linux-kernel - Re: [PATCH 28/30] perf evsel: Provide a new constructor for tracepoints

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Date:	Tue, 25 Sep 2012 13:26:46 +0900
From:	Namhyung Kim <namhyung@...nel.org>
To:	Arnaldo Carvalho de Melo <acme@...radead.org>
Cc:	Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	David Ahern <dsahern@...il.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Jiri Olsa <jolsa@...hat.com>, Mike Galbraith <efault@....de>,
	Paul Mackerras <paulus@...ba.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Stephane Eranian <eranian@...gle.com>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH 28/30] perf evsel: Provide a new constructor for tracepoints

On Mon, 24 Sep 2012 12:59:42 -0300, Arnaldo Carvalho de Melo wrote:
> From: Arnaldo Carvalho de Melo <acme@...hat.com>
>
> The existing constructor receives a perf_event_attr filled with the
> event type and the config.
>
> To reduce the boilerplate for tracepoints, provide a new constructor,
> perf_evsel__newtp() that receives the tracepoint name and will open
> the debugfs file, call into libtraceevent new pevent_parse_format file
> to fill its ->tp_format member, so that users can then just call
> perf_evsel__field() to access its fields.
[snip]
> +static struct event_format *event_format__new(const char *sys, const char *name)
> +{
> +	int fd, n;
> +	char *filename;
> +	void *bf = NULL, *nbf;
> +	size_t size = 0, alloc_size = 0;
> +	struct event_format *format = NULL;
> +
> +	if (asprintf(&filename, "%s/%s/%s/format", tracing_events_path, sys, name) < 0)
> +		goto out;
> +
> +	fd = open(filename, O_RDONLY);
> +	if (fd < 0)
> +		goto out_free_filename;
> +
> +	do {
> +		if (size == alloc_size) {
> +			alloc_size += BUFSIZ;
> +			nbf = realloc(bf, alloc_size);
> +			if (nbf == NULL)
> +				goto out_free_bf;
> +			bf = nbf;
> +		}
> +
> +		n = read(fd, bf + size, BUFSIZ);

Wouldn't it be better doing s/BUFSIZ/alloc_size - size/ ?  Although
there'll be no partial reading issue when working on debugfs I guess.

Thanks,
Namhyung


> +		if (n < 0)
> +			goto out_free_bf;
> +		size += n;
> +	} while (n > 0);
> +
> +	pevent_parse_format(&format, bf, size, sys);
> +
> +out_free_bf:
> +	free(bf);
> +	close(fd);
> +out_free_filename:
> +	free(filename);
> +out:
> +	return format;
> +}
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