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SubjectRe: [PATCH 6/7] smp/hotplug: Differentiate the AP-work lockdep class between up and down
On Thu, Sep 21, 2017 at 1:00 AM, Peter Zijlstra <peterz@infradead.org> 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@infradead.org>
> ---
> 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);
>
>

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