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    SubjectRe: [PATCH v3 0/3] CPU hotplug, Freezer: Fix bugs in CPU hotplug call path
    On Fri, 21 Oct 2011, Srivatsa S. Bhat wrote:

    > When using the CPU hotplug infrastructure to offline/online CPUs, the cpu_up()
    > and cpu_down() functions are used, which internally call _cpu_up() and
    > _cpu_down() with the second argument *always* set to 0. The second argument
    > is "tasks_frozen", which should be correctly set to 1 when tasks have been
    > frozen, even when the freezing of tasks may be due to an event unrelated
    > to CPU hotplug, such as a suspend operation in progress, in which case the
    > freezer subsystem freezes all the freezable tasks.
    > Not giving the correct value for the 'tasks_frozen' argument can potentially
    > lead to a lot of issues since this will send wrong notifications via the
    > notifier mechanism leading to the execution of inappropriate code by the
    > callbacks registered for the notifiers. That is, instead of CPU_ONLINE_FROZEN
    > and CPU_DEAD_FROZEN notifications, it results in CPU_ONLINE and CPU_DEAD
    > notifications to be sent all the time, irrespective of the actual state of
    > the system (i.e., whether tasks are frozen or not).
    > This patch introduces a flag to indicate whether the tasks are frozen are not
    > (by any event) and modifies cpu_up() and cpu_down() functions to check the
    > value of this flag and accordingly call _cpu_up() and _cpu_down() respectively
    > by supplying the correct value as the second argument based on the state of
    > the system. This in turn means that the correct notifications are sent, thus
    > ensuring that all the registered callbacks do the right thing.

    That doesn't make any sense. If tasks were frozen then the task
    calling _cpu_up() or _cpu_down() would be frozen too, and therefore
    wouldn't be able to make the call.

    > Additionally, to ensure that the tasks are not frozen or thawed by the freezer
    > subsystem while the registered callbacks are running, this patch adds a few
    > notifications in the freezer which is then hooked onto by the CPU hotplug
    > code, to avoid this race.

    That's more sensible. Freezing or thawing tasks isn't instantaneous.
    It's possible that _cpu_up() or _cpu_down() could be called while some
    tasks were frozen and others were still running.

    If you're careful to prevent this from happening then there's no reason
    to change the tasks_frozen argument. It should never be anything but
    0 in this situation.

    Alan Stern

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      Last update: 2011-10-21 16:45    [W:0.024 / U:3.744 seconds]
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