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    SubjectRe: [PATCH v3 0/3] CPU hotplug, Freezer: Fix bugs in CPU hotplug call path
    Hi Alan,

    On 10/21/2011 08:12 PM, Alan Stern wrote:
    > 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.

    I can't believe I missed such an obvious thing!
    I must have been carried away while thinking about the race between freezer
    and CPU hotplug. Thanks for pointing this out.

    >> 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.

    Agreed. So patch 1 becomes unnecessary. Patch 2 implements the needed
    I'll fix the code (also incorporating your point that PM_POST_FREEZE and
    PM_THAW_PREPARE are unnecessary) and repost.

    Thank you for the review.

    Srivatsa S. Bhat <>
    Linux Technology Center,
    IBM India Systems and Technology Lab

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