From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Date: Fri, 23 Jun 2017 11:43:30 +0200
Subject: [PATCH] mm, swap: don't disable preemption while taking the per-CPU
 cache

get_cpu_var() disables preemption and returns the per-CPU version of the
variable. Disabling preemption is useful to ensure atomic access to the
variable within the critical section.
In this case however, after the per-CPU version of the variable is
obtained the ->free_lock is acquired. For that reason it seems the raw
accessor could be used. It only seems that ->slots_ret should be
retested (because with disabled preemption this variable can not be set
to NULL otherwise).
This popped up during PREEMPT-RT testing because it tries to take
spinlocks in a preempt disabled section.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 mm/swap_slots.c |    5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

--- a/mm/swap_slots.c
+++ b/mm/swap_slots.c
@@ -267,11 +267,11 @@ int free_swap_slot(swp_entry_t entry)
 {
 	struct swap_slots_cache *cache;
 
-	cache = &get_cpu_var(swp_slots);
+	cache = raw_cpu_ptr(&swp_slots);
 	if (use_swap_slot_cache && cache->slots_ret) {
 		spin_lock_irq(&cache->free_lock);
 		/* Swap slots cache may be deactivated before acquiring lock */
-		if (!use_swap_slot_cache) {
+		if (!use_swap_slot_cache || !cache->slots_ret) {
 			spin_unlock_irq(&cache->free_lock);
 			goto direct_free;
 		}
@@ -291,7 +291,6 @@ int free_swap_slot(swp_entry_t entry)
 direct_free:
 		swapcache_free_entries(&entry, 1);
 	}
-	put_cpu_var(swp_slots);
 
 	return 0;
 }