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SubjectRE: [PATCH] rcu: Add a warnings in sync_sched_exp_online_cleanup()
Date
> 
> Add Cc
>
> Currently, the sync_sched_exp_online_cleanup() is invoked in
> cpuhp per-cpu kthreads when CPU is going online, so the CPU id
> obtained by get_cpu() should always be equal to the CPU id of
> the passed parameter, that is to say, the smp_call_function_single()
> never be invoked, if be invoked, there may be problem with cpu-hotplug,
> this commit add WARN_ON_ONCE() to remind everyone.
>
> Signed-off-by: Zqiang <qiang1.zhang@intel.com>
> ---
> kernel/rcu/tree_exp.h | 2 ++
> 1 file changed, 2 insertions(+)
>
> diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
> index be667583a554..ae8dcfd4486c 100644
> --- a/kernel/rcu/tree_exp.h
> +++ b/kernel/rcu/tree_exp.h
> @@ -865,6 +865,8 @@ static void sync_sched_exp_online_cleanup(int cpu)
> put_cpu();
> return;
> }
> +
> + WARN_ON_ONCE(my_cpu != cpu);

>If we are going to add this sort of warning, why not instead add it
>to rcutree_online_cpu()?
>
>The reason the warning has not been present is the prospect of concurrent
>onlining at boot time, which might have rcutree_online_cpu() invoked
>from CPU 0 for multiple CPUs at boot. However, the for_each_online_cpu()
>loop has recently been removed from rcu_init().

The rcutree_online_cpu() is always invoked in per-cpu cpuhp kthreads,
and even if it is called directly through the for_each_online_cpu() loop in
rcu_init(), since the rcu_init() takes precedence over smp initialization,
only the CPU0 is online at this time, still only called on CPU0.

Thanks
Zqiang

>
>But I would like to hear what others think. Would such a warning
>significantly help debugging?
>
> Thanx, Paul
>
> /* Quiescent state needed on some other CPU, send IPI. */
> ret = smp_call_function_single(cpu, rcu_exp_handler, NULL, 0);
> put_cpu();
> --
> 2.25.1
>

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