src/thread/pthread_cond_timedwait.c

   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);