On Fri, Nov 30, 2018 at 10:19:13AM +0100, David Hildenbrand wrote: >>> 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 :-) > >-- > >Thanks, > >David / dhildenb -- Wei Yang Help you, Help me
