On 21.08.2018 12:44, David Hildenbrand wrote: > This is the same approach as in the first RFC, but this time without > exporting device_hotplug_lock (requested by Greg) and with some more > details and documentation regarding locking. Tested only on x86 so far. > I'll be on vacation for two weeks starting on Saturday. If there are no comments I'll send this as !RFC once I return. Thanks! > -------------------------------------------------------------------------- > > Reading through the code and studying how mem_hotplug_lock is to be used, > I noticed that there are two places where we can end up calling > device_online()/device_offline() - online_pages()/offline_pages() without > the mem_hotplug_lock. And there are other places where we call > device_online()/device_offline() without the device_hotplug_lock. > > While e.g. > echo "online" > /sys/devices/system/memory/memory9/state > is fine, e.g. > echo 1 > /sys/devices/system/memory/memory9/online > Will not take the mem_hotplug_lock. However the device_lock() and > device_hotplug_lock. > > E.g. via memory_probe_store(), we can end up calling > add_memory()->online_pages() without the device_hotplug_lock. So we can > have concurrent callers in online_pages(). We e.g. touch in online_pages() > basically unprotected zone->present_pages then. > > Looks like there is a longer history to that (see Patch #2 for details), > and fixing it to work the way it was intended is not really possible. We > would e.g. have to take the mem_hotplug_lock in device/base/core.c, which > sounds wrong. > > Summary: We had a lock inversion on mem_hotplug_lock and device_lock(). > More details can be found in patch 3 and patch 6. > > I propose the general rules (documentation added in patch 6): > > 1. add_memory/add_memory_resource() must only be called with > device_hotplug_lock. > 2. remove_memory() must only be called with device_hotplug_lock. This is > already documented and holds for all callers. > 3. device_online()/device_offline() must only be called with > device_hotplug_lock. This is already documented and true for now in core > code. Other callers (related to memory hotplug) have to be fixed up. > 4. mem_hotplug_lock is taken inside of add_memory/remove_memory/ > online_pages/offline_pages. > > To me, this looks way cleaner than what we have right now (and easier to > verify). And looking at the documentation of remove_memory, using > lock_device_hotplug also for add_memory() feels natural. > > > RFC -> RFCv2: > - Don't export device_hotplug_lock, provide proper remove_memory/add_memory > wrappers. > - Split up the patches a bit. > - Try to improve powernv memtrace locking > - Add some documentation for locking that matches my knowledge > > David Hildenbrand (6): > mm/memory_hotplug: make remove_memory() take the device_hotplug_lock > mm/memory_hotplug: make add_memory() take the device_hotplug_lock > mm/memory_hotplug: fix online/offline_pages called w.o. > mem_hotplug_lock > powerpc/powernv: hold device_hotplug_lock when calling device_online() > powerpc/powernv: hold device_hotplug_lock in memtrace_offline_pages() > memory-hotplug.txt: Add some details about locking internals > > Documentation/memory-hotplug.txt | 39 +++++++++++- > arch/powerpc/platforms/powernv/memtrace.c | 14 +++-- > .../platforms/pseries/hotplug-memory.c | 8 +-- > drivers/acpi/acpi_memhotplug.c | 4 +- > drivers/base/memory.c | 22 +++---- > drivers/xen/balloon.c | 3 + > include/linux/memory_hotplug.h | 4 +- > mm/memory_hotplug.c | 59 +++++++++++++++---- > 8 files changed, 115 insertions(+), 38 deletions(-) > -- Thanks, David / dhildenb
