Re: [PATCH v3 0/3] CPU hotplug, Freezer: Fix bugs in CPU hotplugcall path

[Srivatsa S. Bhat]

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

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