Re: [PATCH 2/2] CPU hotplug, stop-machine: Plug race-window that leads to "IPI-to-offline-CPU"

[Andrew Morton]

On Tue, 06 May 2014 23:33:03 +0530 "Srivatsa S. Bhat" <srivatsa.bhat@xxxxxxxxxxxxxxxxxx> wrote:

> --- a/kernel/stop_machine.c
> +++ b/kernel/stop_machine.c
> @@ -165,12 +165,21 @@ static void ack_state(struct multi_stop_data *msdata)
>  		set_state(msdata, msdata->state + 1);
>  }
>  
> +/* Holding area for active CPUs, to let all the non-active CPUs go first */
> +static void hold_active_cpus(struct multi_stop_data *msdata,
> +			     int num_active_cpus)
> +{
> +	/* Wait until all the non-active threads ack the state */
> +	while (atomic_read(&msdata->thread_ack) > num_active_cpus)
> +		cpu_relax();
> +}

The code comments are a bit lame.  Can we do a better job of explaining
the overall dynamic behaviour?  Help readers to understand the problem
which hold_active_cpus() is solving and how it solves it?


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