1 /* Copyright Joyent, Inc. and other Node contributors. All rights reserved.
2 * Permission is hereby granted, free of charge, to any person obtaining a copy
3 * of this software and associated documentation files (the "Software"), to
4 * deal in the Software without restriction, including without limitation the
5 * rights to use, copy, modify, merge, publish, distribute, sublicense, and/or
6 * sell copies of the Software, and to permit persons to whom the Software is
7 * furnished to do so, subject to the following conditions:
8 *
9 * The above copyright notice and this permission notice shall be included in
10 * all copies or substantial portions of the Software.
11 *
12 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
13 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
14 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
15 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
16 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
17 * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
18 * IN THE SOFTWARE.
19 */
20
21 /* This file contains both the uv__async internal infrastructure and the
22 * user-facing uv_async_t functions.
23 */
24
25 #include "uv.h"
26 #include "internal.h"
27
28 #include <errno.h>
29 #include <stdatomic.h>
30 #include <stdio.h> /* snprintf() */
31 #include <assert.h>
32 #include <stdlib.h>
33 #include <string.h>
34 #include <unistd.h>
35 #include <sched.h> /* sched_yield() */
36
37 #ifdef __linux__
38 #include <sys/eventfd.h>
39 #endif
40
41 static void uv__async_send(uv_loop_t* loop);
42 static int uv__async_start(uv_loop_t* loop);
43 static void uv__cpu_relax(void);
44
45
uv_async_init(uv_loop_t * loop,uv_async_t * handle,uv_async_cb async_cb)46 int uv_async_init(uv_loop_t* loop, uv_async_t* handle, uv_async_cb async_cb) {
47 int err;
48
49 err = uv__async_start(loop);
50 if (err)
51 return err;
52
53 uv__handle_init(loop, (uv_handle_t*)handle, UV_ASYNC);
54 handle->async_cb = async_cb;
55 handle->pending = 0;
56 handle->u.fd = 0; /* This will be used as a busy flag. */
57
58 uv__queue_insert_tail(&loop->async_handles, &handle->queue);
59 uv__handle_start(handle);
60
61 return 0;
62 }
63
64
uv_async_send(uv_async_t * handle)65 int uv_async_send(uv_async_t* handle) {
66 _Atomic int* pending;
67 _Atomic int* busy;
68
69 pending = (_Atomic int*) &handle->pending;
70 busy = (_Atomic int*) &handle->u.fd;
71
72 /* Do a cheap read first. */
73 if (atomic_load_explicit(pending, memory_order_relaxed) != 0)
74 return 0;
75
76 /* Set the loop to busy. */
77 atomic_fetch_add(busy, 1);
78
79 /* Wake up the other thread's event loop. */
80 if (atomic_exchange(pending, 1) == 0)
81 uv__async_send(handle->loop);
82
83 /* Set the loop to not-busy. */
84 atomic_fetch_add(busy, -1);
85
86 return 0;
87 }
88
89
90 /* Wait for the busy flag to clear before closing.
91 * Only call this from the event loop thread. */
uv__async_spin(uv_async_t * handle)92 static void uv__async_spin(uv_async_t* handle) {
93 _Atomic int* pending;
94 _Atomic int* busy;
95 int i;
96
97 pending = (_Atomic int*) &handle->pending;
98 busy = (_Atomic int*) &handle->u.fd;
99
100 /* Set the pending flag first, so no new events will be added by other
101 * threads after this function returns. */
102 atomic_store(pending, 1);
103
104 for (;;) {
105 /* 997 is not completely chosen at random. It's a prime number, acyclic by
106 * nature, and should therefore hopefully dampen sympathetic resonance.
107 */
108 for (i = 0; i < 997; i++) {
109 if (atomic_load(busy) == 0)
110 return;
111
112 /* Other thread is busy with this handle, spin until it's done. */
113 uv__cpu_relax();
114 }
115
116 /* Yield the CPU. We may have preempted the other thread while it's
117 * inside the critical section and if it's running on the same CPU
118 * as us, we'll just burn CPU cycles until the end of our time slice.
119 */
120 sched_yield();
121 }
122 }
123
124
uv__async_close(uv_async_t * handle)125 void uv__async_close(uv_async_t* handle) {
126 uv__async_spin(handle);
127 uv__queue_remove(&handle->queue);
128 uv__handle_stop(handle);
129 }
130
131
uv__async_io(uv_loop_t * loop,uv__io_t * w,unsigned int events)132 static void uv__async_io(uv_loop_t* loop, uv__io_t* w, unsigned int events) {
133 char buf[1024];
134 ssize_t r;
135 struct uv__queue queue;
136 struct uv__queue* q;
137 uv_async_t* h;
138 _Atomic int *pending;
139
140 assert(w == &loop->async_io_watcher);
141
142 for (;;) {
143 r = read(w->fd, buf, sizeof(buf));
144
145 if (r == sizeof(buf))
146 continue;
147
148 if (r != -1)
149 break;
150
151 if (errno == EAGAIN || errno == EWOULDBLOCK)
152 break;
153
154 if (errno == EINTR)
155 continue;
156
157 abort();
158 }
159
160 uv__queue_move(&loop->async_handles, &queue);
161 while (!uv__queue_empty(&queue)) {
162 q = uv__queue_head(&queue);
163 h = uv__queue_data(q, uv_async_t, queue);
164
165 uv__queue_remove(q);
166 uv__queue_insert_tail(&loop->async_handles, q);
167
168 /* Atomically fetch and clear pending flag */
169 pending = (_Atomic int*) &h->pending;
170 if (atomic_exchange(pending, 0) == 0)
171 continue;
172
173 if (h->async_cb == NULL)
174 continue;
175
176 h->async_cb(h);
177 }
178 }
179
180
uv__async_send(uv_loop_t * loop)181 static void uv__async_send(uv_loop_t* loop) {
182 const void* buf;
183 ssize_t len;
184 int fd;
185 int r;
186
187 buf = "";
188 len = 1;
189 fd = loop->async_wfd;
190
191 #if defined(__linux__)
192 if (fd == -1) {
193 static const uint64_t val = 1;
194 buf = &val;
195 len = sizeof(val);
196 fd = loop->async_io_watcher.fd; /* eventfd */
197 }
198 #endif
199
200 do
201 r = write(fd, buf, len);
202 while (r == -1 && errno == EINTR);
203
204 if (r == len)
205 return;
206
207 if (r == -1)
208 if (errno == EAGAIN || errno == EWOULDBLOCK)
209 return;
210
211 abort();
212 }
213
214
uv__async_start(uv_loop_t * loop)215 static int uv__async_start(uv_loop_t* loop) {
216 int pipefd[2];
217 int err;
218
219 if (loop->async_io_watcher.fd != -1)
220 return 0;
221
222 #ifdef __linux__
223 err = eventfd(0, EFD_CLOEXEC | EFD_NONBLOCK);
224 if (err < 0)
225 return UV__ERR(errno);
226
227 pipefd[0] = err;
228 pipefd[1] = -1;
229 #else
230 err = uv__make_pipe(pipefd, UV_NONBLOCK_PIPE);
231 if (err < 0)
232 return err;
233 #endif
234
235 uv__io_init(&loop->async_io_watcher, uv__async_io, pipefd[0]);
236 uv__io_start(loop, &loop->async_io_watcher, POLLIN);
237 loop->async_wfd = pipefd[1];
238
239 return 0;
240 }
241
242
uv__async_stop(uv_loop_t * loop)243 void uv__async_stop(uv_loop_t* loop) {
244 struct uv__queue queue;
245 struct uv__queue* q;
246 uv_async_t* h;
247
248 if (loop->async_io_watcher.fd == -1)
249 return;
250
251 /* Make sure no other thread is accessing the async handle fd after the loop
252 * cleanup.
253 */
254 uv__queue_move(&loop->async_handles, &queue);
255 while (!uv__queue_empty(&queue)) {
256 q = uv__queue_head(&queue);
257 h = uv__queue_data(q, uv_async_t, queue);
258
259 uv__queue_remove(q);
260 uv__queue_insert_tail(&loop->async_handles, q);
261
262 uv__async_spin(h);
263 }
264
265 if (loop->async_wfd != -1) {
266 if (loop->async_wfd != loop->async_io_watcher.fd)
267 uv__close(loop->async_wfd);
268 loop->async_wfd = -1;
269 }
270
271 uv__io_stop(loop, &loop->async_io_watcher, POLLIN);
272 uv__close(loop->async_io_watcher.fd);
273 loop->async_io_watcher.fd = -1;
274 }
275
276
uv__async_fork(uv_loop_t * loop)277 int uv__async_fork(uv_loop_t* loop) {
278 struct uv__queue queue;
279 struct uv__queue* q;
280 uv_async_t* h;
281
282 if (loop->async_io_watcher.fd == -1) /* never started */
283 return 0;
284
285 uv__queue_move(&loop->async_handles, &queue);
286 while (!uv__queue_empty(&queue)) {
287 q = uv__queue_head(&queue);
288 h = uv__queue_data(q, uv_async_t, queue);
289
290 uv__queue_remove(q);
291 uv__queue_insert_tail(&loop->async_handles, q);
292
293 /* The state of any thread that set pending is now likely corrupt in this
294 * child because the user called fork, so just clear these flags and move
295 * on. Calling most libc functions after `fork` is declared to be undefined
296 * behavior anyways, unless async-signal-safe, for multithreaded programs
297 * like libuv, and nothing interesting in pthreads is async-signal-safe.
298 */
299 h->pending = 0;
300 /* This is the busy flag, and we just abruptly lost all other threads. */
301 h->u.fd = 0;
302 }
303
304 /* Recreate these, since they still exist, but belong to the wrong pid now. */
305 if (loop->async_wfd != -1) {
306 if (loop->async_wfd != loop->async_io_watcher.fd)
307 uv__close(loop->async_wfd);
308 loop->async_wfd = -1;
309 }
310
311 uv__io_stop(loop, &loop->async_io_watcher, POLLIN);
312 uv__close(loop->async_io_watcher.fd);
313 loop->async_io_watcher.fd = -1;
314
315 return uv__async_start(loop);
316 }
317
318
uv__cpu_relax(void)319 static void uv__cpu_relax(void) {
320 #if defined(__i386__) || defined(__x86_64__)
321 __asm__ __volatile__ ("rep; nop" ::: "memory"); /* a.k.a. PAUSE */
322 #elif (defined(__arm__) && __ARM_ARCH >= 7) || defined(__aarch64__)
323 __asm__ __volatile__ ("yield" ::: "memory");
324 #elif (defined(__ppc__) || defined(__ppc64__)) && defined(__APPLE__)
325 __asm volatile ("" : : : "memory");
326 #elif !defined(__APPLE__) && (defined(__powerpc64__) || defined(__ppc64__) || defined(__PPC64__))
327 __asm__ __volatile__ ("or 1,1,1; or 2,2,2" ::: "memory");
328 #endif
329 }
330
