[PATCH] remove preempt workaround in slab.c

Before the irqs_disabled() check in preempt_schedule(), we worked around
some locking issues in slab.c.  Now that we will never preempt with
interrupts disabled, we can remove those and clean things up.

This is courtesy of Manfred Spraul.

mm/slab.c

@@ -1357,11 +1357,7 @@ void* kmem_cache_alloc_batch(kmem_cache_t* cachep, int flags)
 		cc_entry(cc)[cc->avail++] =
 				kmem_cache_alloc_one_tail(cachep, slabp);
 	}
-	/*
-	 * CAREFUL: do not enable preemption yet, the per-CPU
-	 * entries rely on us being atomic.
-	 */
-	_raw_spin_unlock(&cachep->spinlock);
+	spin_unlock(&cachep->spinlock);
 
 	if (cc->avail)
 		return cc_entry(cc)[--cc->avail];
@@ -1389,8 +1385,6 @@ static inline void * __kmem_cache_alloc (kmem_cache_t *cachep, int flags)
 				STATS_INC_ALLOCMISS(cachep);
 				objp = kmem_cache_alloc_batch(cachep,flags);
 				local_irq_restore(save_flags);
-				/* end of non-preemptible region */
-				preempt_enable();
 				if (!objp)
 					goto alloc_new_slab_nolock;
 				return objp;