/*
   * lib/kernel_lock.c
   *
   * This is the traditional BKL - big kernel lock. Largely

   * relegated to obsolescence, but used by various less

   * important (or lazy) subsystems.
   */
  #include <linux/smp_lock.h>
  #include <linux/module.h>
  #include <linux/kallsyms.h>

  #include <linux/semaphore.h>

  /*

   * The 'big kernel lock'

   *

   * This spinlock is taken and released recursively by lock_kernel()

   * and unlock_kernel().  It is transparently dropped and reacquired

   * over schedule().  It is used to protect legacy code that hasn't
   * been migrated to a proper locking design yet.
   *

   * Don't use in new code.
   */

  static  __cacheline_aligned_in_smp DEFINE_SPINLOCK(kernel_flag);

  
  /*

   * Acquire/release the underlying lock from the scheduler.

   *

   * This is called with preemption disabled, and should
   * return an error value if it cannot get the lock and
   * TIF_NEED_RESCHED gets set.

   *

   * If it successfully gets the lock, it should increment
   * the preemption count like any spinlock does.
   *
   * (This works on UP too - _raw_spin_trylock will never
   * return false in that case)

   */
  int __lockfunc __reacquire_kernel_lock(void)
  {

  	while (!_raw_spin_trylock(&kernel_flag)) {
  		if (test_thread_flag(TIF_NEED_RESCHED))
  			return -EAGAIN;
  		cpu_relax();
  	}

  	preempt_disable();

  	return 0;
  }
  
  void __lockfunc __release_kernel_lock(void)
  {

  	_raw_spin_unlock(&kernel_flag);
  	preempt_enable_no_resched();

  }
  
  /*

   * These are the BKL spinlocks - we try to be polite about preemption.
   * If SMP is not on (ie UP preemption), this all goes away because the
   * _raw_spin_trylock() will always succeed.

   */

  #ifdef CONFIG_PREEMPT
  static inline void __lock_kernel(void)

  {

  	preempt_disable();
  	if (unlikely(!_raw_spin_trylock(&kernel_flag))) {
  		/*
  		 * If preemption was disabled even before this
  		 * was called, there's nothing we can be polite
  		 * about - just spin.
  		 */
  		if (preempt_count() > 1) {
  			_raw_spin_lock(&kernel_flag);
  			return;
  		}

  		/*

  		 * Otherwise, let's wait for the kernel lock
  		 * with preemption enabled..

  		 */

  		do {
  			preempt_enable();
  			while (spin_is_locked(&kernel_flag))
  				cpu_relax();
  			preempt_disable();
  		} while (!_raw_spin_trylock(&kernel_flag));
  	}
  }

  #else
  
  /*
   * Non-preemption case - just get the spinlock
   */
  static inline void __lock_kernel(void)
  {
  	_raw_spin_lock(&kernel_flag);

  }

  #endif

  static inline void __unlock_kernel(void)

  {

  	/*
  	 * the BKL is not covered by lockdep, so we open-code the
  	 * unlocking sequence (and thus avoid the dep-chain ops):
  	 */
  	_raw_spin_unlock(&kernel_flag);
  	preempt_enable();
  }

  /*
   * Getting the big kernel lock.
   *
   * This cannot happen asynchronously, so we only need to
   * worry about other CPU's.
   */
  void __lockfunc lock_kernel(void)
  {
  	int depth = current->lock_depth+1;
  	if (likely(!depth))
  		__lock_kernel();
  	current->lock_depth = depth;
  }

  void __lockfunc unlock_kernel(void)
  {
  	BUG_ON(current->lock_depth < 0);
  	if (likely(--current->lock_depth < 0))
  		__unlock_kernel();

  }

  EXPORT_SYMBOL(lock_kernel);
  EXPORT_SYMBOL(unlock_kernel);