On Wed 05-12-18 10:34:26, Wei Yang wrote: > Currently locking for memory hotplug is a little complicated. > > Generally speaking, we leverage the two global lock: > > * device_hotplug_lock > * mem_hotplug_lock > > to serialise the process. > > While for the long term, we are willing to have more fine-grained lock > to provide higher scalability. > > This patch divides Locking Internal section based on these two global > locks to help readers to understand it. Also it adds some new finding to > enrich it. > > [David: words arrangement] > > Signed-off-by: Wei Yang <richard.weiyang@xxxxxxxxx> For a love of mine I cannot find the locking description by Oscar. Maybe it never existed and I just made it up ;) But if it is not imaginary then my recollection is that it was much more comprehensive. If not then even this is a good start. > --- > Documentation/core-api/memory-hotplug.rst | 27 ++++++++++++++++++++++++--- > 1 file changed, 24 insertions(+), 3 deletions(-) > > diff --git a/Documentation/core-api/memory-hotplug.rst b/Documentation/core-api/memory-hotplug.rst > index de7467e48067..95662b283328 100644 > --- a/Documentation/core-api/memory-hotplug.rst > +++ b/Documentation/core-api/memory-hotplug.rst > @@ -89,6 +89,20 @@ NOTIFY_STOP stops further processing of the notification queue. > 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 divided into device_hotplug_lock and mem_hotplug_lock parts > +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: > > @@ -111,13 +125,20 @@ As the device is visible to user space before taking the device_lock(), this > can result in a lock inversion. > > 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) > + > +mem_hotplug_lock > +--------------------- > > 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 > -- > 2.15.1 > -- Michal Hocko SUSE Labs
