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1 /* 2 * arch/arm/include/asm/mutex.h 3 * 4 * ARM optimized mutex locking primitives 5 * 6 * Please look into asm-generic/mutex-xchg.h for a formal definition. 7 */ 8 #ifndef _ASM_MUTEX_H 9 #define _ASM_MUTEX_H 10 11 #if __LINUX_ARM_ARCH__ < 6 12 /* On pre-ARMv6 hardware the swp based implementation is the most efficient. */ 13 # include <asm-generic/mutex-xchg.h> 14 #else 15 16 /* 17 * Attempting to lock a mutex on ARMv6+ can be done with a bastardized 18 * atomic decrement (it is not a reliable atomic decrement but it satisfies 19 * the defined semantics for our purpose, while being smaller and faster 20 * than a real atomic decrement or atomic swap. The idea is to attempt 21 * decrementing the lock value only once. If once decremented it isn't zero, 22 * or if its store-back fails due to a dispute on the exclusive store, we 23 * simply bail out immediately through the slow path where the lock will be 24 * reattempted until it succeeds. 25 */ 26 static inline void 27 __mutex_fastpath_lock(atomic_t *count, void (*fail_fn)(atomic_t *)) 28 { 29 int __ex_flag, __res; 30 31 __asm__ ( 32 33 "ldrex %0, [%2] \n\t" 34 "sub %0, %0, #1 \n\t" 35 "strex %1, %0, [%2] " 36 37 : "=&r" (__res), "=&r" (__ex_flag) 38 : "r" (&(count)->counter) 39 : "cc","memory" ); 40 41 __res |= __ex_flag; 42 if (unlikely(__res != 0)) 43 fail_fn(count); 44 } 45 46 static inline int 47 __mutex_fastpath_lock_retval(atomic_t *count, int (*fail_fn)(atomic_t *)) 48 { 49 int __ex_flag, __res; 50 51 __asm__ ( 52 53 "ldrex %0, [%2] \n\t" 54 "sub %0, %0, #1 \n\t" 55 "strex %1, %0, [%2] " 56 57 : "=&r" (__res), "=&r" (__ex_flag) 58 : "r" (&(count)->counter) 59 : "cc","memory" ); 60 61 __res |= __ex_flag; 62 if (unlikely(__res != 0)) 63 __res = fail_fn(count); 64 return __res; 65 } 66 67 /* 68 * Same trick is used for the unlock fast path. However the original value, 69 * rather than the result, is used to test for success in order to have 70 * better generated assembly. 71 */ 72 static inline void 73 __mutex_fastpath_unlock(atomic_t *count, void (*fail_fn)(atomic_t *)) 74 { 75 int __ex_flag, __res, __orig; 76 77 __asm__ ( 78 79 "ldrex %0, [%3] \n\t" 80 "add %1, %0, #1 \n\t" 81 "strex %2, %1, [%3] " 82 83 : "=&r" (__orig), "=&r" (__res), "=&r" (__ex_flag) 84 : "r" (&(count)->counter) 85 : "cc","memory" ); 86 87 __orig |= __ex_flag; 88 if (unlikely(__orig != 0)) 89 fail_fn(count); 90 } 91 92 /* 93 * If the unlock was done on a contended lock, or if the unlock simply fails 94 * then the mutex remains locked. 95 */ 96 #define __mutex_slowpath_needs_to_unlock() 1 97 98 /* 99 * For __mutex_fastpath_trylock we use another construct which could be 100 * described as a "single value cmpxchg". 101 * 102 * This provides the needed trylock semantics like cmpxchg would, but it is 103 * lighter and less generic than a true cmpxchg implementation. 104 */ 105 static inline int 106 __mutex_fastpath_trylock(atomic_t *count, int (*fail_fn)(atomic_t *)) 107 { 108 int __ex_flag, __res, __orig; 109 110 __asm__ ( 111 112 "1: ldrex %0, [%3] \n\t" 113 "subs %1, %0, #1 \n\t" 114 "strexeq %2, %1, [%3] \n\t" 115 "movlt %0, #0 \n\t" 116 "cmpeq %2, #0 \n\t" 117 "bgt 1b " 118 119 : "=&r" (__orig), "=&r" (__res), "=&r" (__ex_flag) 120 : "r" (&count->counter) 121 : "cc", "memory" ); 122 123 return __orig; 124 } 125 126 #endif 127 #endif 128
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