[PATCH] get/put_cpu methods

This implements a get_cpu() and matching put_cpu() to safely hand out
the current CPU to avoid preempt races.  Andrew and I have been bitching
about the need for such a method.

I also went ahead and replaced an example of current explicit
preempt-off with the new methods, as a case in point.

Documentation/preempt-locking.txt

@@ -31,6 +31,8 @@ require it otherwise.  Second, when a preempted task is finally rescheduled,
 the previous value of smp_processor_id may not equal the current.  You must
 protect these situations by disabling preemption around them.
 
+You can also use put_cpu() and get_cpu(), which will disable preemption.
+
 
 RULE #2: CPU state must be protected.
 

include/linux/smp.h

@@ -96,5 +96,9 @@ static inline void smp_send_reschedule_all(void) { }
 #define per_cpu(var, cpu)			var
 #define this_cpu(var)				var
 
-#endif
-#endif
+#endif /* !SMP */
+
+#define get_cpu()	({ preempt_disable(); smp_processor_id(); })
+#define put_cpu()	preempt_enable()
+
+#endif /* __LINUX_SMP_H */

mm/page-writeback.c

@@ -117,15 +117,14 @@ void balance_dirty_pages_ratelimited(struct address_space *mapping)
 	} ____cacheline_aligned ratelimits[NR_CPUS];
 	int cpu;
 
-	preempt_disable();
-	cpu = smp_processor_id();
+	cpu = get_cpu();
 	if (ratelimits[cpu].count++ >= 1000) {
 		ratelimits[cpu].count = 0;
-		preempt_enable();
+		put_cpu();
 		balance_dirty_pages(mapping);
 		return;
 	}
-	preempt_enable();
+	put_cpu();
 }
 
 /*