#include "pthread_impl.h"
/*
* 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;
};
/* Self-synchronized-destruction-safe lock functions */
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));
}
}
static inline void unlock(volatile int *l)
{
if (a_swap(l, 0)==2)
__wake(l, 1, 1);
}
enum {
WAITING,
SIGNALED,
LEAVING,
};
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;
}
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);
}
}
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 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;
if (ts && ts->tv_nsec >= 1000000000UL)
return EINVAL;
pthread_testcancel();
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);
}
pthread_mutex_unlock(m);
do e = __timedwait(fut, seq, clock, ts, unwait, &node, !node.shared);
while (*fut==seq && (!e || e==EINTR));
if (e == EINTR) e = 0;
unwait(&node);
return node.mutex_ret ? node.mutex_ret : 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;
}
