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

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Date:   Tue, 29 Aug 2023 17:57:31 -0500
From:   Stephen Boyd <swboyd@...omium.org>
To:     Michał Mirosław <mirq-linux@...e.qmqm.pl>
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()

Quoting Michał Mirosław (2023-08-29 14:25:46)
> 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?
>

Squash this in?

---8<---
diff --git a/drivers/regulator/core.c b/drivers/regulator/core.c
index 9736507b62ff..39205cf00fb7 100644
--- a/drivers/regulator/core.c
+++ b/drivers/regulator/core.c
@@ -201,6 +201,7 @@ static int regulator_lock_two(struct regulator_dev *rdev1,
 			      struct regulator_dev *rdev2,
 			      struct ww_acquire_ctx *ww_ctx)
 {
+	struct regulator_dev *held, *contended;
 	int ret;

 	ww_acquire_init(ww_ctx, &regulator_ww_class);
@@ -208,22 +209,24 @@ static int regulator_lock_two(struct regulator_dev *rdev1,
 	ret = regulator_lock_nested(rdev1, ww_ctx);
 	if (WARN_ON(ret))
 		goto exit;
+	held = rdev1;
+	contended = rdev2;

 	while (true) {
-		ret = regulator_lock_nested(rdev2, ww_ctx);
+		ret = regulator_lock_nested(contended, ww_ctx);
 		if (!ret)
-			return 0;
+			break;

-		regulator_unlock(rdev1);
+		regulator_unlock(held);

 		if (WARN_ON(ret != -EDEADLOCK))
 			break;

-		swap(rdev1, rdev2);
+		ww_mutex_lock_slow(&contended->mutex, ww_ctx);
+		contended->ref_cnt++;
+		contended->mutex_owner = current;

-		ww_mutex_lock_slow(&rdev1->mutex, ww_ctx);
-		rdev1->ref_cnt++;
-		rdev1->mutex_owner = current;
+		swap(held, contended);
 	}

 exit: