[PATCH v1] mm/memory_hotplug: Don't take the cpu_hotplug_lock

[David Hildenbrand]

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.

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