[PATCH 2/3] powerpc/64s: Fix hash__change_memory_range preemption warning

[Guenter Roeck]

On Fri, Oct 14, 2022 at 01:16:46AM +1000, Nicholas Piggin wrote:
> stop_machine_cpuslocked takes a mutex so it must be called in a
> preemptible context, so it can't simply be fixed by disabling
> preemption.
> 
> This is not a bug, because CPU hotplug is locked, so this processor will
> call in to the stop machine function. So raw_smp_processor_id() could be
> used. This leaves a small chance that this thread will be migrated to
> another CPU, so the master work would be done by a CPU from a different
> context. Better for test coverage to make that a common case by just
> having the first CPU to call in become the master.
> 
> Signed-off-by: Nicholas Piggin <npiggin at gmail.com>

Tested-by: Guenter Roeck <linux at roeck-us.net>

> ---
>  arch/powerpc/mm/book3s64/hash_pgtable.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/powerpc/mm/book3s64/hash_pgtable.c b/arch/powerpc/mm/book3s64/hash_pgtable.c
> index 747492edb75a..51f48984abca 100644
> --- a/arch/powerpc/mm/book3s64/hash_pgtable.c
> +++ b/arch/powerpc/mm/book3s64/hash_pgtable.c
> @@ -404,7 +404,8 @@ EXPORT_SYMBOL_GPL(hash__has_transparent_hugepage);
>  
>  struct change_memory_parms {
>  	unsigned long start, end, newpp;
> -	unsigned int step, nr_cpus, master_cpu;
> +	unsigned int step, nr_cpus;
> +	atomic_t master_cpu;
>  	atomic_t cpu_counter;
>  };
>  
> @@ -478,7 +479,8 @@ static int change_memory_range_fn(void *data)
>  {
>  	struct change_memory_parms *parms = data;
>  
> -	if (parms->master_cpu != smp_processor_id())
> +	// First CPU goes through, all others wait.
> +	if (atomic_xchg(&parms->master_cpu, 1) == 1)
>  		return chmem_secondary_loop(parms);
>  
>  	// Wait for all but one CPU (this one) to call-in
> @@ -516,7 +518,7 @@ static bool hash__change_memory_range(unsigned long start, unsigned long end,
>  		chmem_parms.end = end;
>  		chmem_parms.step = step;
>  		chmem_parms.newpp = newpp;
> -		chmem_parms.master_cpu = smp_processor_id();
> +		atomic_set(&chmem_parms.master_cpu, 0);
>  
>  		cpus_read_lock();
>  
> -- 
> 2.37.2
>