poll_wait
相关结构:
http://lxr.free-electrons.com/source/include/linux/poll.h?v=3.8
33 /* 34 * Do not touch the structure directly, use the access functions 35 * poll_does_not_wait() and poll_requested_events() instead. 36 */ 37 typedef struct poll_table_struct { 38 poll_queue_proc _qproc; 39 unsigned long _key; 40 } poll_table; 41 42 static inline void poll_wait(struct file * filp, wait_queue_head_t * wait_address, poll_table *p) 43 { 44 if (p && p->_qproc && wait_address) 45 p->_qproc(filp, wait_address, p); 46 } 47 48 /* 49 * Return true if it is guaranteed that poll will not wait. This is the case 50 * if the poll() of another file descriptor in the set got an event, so there 51 * is no need for waiting. 52 */ 53 static inline bool poll_does_not_wait(const poll_table *p) 54 { 55 return p == NULL || p->_qproc == NULL; 56 } 57 58 /* 59 * Return the set of events that the application wants to poll for. 60 * This is useful for drivers that need to know whether a DMA transfer has 61 * to be started implicitly on poll(). You typically only want to do that 62 * if the application is actually polling for POLLIN and/or POLLOUT. 63 */ 64 static inline unsigned long poll_requested_events(const poll_table *p) 65 { 66 return p ? p->_key : ~0UL; 67 } 68 69 static inline void init_poll_funcptr(poll_table *pt, poll_queue_proc qproc) 70 { 71 pt->_qproc = qproc; 72 pt->_key = ~0UL; /* all events enabled */ 73 } 74 75 struct poll_table_entry { 76 struct file *filp; 77 unsigned long key; 78 wait_queue_t wait; 79 wait_queue_head_t *wait_address; 80 }; 81 82 /* 83 * Structures and helpers for select/poll syscall 84 */ 85 struct poll_wqueues { 86 poll_table pt; 87 struct poll_table_page *table; 88 struct task_struct *polling_task; 89 int triggered; 90 int error; 91 int inline_index; 92 struct poll_table_entry inline_entries[N_INLINE_POLL_ENTRIES]; 93 }; 94
poll_wait分析:
在用户空间调用select后转入内核会调用do_select。
do_select中初始化poll接口所需的poll_table结构,调用poll。
http://lxr.free-electrons.com/source/fs/select.c?v=3.8
395 int do_select(int n, fd_set_bits *fds, struct timespec *end_time) 396 { 397 ktime_t expire, *to = NULL; 398 struct poll_wqueues table; 399 poll_table *wait; 400 int retval, i, timed_out = 0; 401 unsigned long slack = 0; 402 403 rcu_read_lock(); 404 retval = max_select_fd(n, fds); 405 rcu_read_unlock(); 406 407 if (retval < 0) 408 return retval; 409 n = retval; 410 411 poll_initwait(&table); //初始化poll_wqueues结构 412 wait = &table.pt;
116 void poll_initwait(struct poll_wqueues *pwq) 117 { 118 init_poll_funcptr(&pwq->pt, __pollwait); 119 pwq->polling_task = current; 120 pwq->triggered = 0; 121 pwq->error = 0; 122 pwq->table = NULL; 123 pwq->inline_index = 0; 124 }
真正的poll_wait调用其实比较简单,其实就是调了__pollwait,虽然叫wait,可是该函数并不阻塞,仅仅是给wait队列增加了个节点。
215 /* Add a new entry */ 216 static void __pollwait(struct file *filp, wait_queue_head_t *wait_address, 217 poll_table *p) 218 { 219 struct poll_wqueues *pwq = container_of(p, struct poll_wqueues, pt); 220 struct poll_table_entry *entry = poll_get_entry(pwq); 221 if (!entry) 222 return; 223 entry->filp = get_file(filp); 224 entry->wait_address = wait_address; 225 entry->key = p->_key; 226 init_waitqueue_func_entry(&entry->wait, pollwake); 227 entry->wait.private = pwq; 228 add_wait_queue(wait_address, &entry->wait); //list_add(&entry->wait.task_list, &wait_address->task_list); 229 }