linux-kernel - RE: [PATCH] rcu: Add a warnings in sync_sched_exp_online

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Date:   Tue, 28 Jun 2022 00:31:12 +0000
From:   "Zhang, Qiang1" <qiang1.zhang@...el.com>
To:     "paulmck@...nel.org" <paulmck@...nel.org>
CC:     "frederic@...nel.org" <frederic@...nel.org>,
        "joel@...lfernandes.org" <joel@...lfernandes.org>,
        "quic_neeraju@...cinc.com" <quic_neeraju@...cinc.com>,
        "rcu@...r.kernel.org" <rcu@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] rcu: Add a warnings in sync_sched_exp_online_cleanup()

> 
> 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@...el.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
>