_FLAG_PROFILE;
> +	}

This is correct, but again, this is not immediately obvious.

Why it is correct to correct to clear TP_FLAG_TRACE? Because we know
that "enabled" was false and thus we remove the single list entry.

So, perhaps,

	if (enabled)
		return 0;

	ret = uprobe_register();
	if (ret) {
		...;
	}

	return ret;

will be a bit more clean.

Oleg.

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