linux-kernel - Re: [PATCH v5 45/45] Documentation/cpu-hotplug: Remove references to stop

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Date:	Fri, 8 Feb 2013 16:16:49 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Cc:	tglx@...utronix.de, peterz@...radead.org, tj@...nel.org,
	oleg@...hat.com, rusty@...tcorp.com.au, mingo@...nel.org,
	akpm@...ux-foundation.org, namhyung@...nel.org,
	rostedt@...dmis.org, wangyun@...ux.vnet.ibm.com,
	xiaoguangrong@...ux.vnet.ibm.com, rjw@...k.pl, sbw@....edu,
	fweisbec@...il.com, linux@....linux.org.uk,
	nikunj@...ux.vnet.ibm.com, linux-pm@...r.kernel.org,
	linux-arch@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linuxppc-dev@...ts.ozlabs.org, netdev@...r.kernel.org,
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 45/45] Documentation/cpu-hotplug: Remove references to
 stop_machine()

On Tue, Jan 22, 2013 at 01:15:48PM +0530, Srivatsa S. Bhat wrote:
> Since stop_machine() is no longer used in the CPU offline path, we cannot
> disable CPU hotplug using preempt_disable()/local_irq_disable() etc. We
> need to use the newly introduced get/put_online_cpus_atomic() APIs.
> Reflect this in the documentation.
> 
> Cc: Rob Landley <rob@...dley.net>
> Cc: linux-doc@...r.kernel.org
> Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@...ux.vnet.ibm.com>

Reviewed-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>

> ---
> 
>  Documentation/cpu-hotplug.txt |   17 +++++++++++------
>  1 file changed, 11 insertions(+), 6 deletions(-)
> 
> diff --git a/Documentation/cpu-hotplug.txt b/Documentation/cpu-hotplug.txt
> index 9f40135..7f907ec 100644
> --- a/Documentation/cpu-hotplug.txt
> +++ b/Documentation/cpu-hotplug.txt
> @@ -113,13 +113,15 @@ Never use anything other than cpumask_t to represent bitmap of CPUs.
>  	#include <linux/cpu.h>
>  	get_online_cpus() and put_online_cpus():
> 
> -The above calls are used to inhibit cpu hotplug operations. While the
> +The above calls are used to inhibit cpu hotplug operations, when invoked from
> +non-atomic context (because the above functions can sleep). While the
>  cpu_hotplug.refcount is non zero, the cpu_online_mask will not change.
> -If you merely need to avoid cpus going away, you could also use
> -preempt_disable() and preempt_enable() for those sections.
> -Just remember the critical section cannot call any
> -function that can sleep or schedule this process away. The preempt_disable()
> -will work as long as stop_machine_run() is used to take a cpu down.
> +
> +However, if you are executing in atomic context (ie., you can't afford to
> +sleep), and you merely need to avoid cpus going offline, you can use
> +get_online_cpus_atomic() and put_online_cpus_atomic() for those sections.
> +Just remember the critical section cannot call any function that can sleep or
> +schedule this process away.
> 
>  CPU Hotplug - Frequently Asked Questions.
> 
> @@ -360,6 +362,9 @@ A: There are two ways.  If your code can be run in interrupt context, use
>  		return err;
>  	}
> 
> +   If my_func_on_cpu() itself cannot block, use get/put_online_cpus_atomic()
> +   instead of get/put_online_cpus() to prevent CPUs from going offline.
> +
>  Q: How do we determine how many CPUs are available for hotplug.
>  A: There is no clear spec defined way from ACPI that can give us that
>     information today. Based on some input from Natalie of Unisys,
> 

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