#include "pthread_impl.h"
-struct cm {
- pthread_cond_t *c;
- pthread_mutex_t *m;
+/*
+ * struct waiter
+ *
+ * Waiter objects have automatic storage on the waiting thread, and
+ * are used in building a linked list representing waiters currently
+ * waiting on the condition variable or a group of waiters woken
+ * together by a broadcast or signal; in the case of signal, this is a
+ * degenerate list of one member.
+ *
+ * Waiter lists attached to the condition variable itself are
+ * protected by the lock on the cv. Detached waiter lists are
+ * protected by the associated mutex. The hand-off between protections
+ * is handled by a "barrier" lock in each node, which disallows
+ * signaled waiters from making forward progress to the code that will
+ * access the list using the mutex until the list is in a consistent
+ * state and the cv lock as been released.
+ *
+ * Since process-shared cond var semantics do not necessarily allow
+ * one thread to see another's automatic storage (they may be in
+ * different processes), the waiter list is not used for the
+ * process-shared case, but the structure is still used to store data
+ * needed by the cancellation cleanup handler.
+ */
+
+struct waiter {
+ struct waiter *prev, *next;
+ int state, barrier, requeued, mutex_ret;
+ int *notify;
+ pthread_mutex_t *mutex;
+ pthread_cond_t *cond;
+ int shared;
};
-static void unwait(pthread_cond_t *c, pthread_mutex_t *m)
-{
- /* Removing a waiter is non-trivial if we could be using requeue
- * based broadcast signals, due to mutex access issues, etc. */
+/* Self-synchronized-destruction-safe lock functions */
- if (c->_c_mutex == (void *)-1) {
- a_dec(&c->_c_waiters);
- if (c->_c_destroy) __wake(&c->_c_waiters, 1, 0);
- return;
+static inline void lock(volatile int *l)
+{
+ if (a_cas(l, 0, 1)) {
+ a_cas(l, 1, 2);
+ do __wait(l, 0, 2, 1);
+ while (a_cas(l, 0, 2));
}
+}
- while (a_swap(&c->_c_lock, 1))
- __wait(&c->_c_lock, &c->_c_lockwait, 1, 1);
+static inline void unlock(volatile int *l)
+{
+ if (a_swap(l, 0)==2)
+ __wake(l, 1, 1);
+}
- if (c->_c_waiters2) c->_c_waiters2--;
- else a_dec(&m->_m_waiters);
+enum {
+ WAITING,
+ SIGNALED,
+ LEAVING,
+};
- a_store(&c->_c_lock, 0);
- if (c->_c_lockwait) __wake(&c->_c_lock, 1, 1);
+static void unwait(void *arg)
+{
+ struct waiter *node = arg, *p;
+
+ if (node->shared) {
+ pthread_cond_t *c = node->cond;
+ pthread_mutex_t *m = node->mutex;
+ if (a_fetch_add(&c->_c_waiters, -1) == -0x7fffffff)
+ __wake(&c->_c_waiters, 1, 0);
+ node->mutex_ret = pthread_mutex_lock(m);
+ return;
+ }
- a_dec(&c->_c_waiters);
- if (c->_c_destroy) __wake(&c->_c_waiters, 1, 1);
-}
+ int oldstate = a_cas(&node->state, WAITING, LEAVING);
+
+ if (oldstate == WAITING) {
+ /* Access to cv object is valid because this waiter was not
+ * yet signaled and a new signal/broadcast cannot return
+ * after seeing a LEAVING waiter without getting notified
+ * via the futex notify below. */
+
+ pthread_cond_t *c = node->cond;
+ lock(&c->_c_lock);
+
+ if (c->_c_head == node) c->_c_head = node->next;
+ else if (node->prev) node->prev->next = node->next;
+ if (c->_c_tail == node) c->_c_tail = node->prev;
+ else if (node->next) node->next->prev = node->prev;
+
+ unlock(&c->_c_lock);
+
+ if (node->notify) {
+ if (a_fetch_add(node->notify, -1)==1)
+ __wake(node->notify, 1, 1);
+ }
+ }
-static void cleanup(void *p)
-{
- struct cm *cm = p;
- unwait(cm->c, cm->m);
- pthread_mutex_lock(cm->m);
+ node->mutex_ret = pthread_mutex_lock(node->mutex);
+
+ if (oldstate == WAITING) return;
+
+ /* If the mutex can't be locked, we're in big trouble because
+ * it's all that protects access to the shared list state.
+ * In order to prevent catastrophic stack corruption from
+ * unsynchronized access, simply deadlock. */
+ if (node->mutex_ret && node->mutex_ret != EOWNERDEAD)
+ for (;;) lock(&(int){0});
+
+ /* Wait until control of the list has been handed over from
+ * the cv lock (signaling thread) to the mutex (waiters). */
+ lock(&node->barrier);
+
+ /* If this thread was requeued to the mutex, undo the extra
+ * waiter count that was added to the mutex. */
+ if (node->requeued) a_dec(&node->mutex->_m_waiters);
+
+ /* Find a thread to requeue to the mutex, starting from the
+ * end of the list (oldest waiters). */
+ for (p=node; p->next; p=p->next);
+ if (p==node) p=node->prev;
+ for (; p && p->requeued; p=p->prev);
+ if (p==node) p=node->prev;
+ if (p) {
+ p->requeued = 1;
+ a_inc(&node->mutex->_m_waiters);
+ /* The futex requeue command cannot requeue from
+ * private to shared, so for process-shared mutexes,
+ * simply wake the target. */
+ int wake = node->mutex->_m_type & 128;
+ __syscall(SYS_futex, &p->state, FUTEX_REQUEUE|128,
+ wake, 1, &node->mutex->_m_lock) != -EINVAL
+ || __syscall(SYS_futex, &p->state, FUTEX_REQUEUE,
+ 0, 1, &node->mutex->_m_lock);
+ }
+
+ /* Remove this thread from the list. */
+ if (node->next) node->next->prev = node->prev;
+ if (node->prev) node->prev->next = node->next;
}
int pthread_cond_timedwait(pthread_cond_t *restrict c, pthread_mutex_t *restrict m, const struct timespec *restrict ts)
{
- struct cm cm = { .c=c, .m=m };
- int r, e=0, seq;
+ struct waiter node = { .cond = c, .mutex = m };
+ int e, seq, *fut, clock = c->_c_clock;
if ((m->_m_type&15) && (m->_m_lock&INT_MAX) != __pthread_self()->tid)
return EPERM;
pthread_testcancel();
- a_inc(&c->_c_waiters);
-
- if (c->_c_mutex != (void *)-1) {
- c->_c_mutex = m;
- while (a_swap(&c->_c_lock, 1))
- __wait(&c->_c_lock, &c->_c_lockwait, 1, 1);
- c->_c_waiters2++;
- a_store(&c->_c_lock, 0);
- if (c->_c_lockwait) __wake(&c->_c_lock, 1, 1);
+ if (c->_c_shared) {
+ node.shared = 1;
+ fut = &c->_c_seq;
+ seq = c->_c_seq;
+ a_inc(&c->_c_waiters);
+ } else {
+ lock(&c->_c_lock);
+
+ node.barrier = 1;
+ fut = &node.state;
+ seq = node.state = WAITING;
+ node.next = c->_c_head;
+ c->_c_head = &node;
+ if (!c->_c_tail) c->_c_tail = &node;
+ else node.next->prev = &node;
+
+ unlock(&c->_c_lock);
}
- seq = c->_c_seq;
-
pthread_mutex_unlock(m);
- do e = __timedwait(&c->_c_seq, seq, c->_c_clock, ts, cleanup, &cm,
- c->_c_mutex != (void *)-1);
- while (c->_c_seq == seq && (!e || e==EINTR));
+ do e = __timedwait(fut, seq, clock, ts, unwait, &node, !node.shared);
+ while (*fut==seq && (!e || e==EINTR));
if (e == EINTR) e = 0;
- unwait(c, m);
+ unwait(&node);
- if ((r=pthread_mutex_lock(m))) return r;
+ return node.mutex_ret ? node.mutex_ret : e;
+}
- return e;
+int __private_cond_signal(pthread_cond_t *c, int n)
+{
+ struct waiter *p, *q=0;
+ int ref = 0, cur;
+
+ lock(&c->_c_lock);
+ for (p=c->_c_tail; n && p; p=p->prev) {
+ /* The per-waiter-node barrier lock is held at this
+ * point, so while the following CAS may allow forward
+ * progress in the target thread, it doesn't allow
+ * access to the waiter list yet. Ideally the target
+ * does not run until the futex wake anyway. */
+ if (a_cas(&p->state, WAITING, SIGNALED) != WAITING) {
+ ref++;
+ p->notify = &ref;
+ } else {
+ n--;
+ if (!q) q=p;
+ }
+ }
+ /* Split the list, leaving any remainder on the cv. */
+ if (p) {
+ if (p->next) p->next->prev = 0;
+ p->next = 0;
+ } else {
+ c->_c_head = 0;
+ }
+ c->_c_tail = p;
+ unlock(&c->_c_lock);
+
+ /* Wait for any waiters in the LEAVING state to remove
+ * themselves from the list before returning or allowing
+ * signaled threads to proceed. */
+ while ((cur = ref)) __wait(&ref, 0, cur, 1);
+
+ /* Wake the first signaled thread and unlock the per-waiter
+ * barriers preventing their forward progress. */
+ for (p=q; p; p=q) {
+ q = p->prev;
+ if (!p->next) __wake(&p->state, 1, 1);
+ unlock(&p->barrier);
+ }
+ return 0;
}
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