linux-kernel - Re: [PATCH 11/12] sched: use for_each

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Date:   Mon, 9 Jul 2018 17:12:32 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        DRI Development <dri-devel@...ts.freedesktop.org>,
        Intel Graphics Development <intel-gfx@...ts.freedesktop.org>,
        Daniel Vetter <daniel.vetter@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 11/12] sched: use for_each_if in topology.h

On Mon, Jul 09, 2018 at 06:03:42PM +0200, Peter Zijlstra wrote:
> On Mon, Jul 09, 2018 at 05:52:04PM +0200, Daniel Vetter wrote:
> > for_each_something(foo)
> > 	if (foo->bla)
> > 		call_bla(foo);
> > 	else
> > 		call_default(foo);
> > 
> > Totally contrived, but this complains. Liberally sprinkling {} also shuts
> > up the compiler, but it's a bit confusing given that a plain for {;;} is
> > totally fine. And it's confusing since at first glance the compiler
> > complaining about nested if and ambigous else doesn't make sense since
> > clearly there's only 1 if there.
> 
> Ah, so the pattern the compiler tries to warn about is:
> 
> 	if (foo)
> 		if (bar)
> 			/* stmts1 */
> 		else
> 			/* stmts2 *
> 
> Because it might not be immediately obvious with which if the else goes.
> Which is fair enough I suppose.
> 
> OK, ACK.

Just to bikeshed, there could be macros other than for_each_*() macros
that will want to use this internally, so perhaps it would be worth the
generic version being named something like if_noelse().

We could always add that as/when required, though.

Mark.