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

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Message-ID: <20170622075104.dlajhtcx7p3sgacs@gmail.com>
Date:   Thu, 22 Jun 2017 09:51:04 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        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 Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] rcu: remove unused variable in boot_cpu_state_init


* Arnd Bergmann <arnd@...db.de> wrote:

> > So I think we should consider it a syntactic construct to avoid.
> 
> Unused variables are relatively harmless compared to used-uninitialized
> variables that are always bugs (though they are provably impossible to
> detect correctly in some cases).

So the thing I was most worried about was that old GCC used to not warn about:

	long __maybe_unused error;

	...

	if (error)
		return error;

... but recent GCC does warn if it's certain that the use is uninitialized, so the 
scenario I outlined should not happen.

But it will supress the warning if the variable is uninitialized but GCC cannot 
prove it for sure, so my point remains that it's a potentially dangerous 
construct.

Thanks,

	Ingo