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 <npig...@gmail.com>
Tested-by: Guenter Roeck <li...@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
>
