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/* libcr_ipc/sema.c - Simple semaphores for libcr (implementation file)
*
* Copyright (C) 2024 Luke T. Shumaker <lukeshu@lukeshu.com>
* SPDX-Licence-Identifier: AGPL-3.0-or-later
*/
#include <assert.h>
#include <libcr_ipc/sema.h>
/** Drain the sema->{head,tail} list. Returns true if cr_getcid() was drained. */
static inline bool drain(cr_sema_t *sema) {
assert(!sema->locked);
cid_t self = cr_getcid();
enum drain_result {
DRAINING,
DRAINED_SELF, /* stopped because drained `self` */
DRAINED_ALL, /* stopped because sema->head == NULL */
DRAINED_SOME, /* stopped because sema->cnt == 0 */
} state = DRAINING;
do {
sema->locked = true;
while (state == DRAINING) {
if (!sema->head) {
state = DRAINED_ALL;
} else if (!sema->cnt) {
state = DRAINED_SOME;
} else {
sema->cnt--;
cid_t cid = sema->head->val;
if (cid == self)
state = DRAINED_SELF;
else
cr_unpause(sema->head->val);
sema->head = sema->head->next;
if (!sema->head)
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wdiscarded-qualifiers"
sema->tail = &sema->head;
#pragma GCC diagnostic pop
}
}
sema->locked = false;
/* If there are still coroutines in sema->head, check
* that sema->cnt wasn't incremented between `if
* (!sema->cnt)` and `sema->locked = false`. */
} while (state == DRAINED_SOME && sema->cnt);
/* If state == DRAINED_SELF, then we better have been the last
* item in the list! */
assert(state != DRAINED_SELF || !sema->head);
return state == DRAINED_SELF;
}
void cr_sema_signal(cr_sema_t *sema) {
sema->cnt++;
if (!sema->locked)
drain(sema);
}
void cr_sema_wait(cr_sema_t *sema) {
struct _cr_ipc_cid_list self = {
.val = cr_getcid(),
.next = NULL,
};
sema->locked = true;
if (!sema->tail)
sema->head = &self;
else
*(sema->tail) = &self;
sema->tail = &(self.next);
sema->locked = false;
if (drain(sema))
/* DRAINED_SELF: (1) No need to pause+yield, (2) we
* better have been the last item in the list! */
assert(!self.next);
else
cr_pause_and_yield();
}
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