linux-kernel - Re: [PATCH] pinctrl: ingenic: Make unreachable path more robust

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Message-ID: <20200217151804.yymflofpbiqjqnnz@treble>
Date:   Mon, 17 Feb 2020 09:18:04 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Paul Cercueil <paul@...pouillou.net>
Cc:     Linus Walleij <linus.walleij@...aro.org>,
        linux-gpio@...r.kernel.org, linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>
Subject: Re: [PATCH] pinctrl: ingenic: Make unreachable path more robust

On Fri, Feb 14, 2020 at 11:37:04PM -0300, Paul Cercueil wrote:
> > >  I don't like the idea that you change this driver's code just to
> > > work around
> > >  a bug in objtool, and I don't like the idea of working around a
> > > future bug
> > >  that shouldn't be introduced in the first place.
> > 
> > It's not an objtool bug.  It's a byproduct of the fact that GCC's
> > undefined behavior is inscrutable, and there's no way to determine that
> > it actually *wants* to jump to a random function.
> > 
> > And anyway, regardless of objtool, the patch is meant to make the code
> > more robust.
> > 
> > Do you not agree that BUG (defined behavior) is more robust than
> > unreachable (undefined behavior)?
> 
> It's a dead code path. That would be an undefined behaviour, if it was
> taken, but it's not.

Given your confidence that humans don't introduce bugs, would you
recommend that we

  s/BUG()/unreachable()/

tree-wide?

Another option would be to remove the unreachable() statement, which
would actually improve the generated code by making it more compact (16
bytes of i-cache savings), on top of removing the "fallthrough to next
function" nastiness.

diff --git a/drivers/pinctrl/pinctrl-ingenic.c b/drivers/pinctrl/pinctrl-ingenic.c
index 96f04d121ebd..13c7d3351ed5 100644
--- a/drivers/pinctrl/pinctrl-ingenic.c
+++ b/drivers/pinctrl/pinctrl-ingenic.c
@@ -2158,7 +2158,8 @@ static int ingenic_pinconf_set(struct pinctrl_dev *pctldev, unsigned int pin,
 			break;
 
 		default:
-			unreachable();
+			/* unreachable */
+			break;
 		}
 	}