Messages in this thread | | | Date | Tue, 25 Jun 2019 16:25:08 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH v2] sched/core: silence a warning in sched_init() |
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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.
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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