Re: [PATCH 6/7] smp/hotplug: Differentiate the AP-work lockdep class between up and down

[Lai Jiangshan]

On Thu, Sep 21, 2017 at 1:00 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> With lockdep-crossrelease we get deadlock reports that span cpu-up and
> cpu-down chains. Such deadlocks cannot possibly happen because cpu-up
> and cpu-down are globally serialized.
>
>   CPU0                  CPU1                    CPU2
>   cpuhp_up_callbacks:   takedown_cpu:           cpuhp_thread_fun:
>
>   cpuhp_state
>                         irq_lock_sparse()
>     irq_lock_sparse()
>                         wait_for_completion()
>                                                 cpuhp_state
>                                                 complete()
>
> Now that we have consistent AP state, we can trivially separate the
> AP-work class between up and down using st->bringup.
>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> ---
>  kernel/cpu.c |   41 ++++++++++++++++++++++++++++++++---------
>  1 file changed, 32 insertions(+), 9 deletions(-)
>
> --- a/kernel/cpu.c
> +++ b/kernel/cpu.c
> @@ -68,9 +68,26 @@ struct cpuhp_cpu_state {
>  static DEFINE_PER_CPU(struct cpuhp_cpu_state, cpuhp_state);
>
>  #if defined(CONFIG_LOCKDEP) && defined(CONFIG_SMP)
> -static struct lock_class_key cpuhp_state_key;
> -static struct lockdep_map cpuhp_state_lock_map =
> -       STATIC_LOCKDEP_MAP_INIT("cpuhp_state", &cpuhp_state_key);
> +static struct lockdep_map cpuhp_state_up_map =
> +       STATIC_LOCKDEP_MAP_INIT("cpuhp_state-up", &cpuhp_state_up_map);
> +static struct lockdep_map cpuhp_state_down_map =
> +       STATIC_LOCKDEP_MAP_INIT("cpuhp_state-down", &cpuhp_state_down_map);
> +
> +
> +static void inline cpuhp_lock_acquire(bool bringup)
> +{
> +       lock_map_acquire(bringup ? &cpuhp_state_up_map : &cpuhp_state_down_map);
> +}
> +
> +static void inline cpuhp_lock_release(bool bringup)
> +{
> +       lock_map_release(bringup ? &cpuhp_state_up_map : &cpuhp_state_down_map);
> +}
> +#else
> +
> +static void inline cpuhp_lock_acquire(bool bringup) { }
> +static void inline cpuhp_lock_release(bool bringup) { }
> +
>  #endif
>
>  /**
> @@ -512,7 +529,7 @@ static void cpuhp_thread_fun(unsigned in
>         if (WARN_ON_ONCE(!st->should_run))
>                 return;
>
> -       lock_map_acquire(&cpuhp_state_lock_map);
> +       cpuhp_lock_acquire(bringup);
>
>         if (st->single) {
>                 state = st->cb_state;
> @@ -564,7 +581,7 @@ static void cpuhp_thread_fun(unsigned in
>         }
>
>  next:
> -       lock_map_release(&cpuhp_state_lock_map);
> +       cpuhp_lock_release(bringup);
>
>         if (!st->should_run)
>                 complete(&st->done);
> @@ -581,8 +598,11 @@ cpuhp_invoke_ap_callback(int cpu, enum c
>         if (!cpu_online(cpu))
>                 return 0;
>
> -       lock_map_acquire(&cpuhp_state_lock_map);
> -       lock_map_release(&cpuhp_state_lock_map);
> +       cpuhp_lock_acquire(false);
> +       cpuhp_lock_release(false);
> +
> +       cpuhp_lock_acquire(true);
> +       cpuhp_lock_release(true);

Hello, Peter,

I'm reading the code in kernel/cpu.c.
I couldn't understand why both lockep_map are acquired here?
Is the lockep_map matching for the argument @bringup enough here?

The log shows that the argument @bringup had been added
when the time this commit was applied. But it was quite probably
non-existed when you wrote the patch since the time was close.

thanks,
Lai.

>
>         /*
>          * If we are up and running, use the hotplug thread. For early calls
> @@ -620,8 +640,11 @@ static int cpuhp_kick_ap_work(unsigned i
>         enum cpuhp_state prev_state = st->state;
>         int ret;
>
> -       lock_map_acquire(&cpuhp_state_lock_map);
> -       lock_map_release(&cpuhp_state_lock_map);
> +       cpuhp_lock_acquire(false);
> +       cpuhp_lock_release(false);
> +
> +       cpuhp_lock_acquire(true);
> +       cpuhp_lock_release(true);
>
>         trace_cpuhp_enter(cpu, st->target, prev_state, cpuhp_kick_ap_work);
>         ret = cpuhp_kick_ap(st, st->target);
>
>