mm/slub: simplify __cmpxchg_double_slab() and slab_[un]lock()

The PREEMPT_RT specific disabling of irqs in __cmpxchg_double_slab()
(through slab_[un]lock()) is unnecessary as bit_spin_lock() disables
preemption and that's sufficient on PREEMPT_RT where no allocation/free
operation is performed in hardirq context and so can't interrupt the
current operation.

That means we no longer need the slab_[un]lock() wrappers, so delete
them and rename the current __slab_[un]lock() to slab_[un]lock().
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: David Rientjes <rientjes@google.com>
Reviewed-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

mm/slub.c

@@ -446,7 +446,7 @@ slub_set_cpu_partial(struct kmem_cache *s, unsigned int nr_objects)
 /*
  * Per slab locking using the pagelock
  */
-static __always_inline void __slab_lock(struct slab *slab)
+static __always_inline void slab_lock(struct slab *slab)
 {
 	struct page *page = slab_page(slab);
 
@@ -454,7 +454,7 @@ static __always_inline void __slab_lock(struct slab *slab)
 	bit_spin_lock(PG_locked, &page->flags);
 }
 
-static __always_inline void __slab_unlock(struct slab *slab)
+static __always_inline void slab_unlock(struct slab *slab)
 {
 	struct page *page = slab_page(slab);
 
@@ -462,24 +462,12 @@ static __always_inline void __slab_unlock(struct slab *slab)
 	__bit_spin_unlock(PG_locked, &page->flags);
 }
 
-static __always_inline void slab_lock(struct slab *slab, unsigned long *flags)
-{
-	if (IS_ENABLED(CONFIG_PREEMPT_RT))
-		local_irq_save(*flags);
-	__slab_lock(slab);
-}
-
-static __always_inline void slab_unlock(struct slab *slab, unsigned long *flags)
-{
-	__slab_unlock(slab);
-	if (IS_ENABLED(CONFIG_PREEMPT_RT))
-		local_irq_restore(*flags);
-}
-
 /*
  * Interrupts must be disabled (for the fallback code to work right), typically
- * by an _irqsave() lock variant. Except on PREEMPT_RT where locks are different
- * so we disable interrupts as part of slab_[un]lock().
+ * by an _irqsave() lock variant. On PREEMPT_RT the preempt_disable(), which is
+ * part of bit_spin_lock(), is sufficient because the policy is not to allow any
+ * allocation/ free operation in hardirq context. Therefore nothing can
+ * interrupt the operation.
  */
 static inline bool __cmpxchg_double_slab(struct kmem_cache *s, struct slab *slab,
 		void *freelist_old, unsigned long counters_old,
@@ -498,18 +486,15 @@ static inline bool __cmpxchg_double_slab(struct kmem_cache *s, struct slab *slab
 	} else
 #endif
 	{
-		/* init to 0 to prevent spurious warnings */
-		unsigned long flags = 0;
-
-		slab_lock(slab, &flags);
+		slab_lock(slab);
 		if (slab->freelist == freelist_old &&
 					slab->counters == counters_old) {
 			slab->freelist = freelist_new;
 			slab->counters = counters_new;
-			slab_unlock(slab, &flags);
+			slab_unlock(slab);
 			return true;
 		}
-		slab_unlock(slab, &flags);
+		slab_unlock(slab);
 	}
 
 	cpu_relax();
@@ -540,16 +525,16 @@ static inline bool cmpxchg_double_slab(struct kmem_cache *s, struct slab *slab,
 		unsigned long flags;
 
 		local_irq_save(flags);
-		__slab_lock(slab);
+		slab_lock(slab);
 		if (slab->freelist == freelist_old &&
 					slab->counters == counters_old) {
 			slab->freelist = freelist_new;
 			slab->counters = counters_new;
-			__slab_unlock(slab);
+			slab_unlock(slab);
 			local_irq_restore(flags);
 			return true;
 		}
-		__slab_unlock(slab);
+		slab_unlock(slab);
 		local_irq_restore(flags);
 	}