cosmopolitan/third_party/nsync/mu_semaphore_futex.c

/*-*- mode:c;indent-tabs-mode:t;c-basic-offset:8;tab-width:8;coding:utf-8   -*-│
│vi: set et ft=c ts=8 tw=8 fenc=utf-8                                       :vi│
╞══════════════════════════════════════════════════════════════════════════════╡
│ Copyright 2016 Google Inc.                                                   │
│                                                                              │
│ Licensed under the Apache License, Version 2.0 (the "License");              │
│ you may not use this file except in compliance with the License.             │
│ You may obtain a copy of the License at                                      │
│                                                                              │
│     http://www.apache.org/licenses/LICENSE-2.0                               │
│                                                                              │
│ Unless required by applicable law or agreed to in writing, software          │
│ distributed under the License is distributed on an "AS IS" BASIS,            │
│ WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.     │
│ See the License for the specific language governing permissions and          │
│ limitations under the License.                                               │
╚─────────────────────────────────────────────────────────────────────────────*/
#include "libc/assert.h"
#include "libc/errno.h"
#include "libc/str/str.h"
#include "libc/thread/thread.h"
#include "third_party/nsync/atomic.h"
#include "third_party/nsync/atomic.internal.h"
#include "third_party/nsync/futex.internal.h"
#include "third_party/nsync/mu_semaphore.internal.h"
// clang-format off

/**
 * @fileoverview Semaphores w/ Linux Futexes API.
 */

#define ASSERT(x) npassert(x)

/* Check that atomic operations on nsync_atomic_uint32_ can be applied to int. */
static const int assert_int_size = 1 /
	(sizeof (assert_int_size) == sizeof (uint32_t) &&
	 sizeof (nsync_atomic_uint32_) == sizeof (uint32_t));

struct futex {
	int i;  /* lo half=count; hi half=waiter count */
};

static nsync_semaphore *sem_big_enough_for_futex = (nsync_semaphore *) (uintptr_t)(1 /
	(sizeof (struct futex) <= sizeof (*sem_big_enough_for_futex)));

/* Initialize *s; the initial value is 0. */
void nsync_mu_semaphore_init_futex (nsync_semaphore *s) {
	struct futex *f = (struct futex *) s;
	f->i = 0;
}

/* Wait until the count of *s exceeds 0, and decrement it. */
errno_t nsync_mu_semaphore_p_futex (nsync_semaphore *s) {
	struct futex *f = (struct futex *) s;
	int e, i;
	errno_t result = 0;
	do {
		i = ATM_LOAD ((nsync_atomic_uint32_ *) &f->i);
		if (i == 0) {
			int futex_result;
			futex_result = -nsync_futex_wait_ (
				(atomic_int *)&f->i, i,
				PTHREAD_PROCESS_PRIVATE, 0);
			ASSERT (futex_result == 0 ||
				futex_result == EINTR ||
				futex_result == EAGAIN ||
				futex_result == ECANCELED ||
				futex_result == EWOULDBLOCK);
			if (futex_result == ECANCELED) {
				result = ECANCELED;
			}
		}
	} while (result == 0 && (i == 0 || !ATM_CAS_ACQ ((nsync_atomic_uint32_ *) &f->i, i, i-1)));
	return result;
}

/* Wait until one of:
   the count of *s is non-zero, in which case decrement *s and return 0;
   or abs_deadline expires, in which case return ETIMEDOUT. */
errno_t nsync_mu_semaphore_p_with_deadline_futex (nsync_semaphore *s, nsync_time abs_deadline) {
	struct futex *f = (struct futex *)s;
	int e, i;
	int result = 0;
	do {
		i = ATM_LOAD ((nsync_atomic_uint32_ *) &f->i);
		if (i == 0) {
			int futex_result;
			struct timespec ts_buf;
			const struct timespec *ts = NULL;
			if (nsync_time_cmp (abs_deadline, nsync_time_no_deadline) != 0) {
				bzero (&ts_buf, sizeof (ts_buf));
				ts_buf.tv_sec = NSYNC_TIME_SEC (abs_deadline);
				ts_buf.tv_nsec = NSYNC_TIME_NSEC (abs_deadline);
				ts = &ts_buf;
			}
			futex_result = nsync_futex_wait_ ((atomic_int *)&f->i, i,
							  PTHREAD_PROCESS_PRIVATE, ts);
			ASSERT (futex_result == 0 ||
				futex_result == -EINTR ||
				futex_result == -EAGAIN ||
				futex_result == -ECANCELED ||
				futex_result == -ETIMEDOUT ||
				futex_result == -EWOULDBLOCK);
			/* Some systems don't wait as long as they are told. */
			if (futex_result == -ETIMEDOUT &&
			    nsync_time_cmp (abs_deadline, nsync_time_now ()) <= 0) {
				result = ETIMEDOUT;
			}
			if (futex_result == -ECANCELED) {
				result = ECANCELED;
			}
		}
	} while (result == 0 && (i == 0 || !ATM_CAS_ACQ ((nsync_atomic_uint32_ *) &f->i, i, i - 1)));
	return (result);
}

/* Ensure that the count of *s is at least 1. */
void nsync_mu_semaphore_v_futex (nsync_semaphore *s) {
	struct futex *f = (struct futex *) s;
        uint32_t old_value;
	do {
		old_value = ATM_LOAD ((nsync_atomic_uint32_ *) &f->i);
	} while (!ATM_CAS_REL ((nsync_atomic_uint32_ *) &f->i, old_value, old_value+1));
	ASSERT (nsync_futex_wake_ ((atomic_int *)&f->i, 1, PTHREAD_PROCESS_PRIVATE) >= 0);
}