1 #include "pthread_impl.h"
6 * Waiter objects have automatic storage on the waiting thread, and
7 * are used in building a linked list representing waiters currently
8 * waiting on the condition variable or a group of waiters woken
9 * together by a broadcast or signal; in the case of signal, this is a
10 * degenerate list of one member.
12 * Waiter lists attached to the condition variable itself are
13 * protected by the lock on the cv. Detached waiter lists are never
14 * modified again, but can only be traversed in reverse order, and are
15 * protected by the "barrier" locks in each node, which are unlocked
16 * in turn to control wake order.
18 * Since process-shared cond var semantics do not necessarily allow
19 * one thread to see another's automatic storage (they may be in
20 * different processes), the waiter list is not used for the
21 * process-shared case, but the structure is still used to store data
22 * needed by the cancellation cleanup handler.
26 struct waiter *prev, *next;
27 int state, barrier, requeued, mutex_ret;
29 pthread_mutex_t *mutex;
34 /* Self-synchronized-destruction-safe lock functions */
36 static inline void lock(volatile int *l)
40 do __wait(l, 0, 2, 1);
41 while (a_cas(l, 0, 2));
45 static inline void unlock(volatile int *l)
57 static void unwait(void *arg)
59 struct waiter *node = arg;
62 pthread_cond_t *c = node->cond;
63 pthread_mutex_t *m = node->mutex;
64 if (a_fetch_add(&c->_c_waiters, -1) == -0x7fffffff)
65 __wake(&c->_c_waiters, 1, 0);
66 node->mutex_ret = pthread_mutex_lock(m);
70 int oldstate = a_cas(&node->state, WAITING, LEAVING);
72 if (oldstate == WAITING) {
73 /* Access to cv object is valid because this waiter was not
74 * yet signaled and a new signal/broadcast cannot return
75 * after seeing a LEAVING waiter without getting notified
76 * via the futex notify below. */
78 pthread_cond_t *c = node->cond;
81 if (c->_c_head == node) c->_c_head = node->next;
82 else if (node->prev) node->prev->next = node->next;
83 if (c->_c_tail == node) c->_c_tail = node->prev;
84 else if (node->next) node->next->prev = node->prev;
89 if (a_fetch_add(node->notify, -1)==1)
90 __wake(node->notify, 1, 1);
93 /* Lock barrier first to control wake order. */
97 node->mutex_ret = pthread_mutex_lock(node->mutex);
99 if (oldstate == WAITING) return;
101 /* If this thread was requeued to the mutex, undo the extra
102 * waiter count that was added to the mutex. */
103 if (node->requeued) a_dec(&node->mutex->_m_waiters);
105 /* Unlock the barrier that's holding back the next waiter,
106 * and either wake it or requeue it to the mutex. */
108 unlock(&node->prev->barrier);
109 node->prev->requeued = 1;
110 a_inc(&node->mutex->_m_waiters);
111 /* The futex requeue command cannot requeue from
112 * private to shared, so for process-shared mutexes,
113 * simply wake the target. */
114 int wake = node->mutex->_m_type & 128;
115 __syscall(SYS_futex, &node->prev->state, FUTEX_REQUEUE|128,
116 wake, 1, &node->mutex->_m_lock) != -EINVAL
117 || __syscall(SYS_futex, &node->prev->state, FUTEX_REQUEUE,
118 0, 1, &node->mutex->_m_lock);
122 int pthread_cond_timedwait(pthread_cond_t *restrict c, pthread_mutex_t *restrict m, const struct timespec *restrict ts)
124 struct waiter node = { .cond = c, .mutex = m };
125 int e, seq, *fut, clock = c->_c_clock;
127 if ((m->_m_type&15) && (m->_m_lock&INT_MAX) != __pthread_self()->tid)
130 if (ts && ts->tv_nsec >= 1000000000UL)
133 pthread_testcancel();
139 a_inc(&c->_c_waiters);
145 seq = node.state = WAITING;
146 node.next = c->_c_head;
148 if (!c->_c_tail) c->_c_tail = &node;
149 else node.next->prev = &node;
154 pthread_mutex_unlock(m);
156 do e = __timedwait(fut, seq, clock, ts, unwait, &node, !node.shared);
157 while (*fut==seq && (!e || e==EINTR));
158 if (e == EINTR) e = 0;
162 return node.mutex_ret ? node.mutex_ret : e;
165 int __private_cond_signal(pthread_cond_t *c, int n)
167 struct waiter *p, *first=0;
171 for (p=c->_c_tail; n && p; p=p->prev) {
172 /* The per-waiter-node barrier lock is held at this
173 * point, so while the following CAS may allow forward
174 * progress in the target thread, it doesn't allow
175 * access to the waiter list yet. Ideally the target
176 * does not run until the futex wake anyway. */
177 if (a_cas(&p->state, WAITING, SIGNALED) != WAITING) {
185 /* Split the list, leaving any remainder on the cv. */
187 if (p->next) p->next->prev = 0;
195 /* Wait for any waiters in the LEAVING state to remove
196 * themselves from the list before returning or allowing
197 * signaled threads to proceed. */
198 while ((cur = ref)) __wait(&ref, 0, cur, 1);
200 /* Allow first signaled waiter, if any, to proceed. */
202 __wake(&first->state, 1, 1);
203 unlock(&first->barrier);