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Date: Wed, 2 Sep 2015 10:55:13 -0300
From: Arnaldo Carvalho de Melo <acme@redhat.com>
To: pi3orama <pi3orama@163.com>
Cc: Jiri Olsa <jolsa@redhat.com>, "Wangnan (F)" <wangnan0@huawei.com>,
        "masami.hiramatsu.pt@hitachi.com" <masami.hiramatsu.pt@hitachi.com>,
        Alexei Starovoitov <ast@plumgrid.com>, Jiri Olsa <jolsa@kernel.org>,
        Namhyung Kim <namhyung@kernel.org>, Zefan Li <lizefan@huawei.com>,
        acme@kernel.org,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] perf tools: Don't write to evsel if parser doesn't
 collect evsel
Message-ID: <20150902135513.GJ22331@redhat.com>
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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@redhat.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.