在dpdk一些代码示例中，有一些使用到了读写锁和原子操作，后者也用于无锁队列的实现。
如原子操作：
139 static inline int
140 rte_atomic32_cmpset(volatile uint32_t *dst, uint32_t exp, uint32_t src)
141 {
142 uint8_t res;
143
144 asm volatile(
145 MPLOCKED
146 "cmpxchgl %[src], %[dst];"
147 "sete %[res];"
148 : [res] "=a" (res), /* output */
149 [dst] "=m" (*dst)
150 : [src] "r" (src), /* input */
151 "a" (exp),
152 "m" (*dst)
153 : "memory"); /* no-clobber list */
154 return res;
155 } //2018-7-14从另一实现修改为当前实现
396 /**
397 * The atomic counter structure.
398 */
399 typedef struct {
400 volatile int32_t cnt; /**< An internal counter value. */
401 } rte_atomic32_t;
164 static inline void
165 rte_atomic32_inc(rte_atomic32_t *v)
166 {
167 int t;
168
169 asm volatile(
170 "1: lwarx %[t],0,%[cnt]\n"
171 "addic %[t],%[t],1\n"
172 "stwcx. %[t],0,%[cnt]\n"
173 "bne- 1b\n"
174 : [t] "=&r" (t), "=m" (v->cnt)
175 : [cnt] "r" (&v->cnt), "m" (v->cnt)
176 : "cc", "xer", "memory");
177 }
178
179 static inline void
180 rte_atomic32_dec(rte_atomic32_t *v)
181 {
182 int t;
183
184 asm volatile(
185 "1: lwarx %[t],0,%[cnt]\n"
186 "addic %[t],%[t],-1\n"
187 "stwcx. %[t],0,%[cnt]\n"
188 "bne- 1b\n"
189 : [t] "=&r" (t), "=m" (v->cnt)
190 : [cnt] "r" (&v->cnt), "m" (v->cnt)
191 : "cc", "xer", "memory");
192 }
内嵌汇编代码，volatile关键字的作用可以看下以前写的文章。
而读写锁是一种特殊的自旋锁，能提高并发性，读锁之间共享资源，写锁之间互斥，读写锁互斥；用于临界资源代码短的情况；线程在等资源期间不能被投入睡眠，只能是忙等，不然睡眠和唤醒也是相当费时的。
56 /**
57 * The rte_rwlock_t type.
58 *
59 * cnt is -1 when write lock is held, and > 0 when read locks are held.
60 */
61 typedef struct {
62 volatile int32_t cnt; /**< -1 when W lock held, > 0 when R locks held. */
63 } rte_rwlock_t;
118 /**
119 * Take a write lock. Loop until the lock is held.
120 *
121 * @param rwl
122 * A pointer to a rwlock structure.
123 */
124 static inline void
125 rte_rwlock_write_lock(rte_rwlock_t *rwl)
126 {
127 int32_t x;
128 int success = 0;
129
130 while (success == 0) {
131 x = rwl->cnt;
132 /* a lock is held */
133 if (x != 0) {
134 rte_pause();
135 continue;
136 }
137 success = rte_atomic32_cmpset((volatile uint32_t *)&rwl->cnt,
138 0, -1);
139 }
140 }
142 /**
143 * Release a write lock.
144 *
145 * @param rwl
146 * A pointer to a rwlock structure.
147 */
148 static inline void
149 rte_rwlock_write_unlock(rte_rwlock_t *rwl)
150 {
151 rte_atomic32_inc((rte_atomic32_t *)(intptr_t)&rwl->cnt);
152 }
82 /**
83 * Take a read lock. Loop until the lock is held.
84 *
85 * @param rwl
86 * A pointer to a rwlock structure.
87 */
88 static inline void
89 rte_rwlock_read_lock(rte_rwlock_t *rwl)
90 {
91 int32_t x;
92 int success = 0;
93
94 while (success == 0) {
95 x = rwl->cnt;
96 /* write lock is held */
97 if (x < 0) {
98 rte_pause();
99 continue;
100 }
101 success = rte_atomic32_cmpset((volatile uint32_t *)&rwl->cnt,
102 x, x + 1);
103 }
104 }
105
106 /**
107 * Release a read lock.
108 *
109 * @param rwl
110 * A pointer to the rwlock structure.
111 */
112 static inline void
113 rte_rwlock_read_unlock(rte_rwlock_t *rwl)
114 {
115 rte_atomic32_dec((rte_atomic32_t *)(intptr_t)&rwl->cnt);
116 }
而普通自旋锁用的比较少，在使用过程中需要注意的是：
不能在获得同一个自旋锁的情况下再获得同一个自旋锁，否则会形成死锁的情况；在获得自旋锁前，禁止中断，防止中断处理程序也获得该自旋锁；
相关实现可参考：http://dpdk.org/doc/api/rte__spinlock_8h.html
https://en.wikipedia.org/wiki/Volatile_(computer_programming)
网友评论