Re: [linux-next:master 3203/9634] arch/powerpc/include/asm/bitops.h:126:26: error: constraint 'n' expects an integer constant expression

[Andrea Righi <arighi@nvidia.com>]

On Sun, Mar 09, 2025 at 04:56:45AM +0800, kernel test robot wrote:
> tree:   https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git master
> head:   0a2f889128969dab41861b6e40111aa03dc57014
> commit: 48849271e66114cb980a3bc44218b04d0f8cdcdd [3203/9634] sched_ext: idle: Per-node idle cpumasks
> config: powerpc-randconfig-r112-20250308 (https://download.01.org/0day-ci/archive/20250309/202503090435.wITkzD2e-lkp@intel.com/config)
> compiler: clang version 18.1.8 (https://github.com/llvm/llvm-project 3b5b5c1ec4a3095ab096dd780e84d7ab81f3d7ff)
> reproduce: (https://download.01.org/0day-ci/archive/20250309/202503090435.wITkzD2e-lkp@intel.com/reproduce)
> 
> If you fix the issue in a separate patch/commit (i.e. not just a new version of
> the same patch/commit), kindly add following tags
> | Reported-by: kernel test robot <lkp@intel.com>
> | Closes: https://lore.kernel.org/oe-kbuild-all/202503090435.wITkzD2e-lkp@intel.com/
> 
> All errors (new ones prefixed by >>):
> 
>    In file included from kernel/sched/build_policy.c:16:
>    In file included from include/linux/sched/clock.h:5:
>    In file included from include/linux/smp.h:13:
>    In file included from include/linux/cpumask.h:11:
>    In file included from include/linux/kernel.h:23:
>    In file included from include/linux/bitops.h:68:
> >> arch/powerpc/include/asm/bitops.h:126:26: error: constraint 'n' expects an integer constant expression
>      126 | DEFINE_CLROP(clear_bits, "")
>          |                          ^
>    1 error generated.

I've expanded the for_each_node_numadist() macro to better understand
what's going on, it seems that clang isn't happy with a node_clear() inside
the for loop, and it's not complaining about a similar node_clear() outside
the for loop.

Also, it seems to happen only with clang (gcc is always happy).

Here's the relevant code (with the macro expanded and comments removed):

===
static DEFINE_PER_CPU(nodemask_t, per_cpu_unvisited);

static s32 pick_idle_cpu_from_online_nodes(const struct cpumask *cpus_allowed, int node, u64 flags)
{
        nodemask_t *unvisited;
        s32 cpu = -EBUSY;

        preempt_disable();
        unvisited = this_cpu_ptr(&per_cpu_unvisited);

        nodes_copy(*unvisited, node_states[N_ONLINE]);
        node_clear(node, *unvisited);

        for (int __start = node,
             node = nearest_node_nodemask(__start, unvisited);
             node < MAX_NUMNODES;
             // node_clear(node, *unvisited), <- clang is happy with this one commented!
             node = nearest_node_nodemask(__start, unvisited)) {
                cpu = pick_idle_cpu_in_node(cpus_allowed, node, flags);
                if (cpu >= 0)
                        break;
        }
        preempt_enable();

        return cpu;
}
===

The code looks correct to me, but I don't know much about powerpc assembly.
Christophe, maybe you have a better idea of what's happening here?

Thanks,
-Andrea

> 
> 
> vim +/n +126 arch/powerpc/include/asm/bitops.h
> 
> fb350784d8d179 Christophe Leroy 2021-09-21  125  
> fb350784d8d179 Christophe Leroy 2021-09-21 @126  DEFINE_CLROP(clear_bits, "")
> fb350784d8d179 Christophe Leroy 2021-09-21  127  DEFINE_CLROP(clear_bits_unlock, PPC_RELEASE_BARRIER)
> fb350784d8d179 Christophe Leroy 2021-09-21  128  
> 
> :::::: The code at line 126 was first introduced by commit
> :::::: fb350784d8d17952afa93383bb47aaa6b715c459 powerpc/bitops: Use immediate operand when possible
> 
> :::::: TO: Christophe Leroy <christophe.leroy@csgroup.eu>
> :::::: CC: Michael Ellerman <mpe@ellerman.id.au>
> 
> -- 
> 0-DAY CI Kernel Test Service
> https://github.com/intel/lkp-tests/wiki