1/* 2 * Assembly implementation of the mutex fastpath, based on atomic 3 * decrement/increment. 4 * 5 * started by Ingo Molnar: 6 * 7 * Copyright (C) 2004, 2005, 2006 Red Hat, Inc., Ingo Molnar <mingo@redhat.com> 8 */ 9#ifndef _ASM_X86_MUTEX_32_H 10#define _ASM_X86_MUTEX_32_H 11 12#include <asm/alternative.h> 13 14/** 15 * __mutex_fastpath_lock - try to take the lock by moving the count 16 * from 1 to a 0 value 17 * @count: pointer of type atomic_t 18 * @fn: function to call if the original value was not 1 19 * 20 * Change the count from 1 to a value lower than 1, and call <fn> if it 21 * wasn't 1 originally. This function MUST leave the value lower than 1 22 * even when the "1" assertion wasn't true. 23 */ 24#define __mutex_fastpath_lock(count, fail_fn) \ 25do { \ 26 unsigned int dummy; \ 27 \ 28 typecheck(atomic_t *, count); \ 29 typecheck_fn(void (*)(atomic_t *), fail_fn); \ 30 \ 31 asm volatile(LOCK_PREFIX " decl (%%eax)\n" \ 32 " jns 1f \n" \ 33 " call " #fail_fn "\n" \ 34 "1:\n" \ 35 : "=a" (dummy) \ 36 : "a" (count) \ 37 : "memory", "ecx", "edx"); \ 38} while (0) 39 40 41/** 42 * __mutex_fastpath_lock_retval - try to take the lock by moving the count 43 * from 1 to a 0 value 44 * @count: pointer of type atomic_t 45 * @fail_fn: function to call if the original value was not 1 46 * 47 * Change the count from 1 to a value lower than 1, and call <fail_fn> if it 48 * wasn't 1 originally. This function returns 0 if the fastpath succeeds, 49 * or anything the slow path function returns 50 */ 51static inline int __mutex_fastpath_lock_retval(atomic_t *count, 52 int (*fail_fn)(atomic_t *)) 53{ 54 if (unlikely(atomic_dec_return(count) < 0)) 55 return fail_fn(count); 56 else 57 return 0; 58} 59 60/** 61 * __mutex_fastpath_unlock - try to promote the mutex from 0 to 1 62 * @count: pointer of type atomic_t 63 * @fail_fn: function to call if the original value was not 0 64 * 65 * try to promote the mutex from 0 to 1. if it wasn't 0, call <fail_fn>. 66 * In the failure case, this function is allowed to either set the value 67 * to 1, or to set it to a value lower than 1. 68 * 69 * If the implementation sets it to a value of lower than 1, the 70 * __mutex_slowpath_needs_to_unlock() macro needs to return 1, it needs 71 * to return 0 otherwise. 72 */ 73#define __mutex_fastpath_unlock(count, fail_fn) \ 74do { \ 75 unsigned int dummy; \ 76 \ 77 typecheck(atomic_t *, count); \ 78 typecheck_fn(void (*)(atomic_t *), fail_fn); \ 79 \ 80 asm volatile(LOCK_PREFIX " incl (%%eax)\n" \ 81 " jg 1f\n" \ 82 " call " #fail_fn "\n" \ 83 "1:\n" \ 84 : "=a" (dummy) \ 85 : "a" (count) \ 86 : "memory", "ecx", "edx"); \ 87} while (0) 88 89#define __mutex_slowpath_needs_to_unlock() 1 90 91/** 92 * __mutex_fastpath_trylock - try to acquire the mutex, without waiting 93 * 94 * @count: pointer of type atomic_t 95 * @fail_fn: fallback function 96 * 97 * Change the count from 1 to a value lower than 1, and return 0 (failure) 98 * if it wasn't 1 originally, or return 1 (success) otherwise. This function 99 * MUST leave the value lower than 1 even when the "1" assertion wasn't true. 100 * Additionally, if the value was < 0 originally, this function must not leave 101 * it to 0 on failure. 102 */ 103static inline int __mutex_fastpath_trylock(atomic_t *count, 104 int (*fail_fn)(atomic_t *)) 105{ 106 /* 107 * We have two variants here. The cmpxchg based one is the best one 108 * because it never induce a false contention state. It is included 109 * here because architectures using the inc/dec algorithms over the 110 * xchg ones are much more likely to support cmpxchg natively. 111 * 112 * If not we fall back to the spinlock based variant - that is 113 * just as efficient (and simpler) as a 'destructive' probing of 114 * the mutex state would be. 115 */ 116#ifdef __HAVE_ARCH_CMPXCHG 117 if (likely(atomic_cmpxchg(count, 1, 0) == 1)) 118 return 1; 119 return 0; 120#else 121 return fail_fn(count); 122#endif 123} 124 125#endif /* _ASM_X86_MUTEX_32_H */ 126