linux-kernel - Re: [PATCH 6/6] regulator: core: simplify lock

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Message-ID: <ZO5iWlczXmX7wNn9@qmqm.qmqm.pl>
Date:   Tue, 29 Aug 2023 23:25:46 +0200
From:   Michał Mirosław <mirq-linux@...e.qmqm.pl>
To:     Stephen Boyd <swboyd@...omium.org>
Cc:     Doug Anderson <dianders@...omium.org>,
        Liam Girdwood <lgirdwood@...il.com>,
        Mark Brown <broonie@...nel.org>, linux-kernel@...r.kernel.org,
        Dmitry Osipenko <digetx@...il.com>
Subject: Re: [PATCH 6/6] regulator: core: simplify lock_two()

On Tue, Aug 29, 2023 at 03:52:19PM -0500, Stephen Boyd wrote:
> Quoting Michał Mirosław (2023-08-28 13:26:54)
> > Indeed they are quite similar. I did remove a bit more code than that,
> > though: in this case there is no early success return before the loop.
> >
> > Instead of saying:
> >
> > lock A
> > lock B
> > if ok return
> > if that failed, loop:
> >   unlock A
> >   lock B harder
> >   lock A
> >   if ok return
> >   swap A <-> B
> >   lock B
> >
> > Now it's:
> >
> > lock A
> > loop forever:
> >   lock B
> >   if ok, return
> >   unlock A
> >   swap them
> >   lock A harder
> >
> > With the same condition 'A held' at the start of an iteration.
> >
> 
> Removing duplicate code is great! I'm primarily concerned with
> readability. The terms 'A' and 'B' doesn't make it easy for me. Can you
> maintain the 'held' and 'contended' names for the variables?
> 
> That would be
> 
> 1.  lock 'held'
> 2.  loop forever:
> 3.    lock 'contended'
> 4.    if ok, return
> 5.    unlock 'held'
> 6.    swap them
> 7.    lock 'held' harder

Doesn't this make it more confusing? The lock is 'held' only in lines
2-5 and looses this trait (but not the name) on the other lines.
'contended' is more problematic: the contended lock is called 'held'
before locking it at line 7.

The algorithm is basically: Take the locks in sequence. If that failed,
swap the order and try again.

Would a comment like the sentence above help with readability?

Or we could wrap the final lines of the iteration in a
'regulator_lock_contended()' to make it self-documenting?

Best Regards
Michał Mirosław