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 #if UV__KQUEUE_EVFILT_USER
42 static uv_once_t kqueue_runtime_detection_guard = UV_ONCE_INIT;
43 static int kqueue_evfilt_user_support = 1;
44
45
uv__kqueue_runtime_detection(void)46 static void uv__kqueue_runtime_detection(void) {
47 int kq;
48 struct kevent ev[2];
49 struct timespec timeout = {0, 0};
50
51 /* Perform the runtime detection to ensure that kqueue with
52 * EVFILT_USER actually works. */
53 kq = kqueue();
54 EV_SET(ev, UV__KQUEUE_EVFILT_USER_IDENT, EVFILT_USER,
55 EV_ADD | EV_CLEAR, 0, 0, 0);
56 EV_SET(ev + 1, UV__KQUEUE_EVFILT_USER_IDENT, EVFILT_USER,
57 0, NOTE_TRIGGER, 0, 0);
58 if (kevent(kq, ev, 2, ev, 1, &timeout) < 1 ||
59 ev[0].filter != EVFILT_USER ||
60 ev[0].ident != UV__KQUEUE_EVFILT_USER_IDENT ||
61 ev[0].flags & EV_ERROR)
62 /* If we wind up here, we can assume that EVFILT_USER is defined but
63 * broken on the current system. */
64 kqueue_evfilt_user_support = 0;
65 uv__close(kq);
66 }
67 #endif
68
69 static void uv__async_send(uv_loop_t* loop);
70 static int uv__async_start(uv_loop_t* loop);
71 static void uv__cpu_relax(void);
72
73
uv_async_init(uv_loop_t * loop,uv_async_t * handle,uv_async_cb async_cb)74 int uv_async_init(uv_loop_t* loop, uv_async_t* handle, uv_async_cb async_cb) {
75 int err;
76
77 err = uv__async_start(loop);
78 if (err)
79 return err;
80
81 uv__handle_init(loop, (uv_handle_t*)handle, UV_ASYNC);
82 handle->async_cb = async_cb;
83 handle->pending = 0;
84 handle->u.fd = 0; /* This will be used as a busy flag. */
85
86 uv__queue_insert_tail(&loop->async_handles, &handle->queue);
87 uv__handle_start(handle);
88
89 return 0;
90 }
91
92
uv_async_send(uv_async_t * handle)93 int uv_async_send(uv_async_t* handle) {
94 _Atomic int* pending;
95 _Atomic int* busy;
96
97 pending = (_Atomic int*) &handle->pending;
98 busy = (_Atomic int*) &handle->u.fd;
99
100 /* Do a cheap read first. */
101 if (atomic_load_explicit(pending, memory_order_relaxed) != 0)
102 return 0;
103
104 /* Set the loop to busy. */
105 atomic_fetch_add(busy, 1);
106
107 /* Wake up the other thread's event loop. */
108 if (atomic_exchange(pending, 1) == 0)
109 uv__async_send(handle->loop);
110
111 /* Set the loop to not-busy. */
112 atomic_fetch_add(busy, -1);
113
114 return 0;
115 }
116
117
118 /* Wait for the busy flag to clear before closing.
119 * Only call this from the event loop thread. */
uv__async_spin(uv_async_t * handle)120 static void uv__async_spin(uv_async_t* handle) {
121 _Atomic int* pending;
122 _Atomic int* busy;
123 int i;
124
125 pending = (_Atomic int*) &handle->pending;
126 busy = (_Atomic int*) &handle->u.fd;
127
128 /* Set the pending flag first, so no new events will be added by other
129 * threads after this function returns. */
130 atomic_store(pending, 1);
131
132 for (;;) {
133 /* 997 is not completely chosen at random. It's a prime number, acyclic by
134 * nature, and should therefore hopefully dampen sympathetic resonance.
135 */
136 for (i = 0; i < 997; i++) {
137 if (atomic_load(busy) == 0)
138 return;
139
140 /* Other thread is busy with this handle, spin until it's done. */
141 uv__cpu_relax();
142 }
143
144 /* Yield the CPU. We may have preempted the other thread while it's
145 * inside the critical section and if it's running on the same CPU
146 * as us, we'll just burn CPU cycles until the end of our time slice.
147 */
148 sched_yield();
149 }
150 }
151
152
uv__async_close(uv_async_t * handle)153 void uv__async_close(uv_async_t* handle) {
154 uv__async_spin(handle);
155 uv__queue_remove(&handle->queue);
156 uv__handle_stop(handle);
157 }
158
159
uv__async_io(uv_loop_t * loop,uv__io_t * w,unsigned int events)160 static void uv__async_io(uv_loop_t* loop, uv__io_t* w, unsigned int events) {
161 char buf[1024];
162 ssize_t r;
163 struct uv__queue queue;
164 struct uv__queue* q;
165 uv_async_t* h;
166 _Atomic int *pending;
167
168 assert(w == &loop->async_io_watcher);
169
170 #if UV__KQUEUE_EVFILT_USER
171 for (;!kqueue_evfilt_user_support;) {
172 #else
173 for (;;) {
174 #endif
175 r = read(w->fd, buf, sizeof(buf));
176
177 if (r == sizeof(buf))
178 continue;
179
180 if (r != -1)
181 break;
182
183 if (errno == EAGAIN || errno == EWOULDBLOCK)
184 break;
185
186 if (errno == EINTR)
187 continue;
188
189 abort();
190 }
191
192 uv__queue_move(&loop->async_handles, &queue);
193 while (!uv__queue_empty(&queue)) {
194 q = uv__queue_head(&queue);
195 h = uv__queue_data(q, uv_async_t, queue);
196
197 uv__queue_remove(q);
198 uv__queue_insert_tail(&loop->async_handles, q);
199
200 /* Atomically fetch and clear pending flag */
201 pending = (_Atomic int*) &h->pending;
202 if (atomic_exchange(pending, 0) == 0)
203 continue;
204
205 if (h->async_cb == NULL)
206 continue;
207
208 h->async_cb(h);
209 }
210 }
211
212
213 static void uv__async_send(uv_loop_t* loop) {
214 const void* buf;
215 ssize_t len;
216 int fd;
217 int r;
218
219 buf = "";
220 len = 1;
221 fd = loop->async_wfd;
222
223 #if defined(__linux__)
224 if (fd == -1) {
225 static const uint64_t val = 1;
226 buf = &val;
227 len = sizeof(val);
228 fd = loop->async_io_watcher.fd; /* eventfd */
229 }
230 #elif UV__KQUEUE_EVFILT_USER
231 struct kevent ev;
232
233 if (kqueue_evfilt_user_support) {
234 fd = loop->async_io_watcher.fd; /* magic number for EVFILT_USER */
235 EV_SET(&ev, fd, EVFILT_USER, 0, NOTE_TRIGGER, 0, 0);
236 r = kevent(loop->backend_fd, &ev, 1, NULL, 0, NULL);
237 if (r == 0)
238 return;
239 abort();
240 }
241 #endif
242
243 do
244 r = write(fd, buf, len);
245 while (r == -1 && errno == EINTR);
246
247 if (r == len)
248 return;
249
250 if (r == -1)
251 if (errno == EAGAIN || errno == EWOULDBLOCK)
252 return;
253
254 abort();
255 }
256
257
258 static int uv__async_start(uv_loop_t* loop) {
259 int pipefd[2];
260 int err;
261 #if UV__KQUEUE_EVFILT_USER
262 struct kevent ev;
263 #endif
264
265 if (loop->async_io_watcher.fd != -1)
266 return 0;
267
268 #ifdef __linux__
269 err = eventfd(0, EFD_CLOEXEC | EFD_NONBLOCK);
270 if (err < 0)
271 return UV__ERR(errno);
272
273 pipefd[0] = err;
274 pipefd[1] = -1;
275 #elif UV__KQUEUE_EVFILT_USER
276 uv_once(&kqueue_runtime_detection_guard, uv__kqueue_runtime_detection);
277 if (kqueue_evfilt_user_support) {
278 /* In order not to break the generic pattern of I/O polling, a valid
279 * file descriptor is required to take up a room in loop->watchers,
280 * thus we create one for that, but this fd will not be actually used,
281 * it's just a placeholder and magic number which is going to be closed
282 * during the cleanup, as other FDs. */
283 err = uv__open_cloexec("/dev/null", O_RDONLY);
284 if (err < 0)
285 return err;
286
287 pipefd[0] = err;
288 pipefd[1] = -1;
289
290 /* When using EVFILT_USER event to wake up the kqueue, this event must be
291 * registered beforehand. Otherwise, calling kevent() to issue an
292 * unregistered EVFILT_USER event will get an ENOENT.
293 * Since uv__async_send() may happen before uv__io_poll() with multi-threads,
294 * we can't defer this registration of EVFILT_USER event as we did for other
295 * events, but must perform it right away. */
296 EV_SET(&ev, err, EVFILT_USER, EV_ADD | EV_CLEAR, 0, 0, 0);
297 err = kevent(loop->backend_fd, &ev, 1, NULL, 0, NULL);
298 if (err < 0)
299 return UV__ERR(errno);
300 } else {
301 err = uv__make_pipe(pipefd, UV_NONBLOCK_PIPE);
302 if (err < 0)
303 return err;
304 }
305 #else
306 err = uv__make_pipe(pipefd, UV_NONBLOCK_PIPE);
307 if (err < 0)
308 return err;
309 #endif
310
311 uv__io_init(&loop->async_io_watcher, uv__async_io, pipefd[0]);
312 uv__io_start(loop, &loop->async_io_watcher, POLLIN);
313 loop->async_wfd = pipefd[1];
314
315 #if UV__KQUEUE_EVFILT_USER
316 /* Prevent the EVFILT_USER event from being added to kqueue redundantly
317 * and mistakenly later in uv__io_poll(). */
318 if (kqueue_evfilt_user_support)
319 loop->async_io_watcher.events = loop->async_io_watcher.pevents;
320 #endif
321
322 return 0;
323 }
324
325
326 void uv__async_stop(uv_loop_t* loop) {
327 struct uv__queue queue;
328 struct uv__queue* q;
329 uv_async_t* h;
330
331 if (loop->async_io_watcher.fd == -1)
332 return;
333
334 /* Make sure no other thread is accessing the async handle fd after the loop
335 * cleanup.
336 */
337 uv__queue_move(&loop->async_handles, &queue);
338 while (!uv__queue_empty(&queue)) {
339 q = uv__queue_head(&queue);
340 h = uv__queue_data(q, uv_async_t, queue);
341
342 uv__queue_remove(q);
343 uv__queue_insert_tail(&loop->async_handles, q);
344
345 uv__async_spin(h);
346 }
347
348 if (loop->async_wfd != -1) {
349 if (loop->async_wfd != loop->async_io_watcher.fd)
350 uv__close(loop->async_wfd);
351 loop->async_wfd = -1;
352 }
353
354 uv__io_stop(loop, &loop->async_io_watcher, POLLIN);
355 uv__close(loop->async_io_watcher.fd);
356 loop->async_io_watcher.fd = -1;
357 }
358
359
360 int uv__async_fork(uv_loop_t* loop) {
361 struct uv__queue queue;
362 struct uv__queue* q;
363 uv_async_t* h;
364
365 if (loop->async_io_watcher.fd == -1) /* never started */
366 return 0;
367
368 uv__queue_move(&loop->async_handles, &queue);
369 while (!uv__queue_empty(&queue)) {
370 q = uv__queue_head(&queue);
371 h = uv__queue_data(q, uv_async_t, queue);
372
373 uv__queue_remove(q);
374 uv__queue_insert_tail(&loop->async_handles, q);
375
376 /* The state of any thread that set pending is now likely corrupt in this
377 * child because the user called fork, so just clear these flags and move
378 * on. Calling most libc functions after `fork` is declared to be undefined
379 * behavior anyways, unless async-signal-safe, for multithreaded programs
380 * like libuv, and nothing interesting in pthreads is async-signal-safe.
381 */
382 h->pending = 0;
383 /* This is the busy flag, and we just abruptly lost all other threads. */
384 h->u.fd = 0;
385 }
386
387 /* Recreate these, since they still exist, but belong to the wrong pid now. */
388 if (loop->async_wfd != -1) {
389 if (loop->async_wfd != loop->async_io_watcher.fd)
390 uv__close(loop->async_wfd);
391 loop->async_wfd = -1;
392 }
393
394 uv__io_stop(loop, &loop->async_io_watcher, POLLIN);
395 uv__close(loop->async_io_watcher.fd);
396 loop->async_io_watcher.fd = -1;
397
398 return uv__async_start(loop);
399 }
400
401
402 static void uv__cpu_relax(void) {
403 #if defined(__i386__) || defined(__x86_64__)
404 __asm__ __volatile__ ("rep; nop" ::: "memory"); /* a.k.a. PAUSE */
405 #elif (defined(__arm__) && __ARM_ARCH >= 7) || defined(__aarch64__)
406 __asm__ __volatile__ ("yield" ::: "memory");
407 #elif (defined(__ppc__) || defined(__ppc64__)) && defined(__APPLE__)
408 __asm volatile ("" : : : "memory");
409 #elif !defined(__APPLE__) && (defined(__powerpc64__) || defined(__ppc64__) || defined(__PPC64__))
410 __asm__ __volatile__ ("or 1,1,1; or 2,2,2" ::: "memory");
411 #endif
412 }
413
