FreeBSD/Linux Kernel Cross Reference
sys/lib/dec_and_lock.c

     #include <linux/module.h>
     #include <linux/spinlock.h>
     #include <asm/atomic.h>
     
     /*
      * This is an architecture-neutral, but slow,
      * implementation of the notion of "decrement
      * a reference count, and return locked if it
      * decremented to zero".
     *
     * NOTE NOTE NOTE! This is _not_ equivalent to
     *
     *      if (atomic_dec_and_test(&atomic)) {
     *              spin_lock(&lock);
     *              return 1;
     *      }
     *      return 0;
     *
     * because the spin-lock and the decrement must be
     * "atomic".
     *
     * This slow version gets the spinlock unconditionally,
     * and releases it if it isn't needed. Architectures
     * are encouraged to come up with better approaches,
     * this is trivially done efficiently using a load-locked
     * store-conditional approach, for example.
     */
    
    #ifndef ATOMIC_DEC_AND_LOCK
    int atomic_dec_and_lock(atomic_t *atomic, spinlock_t *lock)
    {
            spin_lock(lock);
            if (atomic_dec_and_test(atomic))
                    return 1;
            spin_unlock(lock);
            return 0;
    }
    
    EXPORT_SYMBOL(atomic_dec_and_lock);
    #endif

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