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 -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
