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From: Thomas Gleixner <tglx@linutronix.de>
To: "Gustavo A. R. Silva" <garsilva@embeddedor.com>
Cc: Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
	x86@kernel.org, linux-kernel@vger.kernel.org,
	Kees Cook <keescook@chromium.org>
Subject: Re: [PATCH] x86/syscalls: Mark expected switch fall-throughs
Date: Tue, 28 Nov 2017 19:17:37 +0100 (CET)	[thread overview]
Message-ID: <alpine.DEB.2.20.1711281911280.2222@nanos> (raw)
In-Reply-To: <alpine.DEB.2.20.1711281908040.2222@nanos>

On Tue, 28 Nov 2017, Thomas Gleixner wrote:
> On Tue, 28 Nov 2017, Gustavo A. R. Silva wrote:
> > Quoting Thomas Gleixner <tglx@linutronix.de>:
> > 
> > > On Mon, 27 Nov 2017, Gustavo A. R. Silva wrote:
> > > 
> > > > In preparation to enabling -Wimplicit-fallthrough, mark switch cases
> > > > where we are expecting to fall through.
> > > 
> > > > 		case 0:
> > > > 			if (!n--) break;
> > > > 			*args++ = regs->bx;
> > > > +			/* fall through */
> > > 
> > > And these gazillions of pointless comments help enabling of
> > > -Wimplicit-fallthrough in which way?
> > > 
> > 
> > The -Wimplicit-fallthrough option was added to GCC 7. We want to add that
> > option to the top-level Makefile so we can have the compiler help us not make
> > mistakes as missing "break"s or "continue"s. This also documents the intention
> > for humans and provides a way for analyzers to report issues or ignore False
> > Positives.
> > 
> > So prior to adding such option to the Makefile, we have to properly add a code
> > comment wherever the code is intended to fall through.
> > 
> > During the process of placing these comments I have identified actual bugs
> > (missing "break"s/"continue"s) in a variety of components in the kernel, so I
> > think this effort is valuable. Lastly, such a simple comment in the code can
> > save a person plenty of time during a code review.
> 
> To be honest, such comments annoy me during a code review especially when
> the fallthrough is so obvious as in this case. There might be cases where
> its worth to document because it's non obvious, but documenting the obvious
> just for the sake of documenting it is just wrong.

And _IF_ at all then you want a fixed macro for this and not a comment
which will be formatted as people see it fit.

GCC supports: __attribute__ ((fallthrough)) which we can wrap into a macro,
e.g. falltrough()

That'd be useful, but adding all these comments and then having to chase a
gazillion of warning instances to figure out whether there is a comment or
not is just backwards.

Sure, but slapping a comment everywhere is just simpler than reading the
documentation and make something useful and understandable.

Thanks,

	tglx

  reply	other threads:[~2017-11-28 18:17 UTC|newest]

Thread overview: 27+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-11-27 23:52 Gustavo A. R. Silva
2017-11-28 13:49 ` Thomas Gleixner
2017-11-28 18:05   ` Gustavo A. R. Silva
2017-11-28 18:10     ` Thomas Gleixner
2017-11-28 18:17       ` Thomas Gleixner [this message]
2017-11-28 18:22         ` Gustavo A. R. Silva
2017-11-28 18:27           ` Thomas Gleixner
2017-11-28 18:35             ` Thomas Gleixner
2017-11-28 18:45               ` Thomas Gleixner
2017-11-28 18:53                 ` Gustavo A. R. Silva
2017-11-28 19:48                   ` Thomas Gleixner
2017-11-28 19:00               ` Alan Cox
2017-11-28 19:10                 ` Linus Torvalds
2017-11-28 19:59                   ` Joe Perches
2017-11-28 20:08                   ` Thomas Gleixner
2017-11-28 20:34                     ` Kees Cook
2017-11-28 20:37                   ` Gustavo A. R. Silva
2017-11-29  1:07                     ` Joe Perches
2017-11-29  8:20                       ` Geert Uytterhoeven
2017-11-28 20:11                 ` Thomas Gleixner
2017-11-28 20:25                   ` Gustavo A. R. Silva
2017-11-28 21:25                     ` Thomas Gleixner
2017-11-29 15:10                       ` Gustavo A. R. Silva
2017-11-29 15:14                         ` Thomas Gleixner
2017-11-30  0:21                           ` Kees Cook
2019-01-29 23:56 Gustavo A. R. Silva
2019-01-30  0:14 ` Thomas Gleixner

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