1 #include "pthread_impl.h"
2 3 /*
4 * struct waiter
5 *
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.
11 *
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.
17 *
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.
23 */
24 25 struct waiter {
26 struct waiter *prev, *next;
27 volatile int state, barrier;
28 volatile int *notify;
29 };
30 31 /* Self-synchronized-destruction-safe lock functions */
32 33 static inline void lock(volatile int *l)
34 {
35 if (a_cas(l, 0, 1)) {
36 a_cas(l, 1, 2);
37 do __wait(l, 0, 2, 1);
38 while (a_cas(l, 0, 2));
39 }
40 }
41 42 static inline void unlock(volatile int *l)
43 {
44 if (a_swap(l, 0)==2)
45 __wake(l, 1, 1);
46 }
47 48 static inline void unlock_requeue(volatile int *l, volatile int *r, int w)
49 {
50 a_store(l, 0);
51 if (w) __wake(l, 1, 1);
52 else __syscall(SYS_futex, l, FUTEX_REQUEUE|FUTEX_PRIVATE, 0, 1, r) != -ENOSYS
53 || __syscall(SYS_futex, l, FUTEX_REQUEUE, 0, 1, r);
54 }
55 56 enum {
57 WAITING,
58 SIGNALED,
59 LEAVING,
60 };
61 62 int __pthread_cond_timedwait(pthread_cond_t *restrict c, pthread_mutex_t *restrict m, const struct timespec *restrict ts)
63 {
64 struct waiter node = { 0 };
65 int e, seq, clock = c->_c_clock, cs, shared=0, oldstate, tmp;
66 volatile int *fut;
67 68 if ((m->_m_type&15) && (m->_m_lock&INT_MAX) != __pthread_self()->tid)
69 return EPERM;
70 71 if (ts && ts->tv_nsec >= 1000000000UL)
72 return EINVAL;
73 74 __pthread_testcancel();
75 76 if (c->_c_shared) {
77 shared = 1;
78 fut = &c->_c_seq;
79 seq = c->_c_seq;
80 a_inc(&c->_c_waiters);
81 } else {
82 lock(&c->_c_lock);
83 84 seq = node.barrier = 2;
85 fut = &node.barrier;
86 node.state = WAITING;
87 node.next = c->_c_head;
88 c->_c_head = &node;
89 if (!c->_c_tail) c->_c_tail = &node;
90 else node.next->prev = &node;
91 92 unlock(&c->_c_lock);
93 }
94 95 __pthread_mutex_unlock(m);
96 97 __pthread_setcancelstate(PTHREAD_CANCEL_MASKED, &cs);
98 if (cs == PTHREAD_CANCEL_DISABLE) __pthread_setcancelstate(cs, 0);
99 100 do e = __timedwait_cp(fut, seq, clock, ts, !shared);
101 while (*fut==seq && (!e || e==EINTR));
102 if (e == EINTR) e = 0;
103 104 if (shared) {
105 /* Suppress cancellation if a signal was potentially
106 * consumed; this is a legitimate form of spurious
107 * wake even if not. */
108 if (e == ECANCELED && c->_c_seq != seq) e = 0;
109 if (a_fetch_add(&c->_c_waiters, -1) == -0x7fffffff)
110 __wake(&c->_c_waiters, 1, 0);
111 oldstate = WAITING;
112 goto relock;
113 }
114 115 oldstate = a_cas(&node.state, WAITING, LEAVING);
116 117 if (oldstate == WAITING) {
118 /* Access to cv object is valid because this waiter was not
119 * yet signaled and a new signal/broadcast cannot return
120 * after seeing a LEAVING waiter without getting notified
121 * via the futex notify below. */
122 123 lock(&c->_c_lock);
124 125 if (c->_c_head == &node) c->_c_head = node.next;
126 else if (node.prev) node.prev->next = node.next;
127 if (c->_c_tail == &node) c->_c_tail = node.prev;
128 else if (node.next) node.next->prev = node.prev;
129 130 unlock(&c->_c_lock);
131 132 if (node.notify) {
133 if (a_fetch_add(node.notify, -1)==1)
134 __wake(node.notify, 1, 1);
135 }
136 } else {
137 /* Lock barrier first to control wake order. */
138 lock(&node.barrier);
139 }
140 141 relock:
142 /* Errors locking the mutex override any existing error or
143 * cancellation, since the caller must see them to know the
144 * state of the mutex. */
145 if ((tmp = pthread_mutex_lock(m))) e = tmp;
146 147 if (oldstate == WAITING) goto done;
148 149 if (!node.next && !(m->_m_type & 8))
150 a_inc(&m->_m_waiters);
151 152 /* Unlock the barrier that's holding back the next waiter, and
153 * either wake it or requeue it to the mutex. */
154 if (node.prev) {
155 int val = m->_m_lock;
156 if (val>0) a_cas(&m->_m_lock, val, val|0x80000000);
157 unlock_requeue(&node.prev->barrier, &m->_m_lock, m->_m_type & (8|128));
158 } else if (!(m->_m_type & 8)) {
159 a_dec(&m->_m_waiters);
160 }
161 162 /* Since a signal was consumed, cancellation is not permitted. */
163 if (e == ECANCELED) e = 0;
164 165 done:
166 __pthread_setcancelstate(cs, 0);
167 168 if (e == ECANCELED) {
169 __pthread_testcancel();
170 __pthread_setcancelstate(PTHREAD_CANCEL_DISABLE, 0);
171 }
172 173 return e;
174 }
175 176 int __private_cond_signal(pthread_cond_t *c, int n)
177 {
178 struct waiter *p, *first=0;
179 volatile int ref = 0;
180 int cur;
181 182 lock(&c->_c_lock);
183 for (p=c->_c_tail; n && p; p=p->prev) {
184 if (a_cas(&p->state, WAITING, SIGNALED) != WAITING) {
185 ref++;
186 p->notify = &ref;
187 } else {
188 n--;
189 if (!first) first=p;
190 }
191 }
192 /* Split the list, leaving any remainder on the cv. */
193 if (p) {
194 if (p->next) p->next->prev = 0;
195 p->next = 0;
196 } else {
197 c->_c_head = 0;
198 }
199 c->_c_tail = p;
200 unlock(&c->_c_lock);
201 202 /* Wait for any waiters in the LEAVING state to remove
203 * themselves from the list before returning or allowing
204 * signaled threads to proceed. */
205 while ((cur = ref)) __wait(&ref, 0, cur, 1);
206 207 /* Allow first signaled waiter, if any, to proceed. */
208 if (first) unlock(&first->barrier);
209 210 return 0;
211 }
212 213 weak_alias(__pthread_cond_timedwait, pthread_cond_timedwait);
214