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Linux/lib/dec_and_lock.c

  1 #include <linux/export.h>
  2 #include <linux/spinlock.h>
  3 #include <linux/atomic.h>
  4 
  5 /*
  6  * This is an implementation of the notion of "decrement a
  7  * reference count, and return locked if it decremented to zero".
  8  *
  9  * NOTE NOTE NOTE! This is _not_ equivalent to
 10  *
 11  *      if (atomic_dec_and_test(&atomic)) {
 12  *              spin_lock(&lock);
 13  *              return 1;
 14  *      }
 15  *      return 0;
 16  *
 17  * because the spin-lock and the decrement must be
 18  * "atomic".
 19  */
 20 int _atomic_dec_and_lock(atomic_t *atomic, spinlock_t *lock)
 21 {
 22         /* Subtract 1 from counter unless that drops it to 0 (ie. it was 1) */
 23         if (atomic_add_unless(atomic, -1, 1))
 24                 return 0;
 25 
 26         /* Otherwise do it the slow way */
 27         spin_lock(lock);
 28         if (atomic_dec_and_test(atomic))
 29                 return 1;
 30         spin_unlock(lock);
 31         return 0;
 32 }
 33 
 34 EXPORT_SYMBOL(_atomic_dec_and_lock);
 35 

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