linux-kernel - Re: [PATCH] seq_file: remove redundant assignment of index to m->index

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Date:   Sat, 10 Feb 2018 17:02:01 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     Joe Perches <joe@...ches.com>
Cc:     Donglin Peng <dolinux.peng@...il.com>, viro@...iv.linux.org.uk,
        linux-fsdevel@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] seq_file: remove redundant assignment of index to
 m->index

On Sat, Feb 10, 2018 at 10:04:23AM -0800, Joe Perches wrote:
> > @@ -120,14 +120,12 @@ static int traverse(struct seq_file *m, loff_t offset)
> >          if (pos + m->count > offset) {
> >              m->from = offset - pos;
> >              m->count -= m->from;
> > -            m->index = index;
> >              break;
> >          }
> >          pos += m->count;
> >          m->count = 0;
> >          if (pos == offset) {
> >              index++;
> > -            m->index = index;
> >              break;
> >          }
> >          p = m->op->next(m, p, &index);
> 
> Of course this looks correct, but how
> are you _absolutely sure_ about this?
> 
> Perhaps the m->op->stop(m, p) call below
> the break, which takes m as an argument,
> needs an updated m->index.

Not only that, but ->next might also look at m->index.
This is not performance critical; don't try to optimise it.

  Programmers waste enormous amounts of time thinking about, or worrying
  about, the speed of noncritical parts of their programs, and these
  attempts at efficiency actually have a strong negative impact when
  debugging and maintenance are considered. We should forget about small
  efficiencies, say about 97% of the time: premature optimization is the
  root of all evil. Yet we should not pass up our opportunities in that
  critical 3%.  -- Donald Knuth