linux-kernel - Re: [PATCH] perf tools: Don't write to evsel if parser doesn't collect evsel

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Date:	Wed, 2 Sep 2015 10:55:13 -0300
From:	Arnaldo Carvalho de Melo <acme@...hat.com>
To:	pi3orama <pi3orama@....com>
Cc:	Jiri Olsa <jolsa@...hat.com>, "Wangnan (F)" <wangnan0@...wei.com>,
	"masami.hiramatsu.pt@...achi.com" <masami.hiramatsu.pt@...achi.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Jiri Olsa <jolsa@...nel.org>,
	Namhyung Kim <namhyung@...nel.org>,
	Zefan Li <lizefan@...wei.com>, acme@...nel.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] perf tools: Don't write to evsel if parser doesn't
 collect evsel

Em Wed, Sep 02, 2015 at 08:05:54PM +0800, pi3orama escreveu:
> 发自我的 iPhone
> > 在 2015年9月2日，下午7:54，Jiri Olsa <jolsa@...hat.com> 写道：
> >> On Wed, Sep 02, 2015 at 02:53:58PM +0800, Wangnan (F) wrote:
> >>> @@ -1252,7 +1262,13 @@ foreach_evsel_in_last_glob(struct perf_evlist *evlist,
> >>>    struct perf_evsel *last = NULL;
> >>>    int err;
> >>> -    if (evlist->nr_entries > 0)
> >>> +    /*
> >>> +     * Don't return when list_empty, give func a chance to report
> >>> +     * error when it found last == NULL.
> >>> +     *
> >>> +     * So no need to WARN here, let *func do this.
> >>> +     */
> >>> +    if (!list_empty(&evlist->entries))

> > why is it better than to check evlist->nr_entries?
> > evlist->nr_entries is equivalent to !list_empty(&evlist->entries) in here, right?
 
> By checking list we won't rely on the assumption that nr_entries reflects the
> actual number of elements in that list, makes the logic of this code more compact.

But why would we want to break that assumption?

If I see FOO->entries and FOO->nr_entries, it is reasonable to expect
that whatever data structure FOO->entries may be has FOO->nr_entries in
it, lets not break that assumption.

- Arnaldo

> Don't you think so?
> 
> At this point they are equivalent, but the whole patch is preventive action.
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