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  1. /*
  2. * libev linux aio fd activity backend
  3. *
  4. * Copyright (c) 2019 Marc Alexander Lehmann <libev@schmorp.de>
  5. * All rights reserved.
  6. *
  7. * Redistribution and use in source and binary forms, with or without modifica-
  8. * tion, are permitted provided that the following conditions are met:
  9. *
  10. * 1. Redistributions of source code must retain the above copyright notice,
  11. * this list of conditions and the following disclaimer.
  12. *
  13. * 2. Redistributions in binary form must reproduce the above copyright
  14. * notice, this list of conditions and the following disclaimer in the
  15. * documentation and/or other materials provided with the distribution.
  16. *
  17. * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR IMPLIED
  18. * WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MER-
  19. * CHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO
  20. * EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPE-
  21. * CIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
  22. * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS;
  23. * OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
  24. * WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTH-
  25. * ERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED
  26. * OF THE POSSIBILITY OF SUCH DAMAGE.
  27. *
  28. * Alternatively, the contents of this file may be used under the terms of
  29. * the GNU General Public License ("GPL") version 2 or any later version,
  30. * in which case the provisions of the GPL are applicable instead of
  31. * the above. If you wish to allow the use of your version of this file
  32. * only under the terms of the GPL and not to allow others to use your
  33. * version of this file under the BSD license, indicate your decision
  34. * by deleting the provisions above and replace them with the notice
  35. * and other provisions required by the GPL. If you do not delete the
  36. * provisions above, a recipient may use your version of this file under
  37. * either the BSD or the GPL.
  38. */
  39. /*
  40. * general notes about linux aio:
  41. *
  42. * a) at first, the linux aio IOCB_CMD_POLL functionality introduced in
  43. * 4.18 looks too good to be true: both watchers and events can be
  44. * batched, and events can even be handled in userspace using
  45. * a ring buffer shared with the kernel. watchers can be canceled
  46. * regardless of whether the fd has been closed. no problems with fork.
  47. * ok, the ring buffer is 200% undocumented (there isn't even a
  48. * header file), but otherwise, it's pure bliss!
  49. * b) ok, watchers are one-shot, so you have to re-arm active ones
  50. * on every iteration. so much for syscall-less event handling,
  51. * but at least these re-arms can be batched, no big deal, right?
  52. * c) well, linux as usual: the documentation lies to you: io_submit
  53. * sometimes returns EINVAL because the kernel doesn't feel like
  54. * handling your poll mask - ttys can be polled for POLLOUT,
  55. * POLLOUT|POLLIN, but polling for POLLIN fails. just great,
  56. * so we have to fall back to something else (hello, epoll),
  57. * but at least the fallback can be slow, because these are
  58. * exceptional cases, right?
  59. * d) hmm, you have to tell the kernel the maximum number of watchers
  60. * you want to queue when initialising the aio context. but of
  61. * course the real limit is magically calculated in the kernel, and
  62. * is often higher then we asked for. so we just have to destroy
  63. * the aio context and re-create it a bit larger if we hit the limit.
  64. * (starts to remind you of epoll? well, it's a bit more deterministic
  65. * and less gambling, but still ugly as hell).
  66. * e) that's when you find out you can also hit an arbitrary system-wide
  67. * limit. or the kernel simply doesn't want to handle your watchers.
  68. * what the fuck do we do then? you guessed it, in the middle
  69. * of event handling we have to switch to 100% epoll polling. and
  70. * that better is as fast as normal epoll polling, so you practically
  71. * have to use the normal epoll backend with all its quirks.
  72. * f) end result of this train wreck: it inherits all the disadvantages
  73. * from epoll, while adding a number on its own. why even bother to use
  74. * it? because if conditions are right and your fds are supported and you
  75. * don't hit a limit, this backend is actually faster, doesn't gamble with
  76. * your fds, batches watchers and events and doesn't require costly state
  77. * recreates. well, until it does.
  78. * g) all of this makes this backend use almost twice as much code as epoll.
  79. * which in turn uses twice as much code as poll. and that#s not counting
  80. * the fact that this backend also depends on the epoll backend, making
  81. * it three times as much code as poll, or kqueue.
  82. * h) bleah. why can't linux just do kqueue. sure kqueue is ugly, but by now
  83. * it's clear that whatever linux comes up with is far, far, far worse.
  84. */
  85. #include <sys/time.h> /* actually linux/time.h, but we must assume they are compatible */
  86. #include <poll.h>
  87. #include <linux/aio_abi.h>
  88. /*****************************************************************************/
  89. /* syscall wrapdadoop - this section has the raw api/abi definitions */
  90. #include <sys/syscall.h> /* no glibc wrappers */
  91. /* aio_abi.h is not versioned in any way, so we cannot test for its existance */
  92. #define IOCB_CMD_POLL 5
  93. /* taken from linux/fs/aio.c. yup, that's a .c file.
  94. * not only is this totally undocumented, not even the source code
  95. * can tell you what the future semantics of compat_features and
  96. * incompat_features are, or what header_length actually is for.
  97. */
  98. #define AIO_RING_MAGIC 0xa10a10a1
  99. #define EV_AIO_RING_INCOMPAT_FEATURES 0
  100. struct aio_ring
  101. {
  102. unsigned id; /* kernel internal index number */
  103. unsigned nr; /* number of io_events */
  104. unsigned head; /* Written to by userland or by kernel. */
  105. unsigned tail;
  106. unsigned magic;
  107. unsigned compat_features;
  108. unsigned incompat_features;
  109. unsigned header_length; /* size of aio_ring */
  110. struct io_event io_events[0];
  111. };
  112. inline_size
  113. int
  114. evsys_io_setup (unsigned nr_events, aio_context_t *ctx_idp)
  115. {
  116. return ev_syscall2 (SYS_io_setup, nr_events, ctx_idp);
  117. }
  118. inline_size
  119. int
  120. evsys_io_destroy (aio_context_t ctx_id)
  121. {
  122. return ev_syscall1 (SYS_io_destroy, ctx_id);
  123. }
  124. inline_size
  125. int
  126. evsys_io_submit (aio_context_t ctx_id, long nr, struct iocb *cbp[])
  127. {
  128. return ev_syscall3 (SYS_io_submit, ctx_id, nr, cbp);
  129. }
  130. inline_size
  131. int
  132. evsys_io_cancel (aio_context_t ctx_id, struct iocb *cbp, struct io_event *result)
  133. {
  134. return ev_syscall3 (SYS_io_cancel, ctx_id, cbp, result);
  135. }
  136. inline_size
  137. int
  138. evsys_io_getevents (aio_context_t ctx_id, long min_nr, long nr, struct io_event *events, struct timespec *timeout)
  139. {
  140. return ev_syscall5 (SYS_io_getevents, ctx_id, min_nr, nr, events, timeout);
  141. }
  142. /*****************************************************************************/
  143. /* actual backed implementation */
  144. ecb_cold
  145. static int
  146. linuxaio_nr_events (EV_P)
  147. {
  148. /* we start with 16 iocbs and incraese from there
  149. * that's tiny, but the kernel has a rather low system-wide
  150. * limit that can be reached quickly, so let's be parsimonious
  151. * with this resource.
  152. * Rest assured, the kernel generously rounds up small and big numbers
  153. * in different ways (but doesn't seem to charge you for it).
  154. * The 15 here is because the kernel usually has a power of two as aio-max-nr,
  155. * and this helps to take advantage of that limit.
  156. */
  157. /* we try to fill 4kB pages exactly.
  158. * the ring buffer header is 32 bytes, every io event is 32 bytes.
  159. * the kernel takes the io requests number, doubles it, adds 2
  160. * and adds the ring buffer.
  161. * the way we use this is by starting low, and then roughly doubling the
  162. * size each time we hit a limit.
  163. */
  164. int requests = 15 << linuxaio_iteration;
  165. int one_page = (4096
  166. / sizeof (struct io_event) ) / 2; /* how many fit into one page */
  167. int first_page = ((4096 - sizeof (struct aio_ring))
  168. / sizeof (struct io_event) - 2) / 2; /* how many fit into the first page */
  169. /* if everything fits into one page, use count exactly */
  170. if (requests > first_page)
  171. /* otherwise, round down to full pages and add the first page */
  172. requests = requests / one_page * one_page + first_page;
  173. return requests;
  174. }
  175. /* we use out own wrapper structure in case we ever want to do something "clever" */
  176. typedef struct aniocb
  177. {
  178. struct iocb io;
  179. /*int inuse;*/
  180. } *ANIOCBP;
  181. inline_size
  182. void
  183. linuxaio_array_needsize_iocbp (ANIOCBP *base, int offset, int count)
  184. {
  185. while (count--)
  186. {
  187. /* TODO: quite the overhead to allocate every iocb separately, maybe use our own allocator? */
  188. ANIOCBP iocb = (ANIOCBP)ev_malloc (sizeof (*iocb));
  189. /* full zero initialise is probably not required at the moment, but
  190. * this is not well documented, so we better do it.
  191. */
  192. memset (iocb, 0, sizeof (*iocb));
  193. iocb->io.aio_lio_opcode = IOCB_CMD_POLL;
  194. iocb->io.aio_fildes = offset;
  195. base [offset++] = iocb;
  196. }
  197. }
  198. ecb_cold
  199. static void
  200. linuxaio_free_iocbp (EV_P)
  201. {
  202. while (linuxaio_iocbpmax--)
  203. ev_free (linuxaio_iocbps [linuxaio_iocbpmax]);
  204. linuxaio_iocbpmax = 0; /* next resize will completely reallocate the array, at some overhead */
  205. }
  206. static void
  207. linuxaio_modify (EV_P_ int fd, int oev, int nev)
  208. {
  209. array_needsize (ANIOCBP, linuxaio_iocbps, linuxaio_iocbpmax, fd + 1, linuxaio_array_needsize_iocbp);
  210. ANIOCBP iocb = linuxaio_iocbps [fd];
  211. ANFD *anfd = &anfds [fd];
  212. if (ecb_expect_false (iocb->io.aio_reqprio < 0))
  213. {
  214. /* we handed this fd over to epoll, so undo this first */
  215. /* we do it manually because the optimisations on epoll_modify won't do us any good */
  216. epoll_ctl (backend_fd, EPOLL_CTL_DEL, fd, 0);
  217. anfd->emask = 0;
  218. iocb->io.aio_reqprio = 0;
  219. }
  220. else if (ecb_expect_false (iocb->io.aio_buf))
  221. {
  222. /* iocb active, so cancel it first before resubmit */
  223. /* this assumes we only ever get one call per fd per loop iteration */
  224. for (;;)
  225. {
  226. /* on all relevant kernels, io_cancel fails with EINPROGRESS on "success" */
  227. if (ecb_expect_false (evsys_io_cancel (linuxaio_ctx, &iocb->io, (struct io_event *)0) == 0))
  228. break;
  229. if (ecb_expect_true (errno == EINPROGRESS))
  230. break;
  231. /* the EINPROGRESS test is for nicer error message. clumsy. */
  232. if (errno != EINTR)
  233. {
  234. assert (("libev: linuxaio unexpected io_cancel failed", errno != EINTR && errno != EINPROGRESS));
  235. break;
  236. }
  237. }
  238. /* increment generation counter to avoid handling old events */
  239. ++anfd->egen;
  240. }
  241. iocb->io.aio_buf = (nev & EV_READ ? POLLIN : 0)
  242. | (nev & EV_WRITE ? POLLOUT : 0);
  243. if (nev)
  244. {
  245. iocb->io.aio_data = (uint32_t)fd | ((__u64)(uint32_t)anfd->egen << 32);
  246. /* queue iocb up for io_submit */
  247. /* this assumes we only ever get one call per fd per loop iteration */
  248. ++linuxaio_submitcnt;
  249. array_needsize (struct iocb *, linuxaio_submits, linuxaio_submitmax, linuxaio_submitcnt, array_needsize_noinit);
  250. linuxaio_submits [linuxaio_submitcnt - 1] = &iocb->io;
  251. }
  252. }
  253. static void
  254. linuxaio_epoll_cb (EV_P_ struct ev_io *w, int revents)
  255. {
  256. epoll_poll (EV_A_ 0);
  257. }
  258. inline_speed
  259. void
  260. linuxaio_fd_rearm (EV_P_ int fd)
  261. {
  262. anfds [fd].events = 0;
  263. linuxaio_iocbps [fd]->io.aio_buf = 0;
  264. fd_change (EV_A_ fd, EV_ANFD_REIFY);
  265. }
  266. static void
  267. linuxaio_parse_events (EV_P_ struct io_event *ev, int nr)
  268. {
  269. while (nr)
  270. {
  271. int fd = ev->data & 0xffffffff;
  272. uint32_t gen = ev->data >> 32;
  273. int res = ev->res;
  274. assert (("libev: iocb fd must be in-bounds", fd >= 0 && fd < anfdmax));
  275. /* only accept events if generation counter matches */
  276. if (ecb_expect_true (gen == (uint32_t)anfds [fd].egen))
  277. {
  278. /* feed events, we do not expect or handle POLLNVAL */
  279. fd_event (
  280. EV_A_
  281. fd,
  282. (res & (POLLOUT | POLLERR | POLLHUP) ? EV_WRITE : 0)
  283. | (res & (POLLIN | POLLERR | POLLHUP) ? EV_READ : 0)
  284. );
  285. /* linux aio is oneshot: rearm fd. TODO: this does more work than strictly needed */
  286. linuxaio_fd_rearm (EV_A_ fd);
  287. }
  288. --nr;
  289. ++ev;
  290. }
  291. }
  292. /* get any events from ring buffer, return true if any were handled */
  293. static int
  294. linuxaio_get_events_from_ring (EV_P)
  295. {
  296. struct aio_ring *ring = (struct aio_ring *)linuxaio_ctx;
  297. unsigned head, tail;
  298. /* the kernel reads and writes both of these variables, */
  299. /* as a C extension, we assume that volatile use here */
  300. /* both makes reads atomic and once-only */
  301. head = *(volatile unsigned *)&ring->head;
  302. ECB_MEMORY_FENCE_ACQUIRE;
  303. tail = *(volatile unsigned *)&ring->tail;
  304. if (head == tail)
  305. return 0;
  306. /* parse all available events, but only once, to avoid starvation */
  307. if (ecb_expect_true (tail > head)) /* normal case around */
  308. linuxaio_parse_events (EV_A_ ring->io_events + head, tail - head);
  309. else /* wrapped around */
  310. {
  311. linuxaio_parse_events (EV_A_ ring->io_events + head, ring->nr - head);
  312. linuxaio_parse_events (EV_A_ ring->io_events, tail);
  313. }
  314. ECB_MEMORY_FENCE_RELEASE;
  315. /* as an extension to C, we hope that the volatile will make this atomic and once-only */
  316. *(volatile unsigned *)&ring->head = tail;
  317. return 1;
  318. }
  319. inline_size
  320. int
  321. linuxaio_ringbuf_valid (EV_P)
  322. {
  323. struct aio_ring *ring = (struct aio_ring *)linuxaio_ctx;
  324. return ecb_expect_true (ring->magic == AIO_RING_MAGIC)
  325. && ring->incompat_features == EV_AIO_RING_INCOMPAT_FEATURES
  326. && ring->header_length == sizeof (struct aio_ring); /* TODO: or use it to find io_event[0]? */
  327. }
  328. /* read at least one event from kernel, or timeout */
  329. inline_size
  330. void
  331. linuxaio_get_events (EV_P_ ev_tstamp timeout)
  332. {
  333. struct timespec ts;
  334. struct io_event ioev[8]; /* 256 octet stack space */
  335. int want = 1; /* how many events to request */
  336. int ringbuf_valid = linuxaio_ringbuf_valid (EV_A);
  337. if (ecb_expect_true (ringbuf_valid))
  338. {
  339. /* if the ring buffer has any events, we don't wait or call the kernel at all */
  340. if (linuxaio_get_events_from_ring (EV_A))
  341. return;
  342. /* if the ring buffer is empty, and we don't have a timeout, then don't call the kernel */
  343. if (!timeout)
  344. return;
  345. }
  346. else
  347. /* no ringbuffer, request slightly larger batch */
  348. want = sizeof (ioev) / sizeof (ioev [0]);
  349. /* no events, so wait for some
  350. * for fairness reasons, we do this in a loop, to fetch all events
  351. */
  352. for (;;)
  353. {
  354. int res;
  355. EV_RELEASE_CB;
  356. EV_TS_SET (ts, timeout);
  357. res = evsys_io_getevents (linuxaio_ctx, 1, want, ioev, &ts);
  358. EV_ACQUIRE_CB;
  359. if (res < 0)
  360. if (errno == EINTR)
  361. /* ignored, retry */;
  362. else
  363. ev_syserr ("(libev) linuxaio io_getevents");
  364. else if (res)
  365. {
  366. /* at least one event available, handle them */
  367. linuxaio_parse_events (EV_A_ ioev, res);
  368. if (ecb_expect_true (ringbuf_valid))
  369. {
  370. /* if we have a ring buffer, handle any remaining events in it */
  371. linuxaio_get_events_from_ring (EV_A);
  372. /* at this point, we should have handled all outstanding events */
  373. break;
  374. }
  375. else if (res < want)
  376. /* otherwise, if there were fewere events than we wanted, we assume there are no more */
  377. break;
  378. }
  379. else
  380. break; /* no events from the kernel, we are done */
  381. timeout = EV_TS_CONST (0.); /* only wait in the first iteration */
  382. }
  383. }
  384. inline_size
  385. int
  386. linuxaio_io_setup (EV_P)
  387. {
  388. linuxaio_ctx = 0;
  389. return evsys_io_setup (linuxaio_nr_events (EV_A), &linuxaio_ctx);
  390. }
  391. static void
  392. linuxaio_poll (EV_P_ ev_tstamp timeout)
  393. {
  394. int submitted;
  395. /* first phase: submit new iocbs */
  396. /* io_submit might return less than the requested number of iocbs */
  397. /* this is, afaics, only because of errors, but we go by the book and use a loop, */
  398. /* which allows us to pinpoint the erroneous iocb */
  399. for (submitted = 0; submitted < linuxaio_submitcnt; )
  400. {
  401. int res = evsys_io_submit (linuxaio_ctx, linuxaio_submitcnt - submitted, linuxaio_submits + submitted);
  402. if (ecb_expect_false (res < 0))
  403. if (errno == EINVAL)
  404. {
  405. /* This happens for unsupported fds, officially, but in my testing,
  406. * also randomly happens for supported fds. We fall back to good old
  407. * poll() here, under the assumption that this is a very rare case.
  408. * See https://lore.kernel.org/patchwork/patch/1047453/ to see
  409. * discussion about such a case (ttys) where polling for POLLIN
  410. * fails but POLLIN|POLLOUT works.
  411. */
  412. struct iocb *iocb = linuxaio_submits [submitted];
  413. epoll_modify (EV_A_ iocb->aio_fildes, 0, anfds [iocb->aio_fildes].events);
  414. iocb->aio_reqprio = -1; /* mark iocb as epoll */
  415. res = 1; /* skip this iocb - another iocb, another chance */
  416. }
  417. else if (errno == EAGAIN)
  418. {
  419. /* This happens when the ring buffer is full, or some other shit we
  420. * don't know and isn't documented. Most likely because we have too
  421. * many requests and linux aio can't be assed to handle them.
  422. * In this case, we try to allocate a larger ring buffer, freeing
  423. * ours first. This might fail, in which case we have to fall back to 100%
  424. * epoll.
  425. * God, how I hate linux not getting its act together. Ever.
  426. */
  427. evsys_io_destroy (linuxaio_ctx);
  428. linuxaio_submitcnt = 0;
  429. /* rearm all fds with active iocbs */
  430. {
  431. int fd;
  432. for (fd = 0; fd < linuxaio_iocbpmax; ++fd)
  433. if (linuxaio_iocbps [fd]->io.aio_buf)
  434. linuxaio_fd_rearm (EV_A_ fd);
  435. }
  436. ++linuxaio_iteration;
  437. if (linuxaio_io_setup (EV_A) < 0)
  438. {
  439. /* TODO: rearm all and recreate epoll backend from scratch */
  440. /* TODO: might be more prudent? */
  441. /* to bad, we can't get a new aio context, go 100% epoll */
  442. linuxaio_free_iocbp (EV_A);
  443. ev_io_stop (EV_A_ &linuxaio_epoll_w);
  444. ev_ref (EV_A);
  445. linuxaio_ctx = 0;
  446. backend = EVBACKEND_EPOLL;
  447. backend_modify = epoll_modify;
  448. backend_poll = epoll_poll;
  449. }
  450. timeout = EV_TS_CONST (0.);
  451. /* it's easiest to handle this mess in another iteration */
  452. return;
  453. }
  454. else if (errno == EBADF)
  455. {
  456. assert (("libev: event loop rejected bad fd", errno != EBADF));
  457. fd_kill (EV_A_ linuxaio_submits [submitted]->aio_fildes);
  458. res = 1; /* skip this iocb */
  459. }
  460. else if (errno == EINTR) /* not seen in reality, not documented */
  461. res = 0; /* silently ignore and retry */
  462. else
  463. {
  464. ev_syserr ("(libev) linuxaio io_submit");
  465. res = 0;
  466. }
  467. submitted += res;
  468. }
  469. linuxaio_submitcnt = 0;
  470. /* second phase: fetch and parse events */
  471. linuxaio_get_events (EV_A_ timeout);
  472. }
  473. inline_size
  474. int
  475. linuxaio_init (EV_P_ int flags)
  476. {
  477. /* would be great to have a nice test for IOCB_CMD_POLL instead */
  478. /* also: test some semi-common fd types, such as files and ttys in recommended_backends */
  479. /* 4.18 introduced IOCB_CMD_POLL, 4.19 made epoll work, and we need that */
  480. if (ev_linux_version () < 0x041300)
  481. return 0;
  482. if (!epoll_init (EV_A_ 0))
  483. return 0;
  484. linuxaio_iteration = 0;
  485. if (linuxaio_io_setup (EV_A) < 0)
  486. {
  487. epoll_destroy (EV_A);
  488. return 0;
  489. }
  490. ev_io_init (&linuxaio_epoll_w, linuxaio_epoll_cb, backend_fd, EV_READ);
  491. ev_set_priority (&linuxaio_epoll_w, EV_MAXPRI);
  492. ev_io_start (EV_A_ &linuxaio_epoll_w);
  493. ev_unref (EV_A); /* watcher should not keep loop alive */
  494. backend_modify = linuxaio_modify;
  495. backend_poll = linuxaio_poll;
  496. linuxaio_iocbpmax = 0;
  497. linuxaio_iocbps = 0;
  498. linuxaio_submits = 0;
  499. linuxaio_submitmax = 0;
  500. linuxaio_submitcnt = 0;
  501. return EVBACKEND_LINUXAIO;
  502. }
  503. inline_size
  504. void
  505. linuxaio_destroy (EV_P)
  506. {
  507. epoll_destroy (EV_A);
  508. linuxaio_free_iocbp (EV_A);
  509. evsys_io_destroy (linuxaio_ctx); /* fails in child, aio context is destroyed */
  510. }
  511. ecb_cold
  512. static void
  513. linuxaio_fork (EV_P)
  514. {
  515. linuxaio_submitcnt = 0; /* all pointers were invalidated */
  516. linuxaio_free_iocbp (EV_A); /* this frees all iocbs, which is very heavy-handed */
  517. evsys_io_destroy (linuxaio_ctx); /* fails in child, aio context is destroyed */
  518. linuxaio_iteration = 0; /* we start over in the child */
  519. while (linuxaio_io_setup (EV_A) < 0)
  520. ev_syserr ("(libev) linuxaio io_setup");
  521. /* forking epoll should also effectively unregister all fds from the backend */
  522. epoll_fork (EV_A);
  523. /* epoll_fork already did this. hopefully */
  524. /*fd_rearm_all (EV_A);*/
  525. ev_io_stop (EV_A_ &linuxaio_epoll_w);
  526. ev_io_set (EV_A_ &linuxaio_epoll_w, backend_fd, EV_READ);
  527. ev_io_start (EV_A_ &linuxaio_epoll_w);
  528. }