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Date:   Tue, 25 Jun 2019 11:07:09 -0400
From:   Qian Cai <cai@....pw>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     mingo@...hat.com, valentin.schneider@....com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] sched/core: silence a warning in sched_init()

On Tue, 2019-06-25 at 16:25 +0200, Peter Zijlstra wrote:
> On Tue, Jun 25, 2019 at 10:04:19AM -0400, Qian Cai wrote:
> > On Tue, 2019-06-25 at 15:52 +0200, Peter Zijlstra wrote:
> > Yes, -Wmissing-prototype makes no sense, but "-Wunused-but-set-variable" is
> > pretty valid to catch certain developer errors. For example,
> > 
> > https://lists.linuxfoundation.org/pipermail/iommu/2019-May/035680.html
> > 
> > > 
> > > As to this one, ideally the compiler would not be stupid, and understand
> > > the below, but alas.
> > 
> > Pretty sure that won't work, as the compiler will complain something like,
> > 
> > ISO C90 forbids mixed declarations and code
> 
> No, it builds just fine, it's a new block and C allows new variables at
> every block start -- with the scope of that block.

I remember I tried that before but recalled the error code wrong. Here it is,

kernel/sched/core.c:5940:17: warning: unused variable 'ptr' [-Wunused-variable]
                unsigned long ptr = (unsigned long)kzalloc(alloc_size,
GFP_NOWAIT);

> 
> And for our config, alloc_size is an unconditional 0, so it should DCE
> the whole block and with that our variable. But clearly the passes are
> the other way around :/
> 
> > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > > index fa43ce3962e7..cb652e165570 100644
> > > --- a/kernel/sched/core.c
> > > +++ b/kernel/sched/core.c
> > > @@ -6369,7 +6369,7 @@ DECLARE_PER_CPU(cpumask_var_t, select_idle_mask);
> > >  
> > >  void __init sched_init(void)
> > >  {
> > > -	unsigned long alloc_size = 0, ptr;
> > > +	unsigned long alloc_size = 0;
> > >  	int i;
> > >  
> > >  	wait_bit_init();
> > > @@ -6381,7 +6381,7 @@ void __init sched_init(void)
> > >  	alloc_size += 2 * nr_cpu_ids * sizeof(void **);
> > >  #endif
> > >  	if (alloc_size) {
> > > -		ptr = (unsigned long)kzalloc(alloc_size, GFP_NOWAIT);
> > > +		unsigned long ptr = (unsigned long)kzalloc(alloc_size,
> > > GFP_NOWAIT);
> > >  
> > >  #ifdef CONFIG_FAIR_GROUP_SCHED
> > >  		root_task_group.se = (struct sched_entity **)ptr;

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