Am 19.09.18 um 03:22 schrieb Balbir Singh: > On Tue, Sep 18, 2018 at 01:48:16PM +0200, David Hildenbrand wrote: >> 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. >> > > That seems reasonable, but also implies that device_online() would hold > back add/remove memory, could you please also document what mode > read/write the locks need to be held? For example can the device_hotplug_lock > be held in read mode while add/remove memory via (mem_hotplug_lock) is held > in write mode? device_hotplug_lock is an ordinary mutex. So no option there. Only mem_hotplug_lock is a per CPU RW mutex. And as of now it only exists to not require get_online_mems()/put_online_mems() to take the device_hotplug_lock. Which is perfectly valid, because these users only care about memory (not any other devices) not suddenly vanish. And that RW lock makes things fast. Any modifications (online/offline/add/remove) require the mem_hotplug_lock in write. I can add some more details to documentation in patch #6. "... we should always hold the mem_hotplug_lock (via mem_hotplug_begin/mem_hotplug_done) in write mode to serialize memory hotplug" ..." "In addition, mem_hotplug_lock (in contrast to device_hotplug_lock) in read mode allows for a quite efficient get_online_mems/put_online_mems implementation, so code accessing memory can protect from that memory vanishing." Would that work for you? Thanks! > > Balbir Singh. > > -- Thanks, David / dhildenb
