linux-kernel - Re: [PATCH 4/9] ftrace: Add enable/disable ftrace

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Date:	Mon, 28 Nov 2011 15:21:33 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Jiri Olsa <jolsa@...hat.com>
Cc:	fweisbec@...il.com, mingo@...hat.com, paulus@...ba.org,
	acme@...stprotocols.net, linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 4/9] ftrace: Add enable/disable ftrace_ops control
 interface


>  
> @@ -311,6 +321,9 @@ static int __unregister_ftrace_function(struct ftrace_ops *ops)
>  	if (ret < 0)
>  		return ret;
>  
> +	if (ops->flags & FTRACE_OPS_FL_CONTROL)
> +		jump_label_dec(&ftrace_ops_control);
> +
>  	if (ftrace_enabled)
>  		update_ftrace_function();
>  
> @@ -3577,8 +3590,14 @@ ftrace_ops_list_func(unsigned long ip, unsigned long parent_ip)
>  	preempt_disable_notrace();
>  	op = rcu_dereference_raw(ftrace_ops_list);
>  	while (op != &ftrace_list_end) {
> +		if (static_branch(&ftrace_ops_control))

Instead of doing a static_branch() here, which makes me really nervous,
because this is called in function trace context, which has some strict
rules of its own, and is probably prone to recursion, we could add
another "ops" similar to the global_ops.

We could make a control_ops, and add all ops with the
FTRACE_OPS_FL_CONTROL flag set to it. And then this function will have
its own loop that it will check the disabled flag, for the ops
registered to it.

This code doesn't need to be touched, we just add a layer of redirection
for control ops and it will solve the jump_label issue.

-- Steve

> +			if ((op->flags & FTRACE_OPS_FL_CONTROL) &&
> +			    atomic_read(&op->disabled))
> +				goto next;
> +
>  		if (ftrace_ops_test(op, ip))
>  			op->func(ip, parent_ip);
> + next:
>  		op = rcu_dereference_raw(op->next);
>  	};
>  	preempt_enable_notrace();


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