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

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Message-ID: <CAK8P3a3_yRSYq0nezytp632Yz6gHgOqTm+csCXpLvO2XqyXWDg@mail.gmail.com>
Date:   Thu, 22 Jun 2017 10:58:56 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Ingo Molnar <mingo@...nel.org>
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

On Thu, Jun 22, 2017 at 9:59 AM, Ingo Molnar <mingo@...nel.org> wrote:
>
> So, to continue this side thought about uninitialized_var(), it is dangerous
> because the following buggy pattern does not generate a compiler warning:
>
>         long uninitialized_var(error);
>
>         ...
>
>         if (error)
>                 return error;
>
>
> ... and still there are over 290 uses of uninitialized_var() in the kernel - and
> any of them could turn into a silent but real uninitialized variable bugs due to
> subsequent changes.

Right, absolutely agreed on that. A related problem however is blindly
initializing variables to NULL to get rid of uninitialized variable warnings,
such as

      struct subsystem_specific *obj = NULL;
      if (function_argument > 10)
              goto err;
     obj = create_obj();
...
err:
      clean_up(obj->member);


I've seen a couple of variations of that problem, so simply outlawing
uninitialized_var() will only solve a subset of these issues, and ideally
we should also make sure that initializations at declaration time are
used properly, and not just to shut up compiler warnings.

      Arnd