cosmopolitan/libc/thread/once.c

// clang-format off
/* 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/thread/nsync_cpp.h"
#include "libc/thread/platform.h"
#include "libc/thread/compiler.h"
#include "libc/thread/cputype.h"
#include "libc/thread/nsync.h"
#include "libc/thread/dll.h"
#include "libc/thread/sem.h"
#include "libc/thread/wait_internal.h"
#include "libc/thread/common.h"
#include "libc/thread/atomic.h"

NSYNC_CPP_START_

/* An once_sync_s struct contains a lock, and a condition variable on which
   threads may wait for an nsync_once to be initialized by another thread.

   A separate struct is used only to keep nsync_once small.

   A given nsync_once can be associated with any once_sync_s struct, but cannot
   be associated with more than one.  nsync_once instances are mapped to
   once_sync_s instances by a trivial hashing scheme implemented by
   NSYNC_ONCE_SYNC_().

   The number of once_sync_s structs in the following array is greater than one
   only to reduce the probability of contention if a great many distinct
   nsync_once variables are initialized concurrently.  */
static struct once_sync_s {
	nsync_mu once_mu;
	nsync_cv once_cv;
} once_sync[64];

/* Return a pointer to the once_sync_s struct associated with the nsync_once *p. */
#define NSYNC_ONCE_SYNC_(p) &once_sync[(((uintptr_t) (p)) / sizeof (*(p))) % \
				       (sizeof (once_sync) / sizeof (once_sync[0]))]

/* Implement nsync_run_once, nsync_run_once_arg, nsync_run_once_spin, or
   nsync_run_once_arg_spin, chosen as described below.

   If s!=NULL, s is required to point to the once_sync_s associated with *once,
   and the semantics of nsync_run_once or nsync_run_once_arg are provided.
   If s==NULL, the semantics of nsync_run_once_spin, or nsync_run_once_arg_spin
   are provided.
   
   If f!=NULL, the semantics of nsync_run_once or nsync_run_once_spin are
   provided.  Otherwise, farg is required to be non-NULL, and the semantics of
   nsync_run_once_arg or nsync_run_once_arg_spin are provided.  */
static void nsync_run_once_impl (nsync_once *once, struct once_sync_s *s,
				 void (*f) (void), void (*farg) (void *arg), void *arg) {
	uint32_t o = ATM_LOAD_ACQ (once);
	if (o != 2) {
		unsigned attempts = 0;
		if (s != NULL) {
			nsync_mu_lock (&s->once_mu);
		}
		while (o == 0 && !ATM_CAS_ACQ (once, 0, 1)) {
			o = ATM_LOAD (once);
		}
		if (o == 0) {
			if (s != NULL) {
				nsync_mu_unlock (&s->once_mu);
			}
			if (f != NULL) {
				(*f) ();
			} else {
				(*farg) (arg);
			}
			if (s != NULL) {
				nsync_mu_lock (&s->once_mu);
				nsync_cv_broadcast (&s->once_cv);
			}
			ATM_STORE_REL (once, 2);
		}
		while (ATM_LOAD_ACQ (once) != 2) {
			if (s != NULL) {
				nsync_time deadline;
				if (attempts < 50) {
					attempts += 10;
				}
				deadline = nsync_time_add (nsync_time_now (), nsync_time_ms (attempts));
				nsync_cv_wait_with_deadline (&s->once_cv, &s->once_mu, deadline, NULL);
			} else {
				attempts = nsync_spin_delay_ (attempts);
			}
		}
		if (s != NULL) {
			nsync_mu_unlock (&s->once_mu);
		}
	}
}

void nsync_run_once (nsync_once *once, void (*f) (void)) {
	uint32_t o;
	IGNORE_RACES_START ();
	o = ATM_LOAD_ACQ (once);
	if (o != 2) {
		struct once_sync_s *s = NSYNC_ONCE_SYNC_ (once);
		nsync_run_once_impl (once, s, f, NULL, NULL);
	}
	IGNORE_RACES_END ();
}

void nsync_run_once_arg (nsync_once *once, void (*farg) (void *arg), void *arg) {
	uint32_t o;
	IGNORE_RACES_START ();
	o = ATM_LOAD_ACQ (once);
	if (o != 2) {
		struct once_sync_s *s = NSYNC_ONCE_SYNC_ (once);
		nsync_run_once_impl (once, s, NULL, farg, arg);
	}
	IGNORE_RACES_END ();
}

void nsync_run_once_spin (nsync_once *once, void (*f) (void)) {
	uint32_t o;
	IGNORE_RACES_START ();
	o = ATM_LOAD_ACQ (once);
	if (o != 2) {
		nsync_run_once_impl (once, NULL, f, NULL, NULL);
	}
	IGNORE_RACES_END ();
}

void nsync_run_once_arg_spin (nsync_once *once, void (*farg) (void *arg), void *arg) {
	uint32_t o;
	IGNORE_RACES_START ();
	o = ATM_LOAD_ACQ (once);
	if (o != 2) {
		nsync_run_once_impl (once, NULL, NULL, farg, arg);
	}
	IGNORE_RACES_END ();
}

NSYNC_CPP_END_