linux-kernel - Re: PATCH? trace_remove_event_call() should fail if call is active

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Date:	Tue, 02 Jul 2013 18:49:25 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	"zhangwei(Jovi)" <jovi.zhangwei@...wei.com>,
	Jiri Olsa <jolsa@...hat.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: PATCH? trace_remove_event_call() should fail if call is active

On Wed, 2013-07-03 at 00:23 +0200, Oleg Nesterov wrote:
> On 07/02, Oleg Nesterov wrote:
> >
> > So please ignore modules ;)
> 
> Or lets discuss the change above.

I like this better than your first change, as the module code really
can't fail to remove events, as that will complicate things more than
they already are ;-)

> 
> Oleg.
> 
> --- x/kernel/trace/trace_events.c
> +++ x/kernel/trace/trace_events.c
> @@ -1611,14 +1611,40 @@ static void __trace_remove_event_call(st
>  	destroy_preds(call);
>  }
>  
> +static int event_can_remove(struct ftrace_event_call *call)

Should rename this to "event_is_busy" as we are only seeing if the event
is active or not, as we do allow for events to be removed when active.
This may confuse new reviewers.

> +{
> +	struct trace_array *tr;
> +	struct ftrace_event_file *file;
> +
> +#ifdef CONFIG_PERF_EVENTS
> +	if (call->perf_refcount)
> +		return -EBUSY;
> +#endif
> +	do_for_each_event_file(tr, file) {
> +		if (file->event_call != call)
> +			continue;
> +		if (file->flags & FTRACE_EVENT_FL_ENABLED)
> +			return -EBUSY;
> +		break;
> +	} while_for_each_event_file();
> +
> +	return 0;
> +}
> +
>  /* Remove an event_call */
> -void trace_remove_event_call(struct ftrace_event_call *call)
> +int trace_remove_event_call(struct ftrace_event_call *call)

Probably should rename this to trace_probe_remove_event_call() as this
deals only with the probe callers (kprobe and uprobe). Also this code
will need to be documented a bit more.

Other than that, sure, go with it. Masami, you have any comments?

-- Steve

>  {
> +	int err;
> +
>  	mutex_lock(&event_mutex);
>  	down_write(&trace_event_sem);
> -	__trace_remove_event_call(call);
> +	err = event_can_remove(call);
> +	if (!err)
> +		__trace_remove_event_call(call);
>  	up_write(&trace_event_sem);
>  	mutex_unlock(&event_mutex);
> +
> +	return err;
>  }
>  
>  #define for_each_event(event, start, end)			\


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