On Thu 25-07-19 16:35:07, David Hildenbrand wrote: > On 25.07.19 15:57, Michal Hocko wrote: > > On Thu 25-07-19 15:05:02, David Hildenbrand wrote: > >> On 25.07.19 14:56, Michal Hocko wrote: > >>> On Wed 24-07-19 16:30:17, David Hildenbrand wrote: > >>>> We end up calling __add_memory() without the device hotplug lock held. > >>>> (I used a local patch to assert in __add_memory() that the > >>>> device_hotplug_lock is held - I might upstream that as well soon) > >>>> > >>>> [ 26.771684] create_memory_block_devices+0xa4/0x140 > >>>> [ 26.772952] add_memory_resource+0xde/0x200 > >>>> [ 26.773987] __add_memory+0x6e/0xa0 > >>>> [ 26.775161] acpi_memory_device_add+0x149/0x2b0 > >>>> [ 26.776263] acpi_bus_attach+0xf1/0x1f0 > >>>> [ 26.777247] acpi_bus_attach+0x66/0x1f0 > >>>> [ 26.778268] acpi_bus_attach+0x66/0x1f0 > >>>> [ 26.779073] acpi_bus_attach+0x66/0x1f0 > >>>> [ 26.780143] acpi_bus_scan+0x3e/0x90 > >>>> [ 26.780844] acpi_scan_init+0x109/0x257 > >>>> [ 26.781638] acpi_init+0x2ab/0x30d > >>>> [ 26.782248] do_one_initcall+0x58/0x2cf > >>>> [ 26.783181] kernel_init_freeable+0x1bd/0x247 > >>>> [ 26.784345] kernel_init+0x5/0xf1 > >>>> [ 26.785314] ret_from_fork+0x3a/0x50 > >>>> > >>>> So perform the locking just like in acpi_device_hotplug(). > >>> > >>> While playing with the device_hotplug_lock, can we actually document > >>> what it is protecting please? I have a bad feeling that we are adding > >>> this lock just because some other code path does rather than with a good > >>> idea why it is needed. This patch just confirms that. What exactly does > >>> the lock protect from here in an early boot stage. > >> > >> We have plenty of documentation already > >> > >> mm/memory_hotplug.c > >> > >> git grep -C5 device_hotplug mm/memory_hotplug.c > >> > >> Also see > >> > >> Documentation/core-api/memory-hotplug.rst > > > > OK, fair enough. I was more pointing to a documentation right there > > where the lock is declared because that is the place where people > > usually check for documentation. The core-api documentation looks quite > > nice. And based on that doc it seems that this patch is actually not > > needed because neither the online/offline or cpu hotplug should be > > possible that early unless I am missing something. > > I really prefer to stick to locking rules as outlined on the > interfaces if it doesn't hurt. Why it is not needed is not clear. > > > > >> Regarding the early stage: primarily lockdep as I mentioned. > > > > Could you add a lockdep splat that would be fixed by this patch to the > > changelog for reference? > > > > I have one where I enforce what's documented (but that's of course not > upstream and therefore not "real" yet)
Then I suppose to not add locking for something that is not a problem. Really, think about it. People will look at this code and follow the lead without really knowing why the locking is needed. device_hotplug_lock has its purpose and if the code in question doesn't need synchronization for the documented scenarios then the locking simply shouldn't be there. Adding the lock just because of a non-existing, and IMHO dubious, lockdep splats is just wrong. We need to rationalize the locking here, not to add more hacks. -- Michal Hocko SUSE Labs
