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

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Message-ID: <20140809061712.GL9918@twins.programming.kicks-ass.net>
Date:	Sat, 9 Aug 2014 08:17:12 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Bart Van Assche <bvanassche@....org>,
	Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"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, Aug 08, 2014 at 02:52:50PM -0700, David Rientjes wrote:
> 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.

That would stink, having that argument have side effects. I'd call that
a plain old bug that needs fixing.



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