linux/lib/kernel_lock.c
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   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 */
  26static  __cacheline_aligned_in_smp DEFINE_RAW_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 - do_raw_spin_trylock will never
  40 * return false in that case)
  41 */
  42int __lockfunc __reacquire_kernel_lock(void)
  43{
  44        while (!do_raw_spin_trylock(&kernel_flag)) {
  45                if (need_resched())
  46                        return -EAGAIN;
  47                cpu_relax();
  48        }
  49        preempt_disable();
  50        return 0;
  51}
  52
  53void __lockfunc __release_kernel_lock(void)
  54{
  55        do_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 * do_raw_spin_trylock() will always succeed.
  63 */
  64#ifdef CONFIG_PREEMPT
  65static inline void __lock_kernel(void)
  66{
  67        preempt_disable();
  68        if (unlikely(!do_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                        do_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 (raw_spin_is_locked(&kernel_flag))
  86                                cpu_relax();
  87                        preempt_disable();
  88                } while (!do_raw_spin_trylock(&kernel_flag));
  89        }
  90}
  91
  92#else
  93
  94/*
  95 * Non-preemption case - just get the spinlock
  96 */
  97static inline void __lock_kernel(void)
  98{
  99        do_raw_spin_lock(&kernel_flag);
 100}
 101#endif
 102
 103static 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        do_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 */
 119void __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
 132void __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
 141EXPORT_SYMBOL(_lock_kernel);
 142EXPORT_SYMBOL(_unlock_kernel);
 143
 144