>> I suggest adding what you just found out to >> Documentation/admin-guide/mm/memory-hotplug.rst "Locking Internals". >> Maybe a new subsection for mem_hotplug_lock. And eventually also >> pgdat_resize_lock. > > Well, I am not good at document writting. Below is my first trial. Look > forward your comments. I'll have a look, maybe also Oscar and Michal can have a look. I guess we don't have to cover all now, we can add more details as we discover them. > > BTW, in case I would send a new version with this, would I put this into > a separate one or merge this into current one? I would put this into a separate patch. > > diff --git a/Documentation/admin-guide/mm/memory-hotplug.rst b/Documentation/admin-guide/mm/memory-hotplug.rst > index 5c4432c96c4b..1548820a0762 100644 > --- a/Documentation/admin-guide/mm/memory-hotplug.rst > +++ b/Documentation/admin-guide/mm/memory-hotplug.rst > @@ -396,6 +396,20 @@ Need more implementation yet.... > Locking Internals > ================= > > +There are three locks involved in memory-hotplug, two global lock and one local > +lock: > + > +- device_hotplug_lock > +- mem_hotplug_lock > +- device_lock > + > +Currently, they are twisted together for all kinds of reasons. The following > +part is divded into device_hotplug_lock and mem_hotplug_lock parts s/divded/divided/ > +respectively to describe those tricky situations. > + > +device_hotplug_lock > +--------------------- > + > When adding/removing memory that uses memory block devices (i.e. ordinary RAM), > the device_hotplug_lock should be held to: > > @@ -417,14 +431,21 @@ memory faster than expected: > As the device is visible to user space before taking the device_lock(), this > can result in a lock inversion. > > +mem_hotplug_lock > +--------------------- > + I would this section start after the following paragraph, as most of that paragraph belongs to the device_hotplug_lock. > onlining/offlining of memory should be done via device_online()/ > -device_offline() - to make sure it is properly synchronized to actions > -via sysfs. Holding device_hotplug_lock is advised (to e.g. protect online_type) > +device_offline() - to make sure it is properly synchronized to actions via > +sysfs. Even mem_hotplug_lock is used to protect the process, because of the > +lock inversion described above, holding device_hotplug_lock is still advised > +(to e.g. protect online_type) > > When adding/removing/onlining/offlining memory or adding/removing > heterogeneous/device memory, we should always hold the mem_hotplug_lock in > write mode to serialise memory hotplug (e.g. access to global/zone > -variables). > +variables). Currently, we take advantage of this to serialise sparsemem's > +mem_section handling in sparse_add_one_section() and > +sparse_remove_one_section(). > > 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 > >> >> >> Thanks, >> >> David / dhildenb > Apart from that looks good to me, thanks! -- Thanks, David / dhildenb
