linux-kernel - Re: [PATCH] lockdep: avoid bogus clang warning

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Date:   Mon, 25 Mar 2019 09:12:55 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Will Deacon <will.deacon@....com>,
        clang-built-linux@...glegroups.com,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        Bart Van Assche <bvanassche@....org>,
        Waiman Long <longman@...hat.com>,
        "Joel Fernandes (Google)" <joel@...lfernandes.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] lockdep: avoid bogus clang warning

On Mon, 25 Mar 2019 13:57:57 +0100
Arnd Bergmann <arnd@...db.de> wrote:

> When lockdep is enabled, and -Wuninitialized warnings are enabled,
> clang produces a silly warning for every file we compile:
> 
> In file included from  kernel/sched/fair.c:23:
>  kernel/sched/sched.h:1094:15: error: variable 'cookie' is uninitialized when used here [-Werror,-Wuninitialized]
>         rf->cookie = lockdep_pin_lock(&rq->lock);
>                      ^~~~~~~~~~~~~~~~~~~~~~~~~~~

Sad it warns as struct pin_cookie is of size zero. But hey.

>  include/linux/lockdep.h:474:60: note: expanded from macro 'lockdep_pin_lock'
>  #define lockdep_pin_lock(l)                     ({ struct pin_cookie cookie; cookie; })
>                                                                              ^~~~~~
>  kernel/sched/sched.h:1094:15: note: variable 'cookie' is declared here
>  include/linux/lockdep.h:474:34: note: expanded from macro 'lockdep_pin_lock'
>  #define lockdep_pin_lock(l)                     ({ struct pin_cookie cookie; cookie; })
>                                                    ^
> As the 'struct pin_cookie' structure is empty in this configuration,
> there is no need to initialize it for correctness, but it also
> does not hurt to set it to an empty structure, so do that to
> avoid the warning.
> 
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---
>  include/linux/lockdep.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/lockdep.h b/include/linux/lockdep.h
> index 79c3873d58ac..31d7549933eb 100644
> --- a/include/linux/lockdep.h
> +++ b/include/linux/lockdep.h
> @@ -471,7 +471,7 @@ struct pin_cookie { };
>  
>  #define NIL_COOKIE (struct pin_cookie){ }
>  
> -#define lockdep_pin_lock(l)			({ struct pin_cookie cookie; cookie; })
> +#define lockdep_pin_lock(l)			({ struct pin_cookie cookie = {}; cookie; })

Reviewed-by: Steven Rostedt (VMware) <rostedt@...dmis.org>

-- Steve

>  #define lockdep_repin_lock(l, c)		do { (void)(l); (void)(c); } while (0)
>  #define lockdep_unpin_lock(l, c)		do { (void)(l); (void)(c); } while (0)
>