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 #include <linux/smp_lock.h>
  12
  13 #define CREATE_TRACE_POINTS
  14 #include <trace/events/bkl.h>
  15
  16 /*
  17  * The 'big kernel lock'
  18  *
  19  * This spinlock is taken and released recursively by lock_kernel()
  20  * and unlock_kernel().  It is transparently dropped and reacquired
  21  * over schedule().  It is used to protect legacy code that hasn't
  22  * been migrated to a proper locking design yet.
  23  *
  24  * Don't use in new code.
  25  */
  26 static  __cacheline_aligned_in_smp DEFINE_SPINLOCK(kernel_flag);
  27
  28
  29 /*
  30  * Acquire/release the underlying lock from the scheduler.
  31  *
  32  * This is called with preemption disabled, and should
  33  * return an error value if it cannot get the lock and
  34  * TIF_NEED_RESCHED gets set.
  35  *
  36  * If it successfully gets the lock, it should increment
  37  * the preemption count like any spinlock does.
  38  *
  39  * (This works on UP too - _raw_spin_trylock will never
  40  * return false in that case)
  41  */
  42 int __lockfunc __reacquire_kernel_lock(void)
  43 {
  44         while (!_raw_spin_trylock(&kernel_flag)) {
  45                 if (need_resched())
  46                         return -EAGAIN;
  47                 cpu_relax();
  48         }
  49         preempt_disable();
  50         return 0;
  51 }
  52
  53 void __lockfunc __release_kernel_lock(void)
  54 {
  55         _raw_spin_unlock(&kernel_flag);
  56         preempt_enable_no_resched();
  57 }
  58
  59 /*
  60  * These are the BKL spinlocks - we try to be polite about preemption.
  61  * If SMP is not on (ie UP preemption), this all goes away because the
  62  * _raw_spin_trylock() will always succeed.
  63  */
  64 #ifdef CONFIG_PREEMPT
  65 static inline void __lock_kernel(void)
  66 {
  67         preempt_disable();
  68         if (unlikely(!_raw_spin_trylock(&kernel_flag))) {
  69                 /*
  70                  * If preemption was disabled even before this
  71                  * was called, there's nothing we can be polite
  72                  * about - just spin.
  73                  */
  74                 if (preempt_count() > 1) {
  75                         _raw_spin_lock(&kernel_flag);
  76                         return;
  77                 }
  78
  79                 /*
  80                  * Otherwise, let's wait for the kernel lock
  81                  * with preemption enabled..
  82                  */
  83                 do {
  84                         preempt_enable();
  85                         while (spin_is_locked(&kernel_flag))
  86                                 cpu_relax();
  87                         preempt_disable();
  88                 } while (!_raw_spin_trylock(&kernel_flag));
  89         }
  90 }
  91
  92 #else
  93
  94 /*
  95  * Non-preemption case - just get the spinlock
  96  */
  97 static inline void __lock_kernel(void)
  98 {
  99         _raw_spin_lock(&kernel_flag);
 100 }
 101 #endif
 102
 103 static inline void __unlock_kernel(void)
 104 {
 105         /*
 106          * the BKL is not covered by lockdep, so we open-code the
 107          * unlocking sequence (and thus avoid the dep-chain ops):
 108          */
 109         _raw_spin_unlock(&kernel_flag);
 110         preempt_enable();
 111 }
 112
 113 /*
 114  * Getting the big kernel lock.
 115  *
 116  * This cannot happen asynchronously, so we only need to
 117  * worry about other CPU's.
 118  */
 119 void __lockfunc _lock_kernel(const char *func, const char *file, int line)
 120 {
 121         int depth = current->lock_depth + 1;
 122
 123         trace_lock_kernel(func, file, line);
 124
 125         if (likely(!depth)) {
 126                 might_sleep();
 127                 __lock_kernel();
 128         }
 129         current->lock_depth = depth;
 130 }
 131
 132 void __lockfunc _unlock_kernel(const char *func, const char *file, int line)
 133 {
 134         BUG_ON(current->lock_depth < 0);
 135         if (likely(--current->lock_depth < 0))
 136                 __unlock_kernel();
 137
 138         trace_unlock_kernel(func, file, line);
 139 }
 140
 141 EXPORT_SYMBOL(_lock_kernel);
 142 EXPORT_SYMBOL(_unlock_kernel);
 143