memcg: no stock lock for cpu hot-unplug

Previously on the cpu hot-unplug, the kernel would call drain_obj_stock()
with objcg local lock.  However local lock was not needed as the stock
which was accessed belongs to a dead cpu but we kept it there to disable
irqs as drain_obj_stock() may call mod_objcg_mlstate() which required irqs
disabled.  However there is no need to disable irqs now for
mod_objcg_mlstate(), so we can remove the local lock altogether from cpu
hot-unplug path.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Shakeel Butt <[email protected]>
Acked-by: Vlastimil Babka <[email protected]>
Cc: Alexei Starovoitov <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Muchun Song <[email protected]>
Cc: Roman Gushchin <[email protected]>
Cc: Sebastian Andrzej Siewior <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: shakeelb

mm/memcontrol.c

@@ -2023,17 +2023,8 @@ void drain_all_stock(struct mem_cgroup *root_memcg)
 
 static int memcg_hotplug_cpu_dead(unsigned int cpu)
 {
-	struct obj_stock_pcp *obj_st;
-	unsigned long flags;
-
-	obj_st = &per_cpu(obj_stock, cpu);
-
-	/* drain_obj_stock requires objstock.lock */
-	local_lock_irqsave(&obj_stock.lock, flags);
-	drain_obj_stock(obj_st);
-	local_unlock_irqrestore(&obj_stock.lock, flags);
-
 	/* no need for the local lock */
+	drain_obj_stock(&per_cpu(obj_stock, cpu));
 	drain_stock_fully(&per_cpu(memcg_stock, cpu));
 
 	return 0;