On Tue 24-09-19 16:36:15, David Hildenbrand wrote:
> Since commit 3f906ba23689 ("mm/memory-hotplug: switch locking to a percpu
> rwsem") we do a cpus_read_lock() in mem_hotplug_begin(). This was
> introduced to fix a potential deadlock between get_online_mems() and
> get_online_cpus() - the memory and cpu hotplug lock. The root issue was
> that build_all_zonelists() -> stop_machine() required the cpu hotplug lock:
> The reason is that memory hotplug takes the memory hotplug lock and
> then calls stop_machine() which calls get_online_cpus(). That's the
> reverse lock order to get_online_cpus(); get_online_mems(); in
> mm/slub_common.c
>
> So memory hotplug never really required any cpu lock itself, only
> stop_machine() and lru_add_drain_all() required it. Back then,
> stop_machine_cpuslocked() and lru_add_drain_all_cpuslocked() were used
> as the cpu hotplug lock was now obtained in the caller.
>
> Since commit 11cd8638c37f ("mm, page_alloc: remove stop_machine from build
> all_zonelists"), the stop_machine_cpuslocked() call is gone.
> build_all_zonelists() does no longer require the cpu lock and does no
> longer make use of stop_machine().
>
> Since commit 9852a7212324 ("mm: drop hotplug lock from
> lru_add_drain_all()"), lru_add_drain_all() "Doesn't need any cpu hotplug
> locking because we do rely on per-cpu kworkers being shut down before our
> page_alloc_cpu_dead callback is executed on the offlined cpu.". The
> lru_add_drain_all_cpuslocked() variant was removed.
>
> So there is nothing left that requires the cpu hotplug lock. The memory
> hotplug lock and the device hotplug lock are sufficient.
I would love to see this happen. The biggest offenders should be gone.
I really hated how those two locks have been conflated which likely
resulted in some undocumented/unintended dependencies. So for now, I
cannot really tell you whether the patch is correct. It would really
require a lot of testing. I am not sure this is reasonably reviewable.
So please add some testing results (ideally cpu hotplug racing a lot
with the memory hotplug). Then I would be willing to give this a try
and see. First by keeping it in linux-next for a release or two and then
eyes closed, fingers crossed and go to the wild. Do we have a tag for
that Dared-by maybe?
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Oscar Salvador <osalvador@xxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Cc: Pavel Tatashin <pasha.tatashin@xxxxxxxxxx>
> Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> ---
>
> RFC -> v1:
> - Reword and add more details why the cpu hotplug lock was needed here
> in the first place, and why we no longer require it.
>
> ---
> mm/memory_hotplug.c | 2 --
> 1 file changed, 2 deletions(-)
>
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index c3e9aed6023f..5fa30f3010e1 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -88,14 +88,12 @@ __setup("memhp_default_state=", setup_memhp_default_state);
>
> void mem_hotplug_begin(void)
> {
> - cpus_read_lock();
> percpu_down_write(&mem_hotplug_lock);
> }
>
> void mem_hotplug_done(void)
> {
> percpu_up_write(&mem_hotplug_lock);
> - cpus_read_unlock();
> }
>
> u64 max_mem_size = U64_MAX;
> --
> 2.21.0
--
Michal Hocko
SUSE Labs
