LXR linux/lib/kernel

   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
  12/*
  13 * The 'big kernel semaphore'
  14 *
  15 * This mutex is taken and released recursively by lock_kernel()
  16 * and unlock_kernel().  It is transparently dropped and reacquired
  17 * over schedule().  It is used to protect legacy code that hasn't
  18 * been migrated to a proper locking design yet.
  19 *
  20 * Note: code locked by this semaphore will only be serialized against
  21 * other code using the same locking facility. The code guarantees that
  22 * the task remains on the same CPU.
  23 *
  24 * Don't use in new code.
  25 */
  26static DECLARE_MUTEX(kernel_sem);
  27
  28/*
  29 * Re-acquire the kernel semaphore.
  30 *
  31 * This function is called with preemption off.
  32 *
  33 * We are executing in schedule() so the code must be extremely careful
  34 * about recursion, both due to the down() and due to the enabling of
  35 * preemption. schedule() will re-check the preemption flag after
  36 * reacquiring the semaphore.
  37 */
  38int __lockfunc __reacquire_kernel_lock(void)
  39{
  40        struct task_struct *task = current;
  41        int saved_lock_depth = task->lock_depth;
  42
  43        BUG_ON(saved_lock_depth < 0);
  44
  45        task->lock_depth = -1;
  46        preempt_enable_no_resched();
  47
  48        down(&kernel_sem);
  49
  50        preempt_disable();
  51        task->lock_depth = saved_lock_depth;
  52
  53        return 0;
  54}
  55
  56void __lockfunc __release_kernel_lock(void)
  57{
  58        up(&kernel_sem);
  59}
  60
  61/*
  62 * Getting the big kernel semaphore.
  63 */
  64void __lockfunc lock_kernel(void)
  65{
  66        struct task_struct *task = current;
  67        int depth = task->lock_depth + 1;
  68
  69        if (likely(!depth))
  70                /*
  71                 * No recursion worries - we set up lock_depth _after_
  72                 */
  73                down(&kernel_sem);
  74
  75        task->lock_depth = depth;
  76}
  77
  78void __lockfunc unlock_kernel(void)
  79{
  80        struct task_struct *task = current;
  81
  82        BUG_ON(task->lock_depth < 0);
  83
  84        if (likely(--task->lock_depth < 0))
  85                up(&kernel_sem);
  86}
  87
  88EXPORT_SYMBOL(lock_kernel);
  89EXPORT_SYMBOL(unlock_kernel);
  90
  91