1 // SPDX-License-Identifier: GPL-2.0 2 #include <linux/export.h> 3 #include <linux/spinlock.h> 4 #include <linux/atomic.h> 5 6 /* 7 * This is an implementation of the notion of "decrement a 8 * reference count, and return locked if it decremented to zero". 9 * 10 * NOTE NOTE NOTE! This is _not_ equivalent to 11 * 12 * if (atomic_dec_and_test(&atomic)) { 13 * spin_lock(&lock); 14 * return 1; 15 * } 16 * return 0; 17 * 18 * because the spin-lock and the decrement must be 19 * "atomic". 20 */ 21 int _atomic_dec_and_lock(atomic_t *atomic, spinlock_t *lock) 22 { 23 /* Subtract 1 from counter unless that drops it to 0 (ie. it was 1) */ 24 if (atomic_add_unless(atomic, -1, 1)) 25 return 0; 26 27 /* Otherwise do it the slow way */ 28 spin_lock(lock); 29 if (atomic_dec_and_test(atomic)) 30 return 1; 31 spin_unlock(lock); 32 return 0; 33 } 34 35 EXPORT_SYMBOL(_atomic_dec_and_lock); 36