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 synchronization. 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. -- Regards, Srivatsa S. Bhat <srivatsa.bhat@xxxxxxxxxxxxxxxxxx> Linux Technology Center, IBM India Systems and Technology Lab -- 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
