1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
|
/**
* Copyright (C) 2015 MongoDB Inc.
*
* This program is free software: you can redistribute it and/or modify
* it under the terms of the GNU Affero General Public License, version 3,
* as published by the Free Software Foundation.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU Affero General Public License for more details.
*
* You should have received a copy of the GNU Affero General Public License
* along with this program. If not, see <http://www.gnu.org/licenses/>.
*
* As a special exception, the copyright holders give permission to link the
* code of portions of this program with the OpenSSL library under certain
* conditions as described in each individual source file and distribute
* linked combinations including the program with the OpenSSL library. You
* must comply with the GNU Affero General Public License in all respects for
* all of the code used other than as permitted herein. If you modify file(s)
* with this exception, you may extend this exception to your version of the
* file(s), but you are not obligated to do so. If you do not wish to do so,
* delete this exception statement from your version. If you delete this
* exception statement from all source files in the program, then also delete
* it in the license file.
*/
#pragma once
#include <atomic>
#include <condition_variable>
#include <list>
#include "mongo/platform/atomic_word.h"
#include "mongo/stdx/mutex.h"
namespace mongo {
/**
* Notifyable is a slim type meant to allow integration of special kinds of waiters for
* stdx::condition_variable. Specifially, the notify() on this type will be called directly from
* stdx::condition_varibale::notify_(one|all).
*
* See Waitable for the stdx::condition_variable integration.
*/
class Notifyable {
public:
virtual void notify() noexcept = 0;
protected:
~Notifyable() = default;
};
class Waitable;
namespace stdx {
using condition_variable_any = ::std::condition_variable_any; // NOLINT
using cv_status = ::std::cv_status; // NOLINT
using ::std::notify_all_at_thread_exit; // NOLINT
/**
* We wrap std::condition_variable to allow us to register Notifyables which can "wait" on the
* condvar without actually waiting on the std::condition_variable. This allows us to possibly do
* productive work in those types, rather than sleeping in the os.
*/
class condition_variable : private std::condition_variable { // NOLINT
public:
using std::condition_variable::condition_variable; // NOLINT
void notify_one() noexcept {
if (_notifyableCount.load()) {
stdx::lock_guard<stdx::mutex> lk(_mutex);
if (_notifyNextNotifyable(lk)) {
return;
}
}
std::condition_variable::notify_one(); // NOLINT
}
void notify_all() noexcept {
if (_notifyableCount.load()) {
stdx::lock_guard<stdx::mutex> lk(_mutex);
while (_notifyNextNotifyable(lk)) {
}
}
std::condition_variable::notify_all(); // NOLINT
}
using std::condition_variable::wait; // NOLINT
using std::condition_variable::wait_for; // NOLINT
using std::condition_variable::wait_until; // NOLINT
using std::condition_variable::native_handle; // NOLINT
private:
friend class ::mongo::Waitable;
/**
* Runs the callback with the Notifyable registered on the condvar. This ensures that for the
* duration of the callback execution, a notification on the condvar will trigger a notify() to
* the Notifyable.
*
* The scheme here is that list entries are erased from the notification list when notified (so
* that they don't eat multiple notify_one's). We detect that condition by noting that our
* Notifyable* has been overwritten with null (in which case we should avoid a double erase).
*
* The method is private, and accessed via friendship in Waitable.
*/
template <typename Callback>
void _runWithNotifyable(Notifyable& notifyable, Callback&& cb) noexcept {
static_assert(noexcept(std::forward<Callback>(cb)()),
"Only noexcept functions may be invoked with _runWithNotifyable");
// We use this local pad to receive notification that we were notified, rather than timing
// out organically.
//
// Note that n must be guarded by _mutex after its insertion in _notifyables (so that we can
// detect notification in a thread-safe manner).
Notifyable* n = ¬ifyable;
auto iter = [&] {
stdx::lock_guard<stdx::mutex> localMutex(_mutex);
_notifyableCount.addAndFetch(1);
return _notifyables.insert(_notifyables.end(), &n);
}();
std::forward<Callback>(cb)();
stdx::lock_guard<stdx::mutex> localMutex(_mutex);
// if n is null, we were notified, and erased in _notifyNextNotifyable
if (n) {
_notifyableCount.subtractAndFetch(1);
_notifyables.erase(iter);
}
}
/**
* Notifies the next notifyable.
*
* Returns true if there was a notifyable to be notified.
*
* Note that as part of notifying, we zero out pointers allocated on the stack by
* _runWithNotifyable callers. This is safe because we hold _mutex while we do so, and our
* erasure communicates that those waiters need not clear themselves from the notification list
* on wakeup.
*/
bool _notifyNextNotifyable(const stdx::lock_guard<stdx::mutex>&) noexcept {
auto iter = _notifyables.begin();
if (iter == _notifyables.end()) {
return false;
}
_notifyableCount.subtractAndFetch(1);
(**iter)->notify();
// null out iter here, so that the notifyable won't remove itself from the list when it
// wakes up
**iter = nullptr;
_notifyables.erase(iter);
return true;
}
AtomicUInt64 _notifyableCount;
stdx::mutex _mutex;
std::list<Notifyable**> _notifyables;
};
} // namespace stdx
} // namespace mongo
|
