SMP - Big kernel lock (BKL)

- to isolate execution inside kernel we use a big kernel lock
  implemented as a spinlock

- the lock is acquired asap after entering kernel mode and released as
  late as possible. Only one CPU as a time can execute the core kernel
  code

- measurement son real hw show that the overhead of this lock is close
  to 0% of kernel time for the currnet system

- the overhead of this lock may be as high as 45% of kernel time in
  virtual machines depending on the ratio between physical CPUs
  available and emulated CPUs. The performance degradation is
  significant

kernel/spinlock.h

@@ -23,6 +23,7 @@ typedef struct spinlock {
 #define PRIVATE_SPINLOCK_DEFINE(name)	PRIVATE SPINLOCK_DEFINE(name)
 #define SPINLOCK_DECLARE(name)	extern SPINLOCK_DEFINE(name)
 #define spinlock_init(sl) do { (sl)->val = 0; } while (0)
+
 #if CONFIG_MAX_CPUS == 1
 #define spinlock_lock(sl)
 #define spinlock_unlock(sl)
@@ -32,6 +33,9 @@ typedef struct spinlock {
 #endif
 
 
-#endif
+#endif /* CONFIG_SMP */
+
+#define BKL_LOCK()	spinlock_lock(&big_kernel_lock)
+#define BKL_UNLOCK()	spinlock_unlock(&big_kernel_lock)
 
 #endif /* __SPINLOCK_H__ */