lib/kernel_lock.c

   1 /*
   2  * lib/kernel_lock.c
   3  *
   4  * This is the traditional BKL - big kernel lock. Largely
   5  * relegated to obsolescence, but used by various less
   6  * important (or lazy) subsystems.
   7  */
   8 #include <linux/smp_lock.h>
   9 #include <linux/module.h>
  10 #include <linux/kallsyms.h>
  11 #include <linux/semaphore.h>
  12
  13 /*
  14  * The 'big kernel lock'
  15  *
  16  * This spinlock is taken and released recursively by lock_kernel()
  17  * and unlock_kernel().  It is transparently dropped and reacquired
  18  * over schedule().  It is used to protect legacy code that hasn't
  19  * been migrated to a proper locking design yet.
  20  *
  21  * Don't use in new code.
  22  */
  23 static  __cacheline_aligned_in_smp DEFINE_SPINLOCK(kernel_flag);
  24
  25
  26 /*
  27  * Acquire/release the underlying lock from the scheduler.
  28  *
  29  * This is called with preemption disabled, and should
  30  * return an error value if it cannot get the lock and
  31  * TIF_NEED_RESCHED gets set.
  32  *
  33  * If it successfully gets the lock, it should increment
  34  * the preemption count like any spinlock does.
  35  *
  36  * (This works on UP too - _raw_spin_trylock will never
  37  * return false in that case)
  38  */
  39 int __lockfunc __reacquire_kernel_lock(void)
  40 {
  41         while (!_raw_spin_trylock(&kernel_flag)) {
  42                 if (test_thread_flag(TIF_NEED_RESCHED))
  43                         return -EAGAIN;
  44                 cpu_relax();
  45         }
  46         preempt_disable();
  47         return 0;
  48 }
  49
  50 void __lockfunc __release_kernel_lock(void)
  51 {
  52         _raw_spin_unlock(&kernel_flag);
  53         preempt_enable_no_resched();
  54 }
  55
  56 /*
  57  * These are the BKL spinlocks - we try to be polite about preemption.
  58  * If SMP is not on (ie UP preemption), this all goes away because the
  59  * _raw_spin_trylock() will always succeed.
  60  */
  61 #ifdef CONFIG_PREEMPT
  62 static inline void __lock_kernel(void)
  63 {
  64         preempt_disable();
  65         if (unlikely(!_raw_spin_trylock(&kernel_flag))) {
  66                 /*
  67                  * If preemption was disabled even before this
  68                  * was called, there's nothing we can be polite
  69                  * about - just spin.
  70                  */
  71                 if (preempt_count() > 1) {
  72                         _raw_spin_lock(&kernel_flag);
  73                         return;
  74                 }
  75
  76                 /*
  77                  * Otherwise, let's wait for the kernel lock
  78                  * with preemption enabled..
  79                  */
  80                 do {
  81                         preempt_enable();
  82                         while (spin_is_locked(&kernel_flag))
  83                                 cpu_relax();
  84                         preempt_disable();
  85                 } while (!_raw_spin_trylock(&kernel_flag));
  86         }
  87 }
  88
  89 #else
  90
  91 /*
  92  * Non-preemption case - just get the spinlock
  93  */
  94 static inline void __lock_kernel(void)
  95 {
  96         _raw_spin_lock(&kernel_flag);
  97 }
  98 #endif
  99
 100 static inline void __unlock_kernel(void)
 101 {
 102         /*
 103          * the BKL is not covered by lockdep, so we open-code the
 104          * unlocking sequence (and thus avoid the dep-chain ops):
 105          */
 106         _raw_spin_unlock(&kernel_flag);
 107         preempt_enable();
 108 }
 109
 110 /*
 111  * Getting the big kernel lock.
 112  *
 113  * This cannot happen asynchronously, so we only need to
 114  * worry about other CPU's.
 115  */
 116 void __lockfunc lock_kernel(void)
 117 {
 118         int depth = current->lock_depth+1;
 119         if (likely(!depth))
 120                 __lock_kernel();
 121         current->lock_depth = depth;
 122 }
 123
 124 void __lockfunc unlock_kernel(void)
 125 {
 126         BUG_ON(current->lock_depth < 0);
 127         if (likely(--current->lock_depth < 0))
 128                 __unlock_kernel();
 129 }
 130
 131 EXPORT_SYMBOL(lock_kernel);
 132 EXPORT_SYMBOL(unlock_kernel);
 133