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 #ifndef __linux__
162 char buf[1024];
163 ssize_t r;
164 #endif
165 struct uv__queue queue;
166 struct uv__queue* q;
167 uv_async_t* h;
168 _Atomic int *pending;
169
170 assert(w == &loop->async_io_watcher);
171
172 #ifndef __linux__
173 #if UV__KQUEUE_EVFILT_USER
174 for (;!kqueue_evfilt_user_support;) {
175 #else
176 for (;;) {
177 #endif
178 r = read(w->fd, buf, sizeof(buf));
179
180 if (r == sizeof(buf))
181 continue;
182
183 if (r != -1)
184 break;
185
186 if (errno == EAGAIN || errno == EWOULDBLOCK)
187 break;
188
189 if (errno == EINTR)
190 continue;
191
192 abort();
193 }
194 #endif /* !__linux__ */
195
196 uv__queue_move(&loop->async_handles, &queue);
197 while (!uv__queue_empty(&queue)) {
198 q = uv__queue_head(&queue);
199 h = uv__queue_data(q, uv_async_t, queue);
200
201 uv__queue_remove(q);
202 uv__queue_insert_tail(&loop->async_handles, q);
203
204 /* Atomically fetch and clear pending flag */
205 pending = (_Atomic int*) &h->pending;
206 if (atomic_exchange(pending, 0) == 0)
207 continue;
208
209 if (h->async_cb == NULL)
210 continue;
211
212 h->async_cb(h);
213 }
214 }
215
216
217 static void uv__async_send(uv_loop_t* loop) {
218 int fd;
219 ssize_t r;
220 #ifdef __linux__
221 uint64_t val;
222
223 fd = loop->async_io_watcher.fd; /* eventfd */
224 for (val = 1; /* empty */; val = 1) {
225 r = write(fd, &val, sizeof(uint64_t));
226 if (r < 0) {
227 /* When EAGAIN occurs, the eventfd counter hits the maximum value of the unsigned 64-bit.
228 * We need to first drain the eventfd and then write again.
229 *
230 * Check out https://man7.org/linux/man-pages/man2/eventfd.2.html for details.
231 */
232 if (errno == EAGAIN) {
233 /* It's ready to retry. */
234 if (read(fd, &val, sizeof(uint64_t)) > 0 || errno == EAGAIN) {
235 continue;
236 }
237 }
238 /* Unknown error occurs. */
239 break;
240 }
241 return;
242 }
243 #else
244 #if UV__KQUEUE_EVFILT_USER
245 struct kevent ev;
246
247 if (kqueue_evfilt_user_support) {
248 fd = loop->async_io_watcher.fd; /* magic number for EVFILT_USER */
249 EV_SET(&ev, fd, EVFILT_USER, 0, NOTE_TRIGGER, 0, 0);
250 r = kevent(loop->backend_fd, &ev, 1, NULL, 0, NULL);
251 if (r == 0)
252 return;
253 else
254 abort();
255 }
256 #endif
257
258 fd = loop->async_wfd; /* write end of the pipe */
259 do
260 r = write(fd, "x", 1);
261 while (r == -1 && errno == EINTR);
262
263 if (r == 1)
264 return;
265
266 if (r == -1)
267 if (errno == EAGAIN || errno == EWOULDBLOCK)
268 return;
269 #endif
270
271 abort();
272 }
273
274
275 static int uv__async_start(uv_loop_t* loop) {
276 int pipefd[2];
277 int err;
278 #if UV__KQUEUE_EVFILT_USER
279 struct kevent ev;
280 #endif
281
282 if (loop->async_io_watcher.fd != -1)
283 return 0;
284
285 #ifdef __linux__
286 err = eventfd(0, EFD_CLOEXEC | EFD_NONBLOCK);
287 if (err < 0)
288 return UV__ERR(errno);
289
290 pipefd[0] = err;
291 pipefd[1] = -1;
292 #elif UV__KQUEUE_EVFILT_USER
293 uv_once(&kqueue_runtime_detection_guard, uv__kqueue_runtime_detection);
294 if (kqueue_evfilt_user_support) {
295 /* In order not to break the generic pattern of I/O polling, a valid
296 * file descriptor is required to take up a room in loop->watchers,
297 * thus we create one for that, but this fd will not be actually used,
298 * it's just a placeholder and magic number which is going to be closed
299 * during the cleanup, as other FDs. */
300 err = uv__open_cloexec("/dev/null", O_RDONLY);
301 if (err < 0)
302 return err;
303
304 pipefd[0] = err;
305 pipefd[1] = -1;
306
307 /* When using EVFILT_USER event to wake up the kqueue, this event must be
308 * registered beforehand. Otherwise, calling kevent() to issue an
309 * unregistered EVFILT_USER event will get an ENOENT.
310 * Since uv__async_send() may happen before uv__io_poll() with multi-threads,
311 * we can't defer this registration of EVFILT_USER event as we did for other
312 * events, but must perform it right away. */
313 EV_SET(&ev, err, EVFILT_USER, EV_ADD | EV_CLEAR, 0, 0, 0);
314 err = kevent(loop->backend_fd, &ev, 1, NULL, 0, NULL);
315 if (err < 0)
316 return UV__ERR(errno);
317 } else {
318 err = uv__make_pipe(pipefd, UV_NONBLOCK_PIPE);
319 if (err < 0)
320 return err;
321 }
322 #else
323 err = uv__make_pipe(pipefd, UV_NONBLOCK_PIPE);
324 if (err < 0)
325 return err;
326 #endif
327
328 uv__io_init(&loop->async_io_watcher, uv__async_io, pipefd[0]);
329 uv__io_start(loop, &loop->async_io_watcher, POLLIN);
330 loop->async_wfd = pipefd[1];
331
332 #if UV__KQUEUE_EVFILT_USER
333 /* Prevent the EVFILT_USER event from being added to kqueue redundantly
334 * and mistakenly later in uv__io_poll(). */
335 if (kqueue_evfilt_user_support)
336 loop->async_io_watcher.events = loop->async_io_watcher.pevents;
337 #endif
338
339 return 0;
340 }
341
342
343 void uv__async_stop(uv_loop_t* loop) {
344 struct uv__queue queue;
345 struct uv__queue* q;
346 uv_async_t* h;
347
348 if (loop->async_io_watcher.fd == -1)
349 return;
350
351 /* Make sure no other thread is accessing the async handle fd after the loop
352 * cleanup.
353 */
354 uv__queue_move(&loop->async_handles, &queue);
355 while (!uv__queue_empty(&queue)) {
356 q = uv__queue_head(&queue);
357 h = uv__queue_data(q, uv_async_t, queue);
358
359 uv__queue_remove(q);
360 uv__queue_insert_tail(&loop->async_handles, q);
361
362 uv__async_spin(h);
363 }
364
365 if (loop->async_wfd != -1) {
366 if (loop->async_wfd != loop->async_io_watcher.fd)
367 uv__close(loop->async_wfd);
368 loop->async_wfd = -1;
369 }
370
371 uv__io_stop(loop, &loop->async_io_watcher, POLLIN);
372 uv__close(loop->async_io_watcher.fd);
373 loop->async_io_watcher.fd = -1;
374 }
375
376
377 int uv__async_fork(uv_loop_t* loop) {
378 struct uv__queue queue;
379 struct uv__queue* q;
380 uv_async_t* h;
381
382 if (loop->async_io_watcher.fd == -1) /* never started */
383 return 0;
384
385 uv__queue_move(&loop->async_handles, &queue);
386 while (!uv__queue_empty(&queue)) {
387 q = uv__queue_head(&queue);
388 h = uv__queue_data(q, uv_async_t, queue);
389
390 uv__queue_remove(q);
391 uv__queue_insert_tail(&loop->async_handles, q);
392
393 /* The state of any thread that set pending is now likely corrupt in this
394 * child because the user called fork, so just clear these flags and move
395 * on. Calling most libc functions after `fork` is declared to be undefined
396 * behavior anyways, unless async-signal-safe, for multithreaded programs
397 * like libuv, and nothing interesting in pthreads is async-signal-safe.
398 */
399 h->pending = 0;
400 /* This is the busy flag, and we just abruptly lost all other threads. */
401 h->u.fd = 0;
402 }
403
404 /* Recreate these, since they still exist, but belong to the wrong pid now. */
405 if (loop->async_wfd != -1) {
406 if (loop->async_wfd != loop->async_io_watcher.fd)
407 uv__close(loop->async_wfd);
408 loop->async_wfd = -1;
409 }
410
411 uv__io_stop(loop, &loop->async_io_watcher, POLLIN);
412 uv__close(loop->async_io_watcher.fd);
413 loop->async_io_watcher.fd = -1;
414
415 return uv__async_start(loop);
416 }
417
418
419 static void uv__cpu_relax(void) {
420 #if defined(__i386__) || defined(__x86_64__)
421 __asm__ __volatile__ ("rep; nop" ::: "memory"); /* a.k.a. PAUSE */
422 #elif (defined(__arm__) && __ARM_ARCH >= 7) || defined(__aarch64__)
423 __asm__ __volatile__ ("yield" ::: "memory");
424 #elif (defined(__ppc__) || defined(__ppc64__)) && defined(__APPLE__)
425 __asm volatile ("" : : : "memory");
426 #elif !defined(__APPLE__) && (defined(__powerpc64__) || defined(__ppc64__) || defined(__PPC64__))
427 __asm__ __volatile__ ("or 1,1,1; or 2,2,2" ::: "memory");
428 #endif
429 }
430