lib/kernel_lock.c

   1 /*
   2  * lib/kernel_lock.c
   3  *
   4  * This is the traditional BKL - big kernel lock. Largely
   5  * relegated to obsolescense, but used by various less
   6  * important (or lazy) subsystems.
   7  */
   8 #include <linux/smp_lock.h>
   9 #include <linux/module.h>
  10
  11 /*
  12  * The 'big kernel lock'
  13  *
  14  * This spinlock is taken and released recursively by lock_kernel()
  15  * and unlock_kernel().  It is transparently dropped and reaquired
  16  * over schedule().  It is used to protect legacy code that hasn't
  17  * been migrated to a proper locking design yet.
  18  *
  19  * Don't use in new code.
  20  */
  21 static spinlock_t kernel_flag __cacheline_aligned_in_smp = SPIN_LOCK_UNLOCKED;
  22
  23
  24 /*
  25  * Acquire/release the underlying lock from the scheduler.
  26  *
  27  * This is called with preemption disabled, and should
  28  * return an error value if it cannot get the lock and
  29  * TIF_NEED_RESCHED gets set.
  30  *
  31  * If it successfully gets the lock, it should increment
  32  * the preemption count like any spinlock does.
  33  *
  34  * (This works on UP too - _raw_spin_trylock will never
  35  * return false in that case)
  36  */
  37 int __lockfunc get_kernel_lock(void)
  38 {
  39         while (!_raw_spin_trylock(&kernel_flag)) {
  40                 if (test_thread_flag(TIF_NEED_RESCHED))
  41                         return -EAGAIN;
  42                 cpu_relax();
  43         }
  44         preempt_disable();
  45         return 0;
  46 }
  47
  48 void __lockfunc put_kernel_lock(void)
  49 {
  50         _raw_spin_unlock(&kernel_flag);
  51         preempt_enable_no_resched();
  52 }
  53
  54 /*
  55  * These are the BKL spinlocks - we try to be polite about preemption.
  56  * If SMP is not on (ie UP preemption), this all goes away because the
  57  * _raw_spin_trylock() will always succeed.
  58  */
  59 #ifdef CONFIG_PREEMPT
  60 static inline void __lock_kernel(void)
  61 {
  62         preempt_disable();
  63         if (unlikely(!_raw_spin_trylock(&kernel_flag))) {
  64                 /*
  65                  * If preemption was disabled even before this
  66                  * was called, there's nothing we can be polite
  67                  * about - just spin.
  68                  */
  69                 if (preempt_count() > 1) {
  70                         _raw_spin_lock(&kernel_flag);
  71                         return;
  72                 }
  73
  74                 /*
  75                  * Otherwise, let's wait for the kernel lock
  76                  * with preemption enabled..
  77                  */
  78                 do {
  79                         preempt_enable();
  80                         while (spin_is_locked(&kernel_flag))
  81                                 cpu_relax();
  82                         preempt_disable();
  83                 } while (!_raw_spin_trylock(&kernel_flag));
  84         }
  85 }
  86
  87 #else
  88
  89 /*
  90  * Non-preemption case - just get the spinlock
  91  */
  92 static inline void __lock_kernel(void)
  93 {
  94         _raw_spin_lock(&kernel_flag);
  95 }
  96 #endif
  97
  98 static inline void __unlock_kernel(void)
  99 {
 100         _raw_spin_unlock(&kernel_flag);
 101         preempt_enable();
 102 }
 103
 104 /*
 105  * Getting the big kernel lock.
 106  *
 107  * This cannot happen asynchronously, so we only need to
 108  * worry about other CPU's.
 109  */
 110 void __lockfunc lock_kernel(void)
 111 {
 112         int depth = current->lock_depth+1;
 113         if (likely(!depth))
 114                 __lock_kernel();
 115         current->lock_depth = depth;
 116 }
 117
 118 void __lockfunc unlock_kernel(void)
 119 {
 120         BUG_ON(current->lock_depth < 0);
 121         if (likely(--current->lock_depth < 0))
 122                 __unlock_kernel();
 123 }
 124
 125 EXPORT_SYMBOL(lock_kernel);
 126 EXPORT_SYMBOL(unlock_kernel);