linux-kernel - Re: [PATCH] rcu: remove unused variable in boot_cpu_state

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Message-ID: <20170622072644.so23jacjhvrox5pd@gmail.com>
Date:   Thu, 22 Jun 2017 09:26:44 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Anna-Maria Gleixner <anna-maria@...utronix.de>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rcu: remove unused variable in boot_cpu_state_init


* Paul E. McKenney <paulmck@...ux.vnet.ibm.com> wrote:

> On Wed, Jun 21, 2017 at 11:57:28PM +0200, Arnd Bergmann wrote:
> > Without CONFIG_SMP, we get a harmless warning about
> > an unused variable:
> > 
> > kernel/cpu.c: In function 'boot_cpu_state_init':
> > kernel/cpu.c:1778:6: error: unused variable 'cpu' [-Werror=unused-variable]
> > 
> > This reworks the function to have the declaration inside
> > of the #ifdef.
> > 
> > Fixes: faeb334286b7 ("rcu: Migrate callbacks earlier in the CPU-offline timeline")
> > Signed-off-by: Arnd Bergmann <arnd@...db.de>
> 
> I simply added a __maybe_unused in 6441c656acde ("rcu: Migrate callbacks
> earlier in the CPU-offline timeline") in my -rcu tree.  However, your
> approach does have the advantage of complaining if the code using that
> variable is removed.
> 
> So, would you be OK with my folding your approach into my commit with
> attribution?

Also, note that __maybe_unused can be dangerous: it can hide a build warning where 
there's a _real_ unused variable bug now or due to future changes, causing a real 
runtime bug.

So I think we should consider it a syntactic construct to avoid.

Thanks,

	Ingo