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

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Message-Id: <20140506134054.8cd531296c8aa5d9c3eff958@linux-foundation.org>
Date:	Tue, 6 May 2014 13:40:54 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Cc:	peterz@...radead.org, tglx@...utronix.de, mingo@...nel.org,
	tj@...nel.org, rusty@...tcorp.com.au, fweisbec@...il.com,
	hch@...radead.org, mgorman@...e.de, riel@...hat.com, bp@...e.de,
	rostedt@...dmis.org, mgalbraith@...e.de, ego@...ux.vnet.ibm.com,
	paulmck@...ux.vnet.ibm.com, oleg@...hat.com, rjw@...ysocki.net,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] CPU hotplug, stop-machine: Plug race-window that
 leads to "IPI-to-offline-CPU"

On Tue, 06 May 2014 23:33:03 +0530 "Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com> 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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