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    SubjectRe: [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@landley.net>
    > Cc: linux-doc@vger.kernel.org
    > Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>

    Reviewed-by: Paul E. McKenney <paulmck@linux.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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