linux-kernel - Re: [PATCH v3] spin_lock_nested(): Always evaluate second argument

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Date:	Fri, 8 Aug 2014 14:52:50 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Bart Van Assche <bvanassche@....org>
cc:	Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"David S. Miller" <davem@...emloft.net>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3] spin_lock_nested(): Always evaluate second argument

On Fri, 8 Aug 2014, Bart Van Assche wrote:

> diff --git a/include/linux/spinlock.h b/include/linux/spinlock.h
> index 3f2867f..262ba4e 100644
> --- a/include/linux/spinlock.h
> +++ b/include/linux/spinlock.h
> @@ -197,7 +197,13 @@ static inline void do_raw_spin_unlock(raw_spinlock_t *lock) __releases(lock)
>  		 _raw_spin_lock_nest_lock(lock, &(nest_lock)->dep_map);	\
>  	 } while (0)
>  #else
> -# define raw_spin_lock_nested(lock, subclass)		_raw_spin_lock(lock)
> +/*
> + * Always evaluate the 'subclass' argument to avoid that the compiler
> + * warns about set-but-not-used variables when building with
> + * CONFIG_DEBUG_LOCK_ALLOC=n and with W=1.
> + */

I was hoping there was going to be a more important reason for this change 
than to avoid compiler warnings, such as an example where someone is doing 
spin_lock_nested(lock, subclass) and the expression for "subclass" 
requires evaluation in all configs.

> +# define raw_spin_lock_nested(lock, subclass)		\
> +	_raw_spin_lock(((void)(subclass), (lock)))
>  # define raw_spin_lock_nest_lock(lock, nest_lock)	_raw_spin_lock(lock)
>  #endif
>  
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