linux-kernel - Re: [PATCH v2] sched/core: silence a warning in sched

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Message-ID: <1561471459.5154.70.camel@lca.pw>
Date:   Tue, 25 Jun 2019 10:04:19 -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 15:52 +0200, Peter Zijlstra wrote:
> On Tue, Jun 25, 2019 at 08:44:22AM -0400, Qian Cai wrote:
> > Compiling a kernel with both FAIR_GROUP_SCHED=n and RT_GROUP_SCHED=n
> > will generate a warning using W=1,
> > 
> > kernel/sched/core.c: In function 'sched_init':
> > kernel/sched/core.c:5906:32: warning: variable 'ptr' set but not used
> > [-Wunused-but-set-variable]
> >   unsigned long alloc_size = 0, ptr;
> >                                 ^~~
> > 
> > It apparently the maintainers don't like the previous fix [1] which
> > contains ugly idefs, so silence it by appending the __maybe_unused
> > attribute for it instead.
> > 
> > Reviewed-by: Valentin Schneider <valentin.schneider@....com>
> > Signed-off-by: Qian Cai <cai@....pw>
> > ---
> > 
> > v2: Incorporate the feedback from Valentin.
> > 
> >  kernel/sched/core.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index 874c427742a9..12b9b69c8a66 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -5903,7 +5903,8 @@ int in_sched_functions(unsigned long addr)
> >  void __init sched_init(void)
> >  {
> >  	int i, j;
> > -	unsigned long alloc_size = 0, ptr;
> > +	unsigned long alloc_size = 0;
> > +	unsigned long __maybe_unused ptr;
> >  
> 
> That still isn't particularly pretty.
> 
> Why do we care about W=1 build noise? Some of that seems rather silly,
> like that -Wmissing-prototype nonsense.

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

> 
> 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;