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Date:   Wed, 2 Mar 2022 20:18:06 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Kees Cook <keescook@chromium.org>, Ingo Molnar <mingo@redhat.com>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 00/19] Enable -Wshadow=local for kernel/sched
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 <202203021030.EEEF58C2@keescook>
 <Yh+67VF0/OFSp15y@casper.infradead.org>
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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.