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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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