linux-kernel - Re: [PATCH] kernel.h: Add for_each

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Date:   Mon, 9 Jul 2018 16:30:01 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Daniel Vetter <daniel.vetter@...ll.ch>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        DRI Development <dri-devel@...ts.freedesktop.org>,
        Intel Graphics Development <intel-gfx@...ts.freedesktop.org>,
        Gustavo Padovan <gustavo@...ovan.org>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Sean Paul <seanpaul@...omium.org>,
        David Airlie <airlied@...ux.ie>,
        Kees Cook <keescook@...omium.org>,
        Ingo Molnar <mingo@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        NeilBrown <neilb@...e.com>, Wei Wang <wvw@...gle.com>,
        Stefan Agner <stefan@...er.ch>,
        Andrei Vagin <avagin@...nvz.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Yisheng Xie <ysxie@...mail.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Daniel Vetter <daniel.vetter@...el.com>
Subject: Re: [PATCH] kernel.h: Add for_each_if()

On Mon,  9 Jul 2018 18:25:09 +0200 Daniel Vetter <daniel.vetter@...ll.ch> wrote:

> To avoid compilers complainig about ambigious else blocks when putting
> an if condition into a for_each macro one needs to invert the
> condition and add a dummy else. We have a nice little convenience
> macro for that in drm headers, let's move it out. Subsequent patches
> will roll it out to other places.
> 
> The issue the compilers complain about are nested if with an else
> block and no {} to disambiguate which if the else belongs to. The C
> standard is clear, but in practice people forget:
> 
> if (foo)
> 	if (bar)
> 		/* something */
> 	else
> 		/* something else

um, yeah, don't do that.  Kernel coding style is very much to do

	if (foo) {
		if (bar)
			/* something */
		else
			/* something else
	}

And if not doing that generates a warning then, well, do that.

> The same can happen in a for_each macro when it also contains an if
> condition at the end, except the compiler message is now really
> confusing since there's only 1 if:
> 
> for_each_something()
> 	if (bar)
> 		/* something */
> 	else
> 		/* something else
> 
> The for_each_if() macro, by inverting the condition and adding an
> else, avoids the compiler warning.

Ditto.

> Motivated by a discussion with Andy and Yisheng, who want to add
> another for_each_macro which would benefit from for_each_if() instead
> of hand-rolling it.

Ditto.

> v2: Explain a bit better what this is good for, after the discussion
> with Peter Z.

Presumably the above was discussed in whatever-thread-that-was.