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/module.h>
   9 #include <linux/kallsyms.h>
  10 #include <linux/semaphore.h>
  11 #define CREATE_TRACE_POINTS
  12 #include <linux/smp_lock.h>
  13
  14 /*
  15  * The 'big kernel lock'
  16  *
  17  * This spinlock is taken and released recursively by lock_kernel()
  18  * and unlock_kernel().  It is transparently dropped and reacquired
  19  * over schedule().  It is used to protect legacy code that hasn't
  20  * been migrated to a proper locking design yet.
  21  *
  22  * Don't use in new code.
  23  */
  24 static  __cacheline_aligned_in_smp DEFINE_SPINLOCK(kernel_flag);
  25
  26
  27 /*
  28  * Acquire/release the underlying lock from the scheduler.
  29  *
  30  * This is called with preemption disabled, and should
  31  * return an error value if it cannot get the lock and
  32  * TIF_NEED_RESCHED gets set.
  33  *
  34  * If it successfully gets the lock, it should increment
  35  * the preemption count like any spinlock does.
  36  *
  37  * (This works on UP too - _raw_spin_trylock will never
  38  * return false in that case)
  39  */
  40 int __lockfunc __reacquire_kernel_lock(void)
  41 {
  42         while (!_raw_spin_trylock(&kernel_flag)) {
  43                 if (need_resched())
  44                         return -EAGAIN;
  45                 cpu_relax();
  46         }
  47         preempt_disable();
  48         return 0;
  49 }
  50
  51 void __lockfunc __release_kernel_lock(void)
  52 {
  53         _raw_spin_unlock(&kernel_flag);
  54         preempt_enable_no_resched();
  55 }
  56
  57 /*
  58  * These are the BKL spinlocks - we try to be polite about preemption.
  59  * If SMP is not on (ie UP preemption), this all goes away because the
  60  * _raw_spin_trylock() will always succeed.
  61  */
  62 #ifdef CONFIG_PREEMPT
  63 static inline void __lock_kernel(void)
  64 {
  65         preempt_disable();
  66         if (unlikely(!_raw_spin_trylock(&kernel_flag))) {
  67                 /*
  68                  * If preemption was disabled even before this
  69                  * was called, there's nothing we can be polite
  70                  * about - just spin.
  71                  */
  72                 if (preempt_count() > 1) {
  73                         _raw_spin_lock(&kernel_flag);
  74                         return;
  75                 }
  76
  77                 /*
  78                  * Otherwise, let's wait for the kernel lock
  79                  * with preemption enabled..
  80                  */
  81                 do {
  82                         preempt_enable();
  83                         while (spin_is_locked(&kernel_flag))
  84                                 cpu_relax();
  85                         preempt_disable();
  86                 } while (!_raw_spin_trylock(&kernel_flag));
  87         }
  88 }
  89
  90 #else
  91
  92 /*
  93  * Non-preemption case - just get the spinlock
  94  */
  95 static inline void __lock_kernel(void)
  96 {
  97         _raw_spin_lock(&kernel_flag);
  98 }
  99 #endif
 100
 101 static inline void __unlock_kernel(void)
 102 {
 103         /*
 104          * the BKL is not covered by lockdep, so we open-code the
 105          * unlocking sequence (and thus avoid the dep-chain ops):
 106          */
 107         _raw_spin_unlock(&kernel_flag);
 108         preempt_enable();
 109 }
 110
 111 /*
 112  * Getting the big kernel lock.
 113  *
 114  * This cannot happen asynchronously, so we only need to
 115  * worry about other CPU's.
 116  */
 117 void __lockfunc _lock_kernel(void)
 118 {
 119         int depth = current->lock_depth+1;
 120         if (likely(!depth))
 121                 __lock_kernel();
 122         current->lock_depth = depth;
 123 }
 124
 125 void __lockfunc _unlock_kernel(void)
 126 {
 127         BUG_ON(current->lock_depth < 0);
 128         if (likely(--current->lock_depth < 0))
 129                 __unlock_kernel();
 130 }
 131
 132 EXPORT_SYMBOL(_lock_kernel);
 133 EXPORT_SYMBOL(_unlock_kernel);
 134