Re: [PATCH] mm/hotplug: Remove stop_machine() fromtry_offline_node()

[Toshi Kani]

On Wed, 2013-08-14 at 21:21 -0400, KOSAKI Motohiro wrote:
> (8/12/13 3:34 PM), Toshi Kani wrote:
> > lock_device_hotplug() serializes hotplug & online/offline operations.
> > The lock is held in common sysfs online/offline interfaces and ACPI
> > hotplug code paths.
> > 
> > try_offline_node() off-lines a node if all memory sections and cpus
> > are removed on the node.  It is called from acpi_processor_remove()
> > and acpi_memory_remove_memory()->remove_memory() paths, both of which
> > are in the ACPI hotplug code.
> > 
> > try_offline_node() calls stop_machine() to stop all cpus while checking
> > all cpu status with the assumption that the caller is not protected from
> > CPU hotplug or CPU online/offline operations.  However, the caller is
> > always serialized with lock_device_hotplug().  Also, the code needs to
> > be properly serialized with a lock, not by stopping all cpus at a random
> > place with stop_machine().
> > 
> > This patch removes the use of stop_machine() in try_offline_node() and
> > adds comments to try_offline_node() and remove_memory() that
> > lock_device_hotplug() is required.
> 
> This patch need more verbose explanation. check_cpu_on_node() traverse cpus
> and cpu hotplug seems to use cpu_hotplug_driver_lock() instead of lock_device_hotplug().

As described: 

| lock_device_hotplug() serializes hotplug & online/offline operations.
| The lock is held in common sysfs online/offline interfaces and ACPI
| hotplug code paths.

And here are their code paths.

- CPU & Mem online/offline via sysfs online
	store_online()->lock_device_hotplug()

- Mem online via sysfs state:
	store_mem_state()->lock_device_hotplug()

- ACPI CPU & Mem hot-add:
	acpi_scan_bus_device_check()->lock_device_hotplug()

- ACPI CPU & Mem hot-delete:
	acpi_scan_hot_remove()->lock_device_hotplug()

> That said, the race is not happen against another memeory happen. It's likely happen
> another cpu hotplug. So commenting remove_memory() doesn't make much sense.

This lock scheme protects from both CPU and memory hotplug, and requires
the caller to call lock_device_hotplug().

Thanks,
-Toshi


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