1/* Copyright 2016 Google Inc.
2
3 Licensed under the Apache License, Version 2.0 (the "License");
4 you may not use this file except in compliance with the License.
5 You may obtain a copy of the License at
6
7 http://www.apache.org/licenses/LICENSE-2.0
8
9 Unless required by applicable law or agreed to in writing, software
10 distributed under the License is distributed on an "AS IS" BASIS,
11 WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
12 See the License for the specific language governing permissions and
13 limitations under the License. */
14
15#include "nsync_cpp.h"
16#include "platform.h"
17#include "compiler.h"
18#include "cputype.h"
19#include "nsync.h"
20#include "dll.h"
21#include "sem.h"
22#include "wait_internal.h"
23#include "common.h"
24#include "atomic.h"
25
26NSYNC_CPP_START_
27
28/* An once_sync_s struct contains a lock, and a condition variable on which
29 threads may wait for an nsync_once to be initialized by another thread.
30
31 A separate struct is used only to keep nsync_once small.
32
33 A given nsync_once can be associated with any once_sync_s struct, but cannot
34 be associated with more than one. nsync_once instances are mapped to
35 once_sync_s instances by a trivial hashing scheme implemented by
36 NSYNC_ONCE_SYNC_().
37
38 The number of once_sync_s structs in the following array is greater than one
39 only to reduce the probability of contention if a great many distinct
40 nsync_once variables are initialized concurrently. */
41static struct once_sync_s {
42 nsync_mu once_mu;
43 nsync_cv once_cv;
44} once_sync[64];
45
46/* Return a pointer to the once_sync_s struct associated with the nsync_once *p. */
47#define NSYNC_ONCE_SYNC_(p) &once_sync[(((uintptr_t) (p)) / sizeof (*(p))) % \
48 (sizeof (once_sync) / sizeof (once_sync[0]))]
49
50/* Implement nsync_run_once, nsync_run_once_arg, nsync_run_once_spin, or
51 nsync_run_once_arg_spin, chosen as described below.
52
53 If s!=NULL, s is required to point to the once_sync_s associated with *once,
54 and the semantics of nsync_run_once or nsync_run_once_arg are provided.
55 If s==NULL, the semantics of nsync_run_once_spin, or nsync_run_once_arg_spin
56 are provided.
57
58 If f!=NULL, the semantics of nsync_run_once or nsync_run_once_spin are
59 provided. Otherwise, farg is required to be non-NULL, and the semantics of
60 nsync_run_once_arg or nsync_run_once_arg_spin are provided. */
61static void nsync_run_once_impl (nsync_once *once, struct once_sync_s *s,
62 void (*f) (void), void (*farg) (void *arg), void *arg) {
63 uint32_t o = ATM_LOAD_ACQ (once);
64 if (o != 2) {
65 unsigned attempts = 0;
66 if (s != NULL) {
67 nsync_mu_lock (&s->once_mu);
68 }
69 while (o == 0 && !ATM_CAS_ACQ (once, 0, 1)) {
70 o = ATM_LOAD (once);
71 }
72 if (o == 0) {
73 if (s != NULL) {
74 nsync_mu_unlock (&s->once_mu);
75 }
76 if (f != NULL) {
77 (*f) ();
78 } else {
79 (*farg) (arg);
80 }
81 if (s != NULL) {
82 nsync_mu_lock (&s->once_mu);
83 nsync_cv_broadcast (&s->once_cv);
84 }
85 ATM_STORE_REL (once, 2);
86 }
87 while (ATM_LOAD_ACQ (once) != 2) {
88 if (s != NULL) {
89 nsync_time deadline;
90 if (attempts < 50) {
91 attempts += 10;
92 }
93 deadline = nsync_time_add (nsync_time_now (), nsync_time_ms (attempts));
94 nsync_cv_wait_with_deadline (&s->once_cv, &s->once_mu, deadline, NULL);
95 } else {
96 attempts = nsync_spin_delay_ (attempts);
97 }
98 }
99 if (s != NULL) {
100 nsync_mu_unlock (&s->once_mu);
101 }
102 }
103}
104
105void nsync_run_once (nsync_once *once, void (*f) (void)) {
106 uint32_t o;
107 IGNORE_RACES_START ();
108 o = ATM_LOAD_ACQ (once);
109 if (o != 2) {
110 struct once_sync_s *s = NSYNC_ONCE_SYNC_ (once);
111 nsync_run_once_impl (once, s, f, NULL, NULL);
112 }
113 IGNORE_RACES_END ();
114}
115
116void nsync_run_once_arg (nsync_once *once, void (*farg) (void *arg), void *arg) {
117 uint32_t o;
118 IGNORE_RACES_START ();
119 o = ATM_LOAD_ACQ (once);
120 if (o != 2) {
121 struct once_sync_s *s = NSYNC_ONCE_SYNC_ (once);
122 nsync_run_once_impl (once, s, NULL, farg, arg);
123 }
124 IGNORE_RACES_END ();
125}
126
127void nsync_run_once_spin (nsync_once *once, void (*f) (void)) {
128 uint32_t o;
129 IGNORE_RACES_START ();
130 o = ATM_LOAD_ACQ (once);
131 if (o != 2) {
132 nsync_run_once_impl (once, NULL, f, NULL, NULL);
133 }
134 IGNORE_RACES_END ();
135}
136
137void nsync_run_once_arg_spin (nsync_once *once, void (*farg) (void *arg), void *arg) {
138 uint32_t o;
139 IGNORE_RACES_START ();
140 o = ATM_LOAD_ACQ (once);
141 if (o != 2) {
142 nsync_run_once_impl (once, NULL, NULL, farg, arg);
143 }
144 IGNORE_RACES_END ();
145}
146
147NSYNC_CPP_END_
148