linux-kernel - Re: [PATCH 00/19] Enable -Wshadow=local for kernel/sched

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Message-ID: <Yh/C7nnElPOL3G/k@hirez.programming.kicks-ass.net>
Date:   Wed, 2 Mar 2022 20:18:06 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Kees Cook <keescook@...omium.org>, Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 00/19] Enable -Wshadow=local for kernel/sched

On Wed, Mar 02, 2022 at 06:43:57PM +0000, Matthew Wilcox wrote:
> ie "__ret = freezable_schedule_timeout(__ret)" is supposed to refer to
> the inner __ret, not the outer __ret.  Which was the opposite of what
> I thought was supposed to happen.
> 
> We can fix this, of course.  Something like ...
> 
> #define ___wait_event_freezable_timeout(wq_head, condition, timeout, ret) \
> 	___wait_event(wq_head, ___wait_cond_timeout(condition, ret),      \
> 		TASK_INTERRUPTIBLE, 0, timeout,				  \
> 		ret = freezable_schedule_timeout(ret), ret)
> 
> #define __wait_event_freezable_timeout(wq_head, condition, timeout) \
> 	___wait_event_freezable_timeout(wq_head, condition, timeout, UNIQUE_ID)
> 
> ... and now all the 'ret' refer to the thing that they look like they're
> referring to.

Right; so the trick is to make sure all ___wait_event() users will have
a ret and then the inner ret can go away. The interruptible/timeout
variants all already have a ret variable, but the unconditional things
like wait_event() do not (which is where all the trouble started).

By simply adding a ret variable, even to the variants without return
value, the inner variable can go away and the shadowing goes away.