Re: [PATCH v2 1/3] powerpc/rtas: use device model APIs and serialization during LPM

[Gautham R Shenoy <ego.lkml@gmail.com>]

Hello Nathan,

On Sat, Aug 3, 2019 at 1:06 AM Nathan Lynch <nathanl@linux.ibm.com> wrote:
>
> The LPAR migration implementation and userspace-initiated cpu hotplug
> can interleave their executions like so:
>
> 1. Set cpu 7 offline via sysfs.
>
> 2. Begin a partition migration, whose implementation requires the OS
>    to ensure all present cpus are online; cpu 7 is onlined:
>
>      rtas_ibm_suspend_me -> rtas_online_cpus_mask -> cpu_up
>
>    This sets cpu 7 online in all respects except for the cpu's
>    corresponding struct device; dev->offline remains true.
>
> 3. Set cpu 7 online via sysfs. _cpu_up() determines that cpu 7 is
>    already online and returns success. The driver core (device_online)
>    sets dev->offline = false.
>
> 4. The migration completes and restores cpu 7 to offline state:
>
>      rtas_ibm_suspend_me -> rtas_offline_cpus_mask -> cpu_down
>
> This leaves cpu7 in a state where the driver core considers the cpu
> device online, but in all other respects it is offline and
> unused. Attempts to online the cpu via sysfs appear to succeed but the
> driver core actually does not pass the request to the lower-level
> cpuhp support code. This makes the cpu unusable until the cpu device
> is manually set offline and then online again via sysfs.
>
> Instead of directly calling cpu_up/cpu_down, the migration code should
> use the higher-level device core APIs to maintain consistent state and
> serialize operations.
>
> Fixes: 120496ac2d2d ("powerpc: Bring all threads online prior to migration/hibernation")
> Signed-off-by: Nathan Lynch <nathanl@linux.ibm.com>

Looks good to me. This locking scheme makes the code consistent with
dlpar_cpu() which also uses the  high-level device APIs.

Reviewed-by: Gautham R. Shenoy <ego@linux.vnet.ibm.com>
> ---
>  arch/powerpc/kernel/rtas.c | 11 ++++++++---
>  1 file changed, 8 insertions(+), 3 deletions(-)
>
> diff --git a/arch/powerpc/kernel/rtas.c b/arch/powerpc/kernel/rtas.c
> index 5faf0a64c92b..05824eb4323b 100644
> --- a/arch/powerpc/kernel/rtas.c
> +++ b/arch/powerpc/kernel/rtas.c
> @@ -871,15 +871,17 @@ static int rtas_cpu_state_change_mask(enum rtas_cpu_state state,
>                 return 0;
>
>         for_each_cpu(cpu, cpus) {
> +               struct device *dev = get_cpu_device(cpu);
> +
>                 switch (state) {
>                 case DOWN:
> -                       cpuret = cpu_down(cpu);
> +                       cpuret = device_offline(dev);
>                         break;
>                 case UP:
> -                       cpuret = cpu_up(cpu);
> +                       cpuret = device_online(dev);
>                         break;
>                 }
> -               if (cpuret) {
> +               if (cpuret < 0) {
>                         pr_debug("%s: cpu_%s for cpu#%d returned %d.\n",
>                                         __func__,
>                                         ((state == UP) ? "up" : "down"),
> @@ -968,6 +970,8 @@ int rtas_ibm_suspend_me(u64 handle)
>         data.token = rtas_token("ibm,suspend-me");
>         data.complete = &done;
>
> +       lock_device_hotplug();
> +
>         /* All present CPUs must be online */
>         cpumask_andnot(offline_mask, cpu_present_mask, cpu_online_mask);
>         cpuret = rtas_online_cpus_mask(offline_mask);
> @@ -1006,6 +1010,7 @@ int rtas_ibm_suspend_me(u64 handle)
>                                 __func__);
>
>  out:
> +       unlock_device_hotplug();
>         free_cpumask_var(offline_mask);
>         return atomic_read(&data.error);
>  }
> --
> 2.20.1
>


-- 
Thanks and Regards
gautham.