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    Subject[PATCH v5 45/45] Documentation/cpu-hotplug: Remove references to stop_machine()
    Date
    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>
    ---

    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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