Re: [PATCH v2] sched/core: silence a warning in sched_init()

[Qian Cai]

> On Jun 25, 2019, at 11:57 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> 
> On Tue, Jun 25, 2019 at 11:07:09AM -0400, Qian Cai wrote:
>> 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);
> 
> Yes, I know, I tried. And GCC is a moron because of it.

Actually, not only GCC but clang also donât understand your patch.

# make CC=clang W=1 kernel/sched/core.o

kernel/sched/core.c:6384:17: warning: unused variable 'ptr' [-Wunused-variable]
                unsigned long ptr = (unsigned long)kzalloc(alloc_size, GFP_NOWAIT);

Maybe adding a â__maybe_unusedâ until the day that compilers are getting smarter.