linux-kernel - [PATCH v6 46/46] Documentation/cpu-hotplug: Remove references to stop

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Message-ID: <20130218124448.26245.60959.stgit@srivatsabhat.in.ibm.com>
Date:	Mon, 18 Feb 2013 18:14:49 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	tglx@...utronix.de, peterz@...radead.org, tj@...nel.org,
	oleg@...hat.com, paulmck@...ux.vnet.ibm.com, rusty@...tcorp.com.au,
	mingo@...nel.org, akpm@...ux-foundation.org, namhyung@...nel.org
Cc:	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, srivatsa.bhat@...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,
	walken@...gle.com, vincent.guittot@...aro.org
Subject: [PATCH v6 46/46] Documentation/cpu-hotplug: Remove references to
 stop_machine()

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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