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Revision: 1.4
Committed: Wed Aug 28 10:05:23 2019 UTC (4 years, 10 months ago) by root
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Branch: MAIN
Changes since 1.3: +1 -1 lines
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# User Rev Content
1 root 1.1 /*
2     * libev linux io_uring 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     /*
41     * general notes about linux io_uring:
42     *
43     * a) it's the best interface I have seen so far. on linux.
44     * b) best is not necessarily very good.
45     * c) it's better than the aio mess, doesn't suffer from the fork problems
46     * of linux aio or epoll and so on and so on. and you could do event stuff
47     * without any syscalls. what's not to like?
48     * d) ok, it's vastly more complex, but that's ok, really.
49     * e) why 3 mmaps instead of one? one would be more space-efficient,
50     * and I can't see what benefit three would have (other than being
51     * somehow resizable/relocatable, but that's apparently not possible).
52     * f) hmm, it's practiclaly undebuggable (gdb can't access the memory, and
53     the bizarre way structure offsets are commuinicated makes it hard to
54     * just print the ring buffer heads, even *iff* the memory were visible
55     * in gdb. but then, that's also ok, really.
56     * g) well, you cannot specify a timeout when waiting for events. no,
57     * seriously, the interface doesn't support a timeout. never seen _that_
58     * before. sure, you can use a timerfd, but that's another syscall
59     * you could have avoided. overall, this bizarre omission smells
60     * like a ยต-optimisation by the io_uring author for his personal
61     * applications, to the detriment of everybody else who just wants
62     * an event loop. but, umm, ok, if that's all, it could be worse.
63     * h) there is a hardcoded limit of 4096 outstanding events. okay,
64     * at least there is no arbitrary low system-wide limit...
65     * i) unlike linux aio, you *can* register more then the limit
66     * of fd events, and the kernel will "gracefully" signal an
67     * overflow, after which you could destroy and recreate the kernel
68     * state, a bit bigger, or fall back to e.g. poll. thats not
69     * totally insane, but kind of questions the point a high
70     * performance I/O framework when it doesn't really work
71     * under stress.
72     * j) but, oh my! is has exactly the same bugs as the linux aio backend,
73     * where some undocumented poll combinations just fail.
74     * so we need epoll AGAIN as a fallback. AGAIN! epoll!! and of course,
75     * this is completely undocumented, have I mantioned this already?
76     * k) overall, the *API* itself is, I dare to say, not a total trainwreck.
77     * the big isuess with it are the bugs requiring epoll, which might
78     * or might not get fixed (do I hold my breath?).
79     */
80    
81     #include <sys/timerfd.h>
82     #include <sys/mman.h>
83     #include <poll.h>
84    
85     #define IOURING_INIT_ENTRIES 32
86    
87     /*****************************************************************************/
88     /* syscall wrapdadoop - this section has the raw api/abi definitions */
89    
90     #include <linux/fs.h>
91     #include <linux/types.h>
92    
93     /* mostly directly taken from the kernel or documentation */
94    
95     struct io_uring_sqe
96     {
97     __u8 opcode;
98     __u8 flags;
99     __u16 ioprio;
100     __s32 fd;
101     __u64 off;
102     __u64 addr;
103     __u32 len;
104     union {
105     __kernel_rwf_t rw_flags;
106     __u32 fsync_flags;
107     __u16 poll_events;
108     __u32 sync_range_flags;
109     __u32 msg_flags;
110     };
111     __u64 user_data;
112     union {
113     __u16 buf_index;
114     __u64 __pad2[3];
115     };
116     };
117    
118     struct io_uring_cqe
119     {
120     __u64 user_data;
121     __s32 res;
122     __u32 flags;
123     };
124    
125     struct io_sqring_offsets
126     {
127     __u32 head;
128     __u32 tail;
129     __u32 ring_mask;
130     __u32 ring_entries;
131     __u32 flags;
132     __u32 dropped;
133     __u32 array;
134     __u32 resv1;
135     __u64 resv2;
136     };
137    
138     struct io_cqring_offsets
139     {
140     __u32 head;
141     __u32 tail;
142     __u32 ring_mask;
143     __u32 ring_entries;
144     __u32 overflow;
145     __u32 cqes;
146     __u64 resv[2];
147     };
148    
149     struct io_uring_params
150     {
151     __u32 sq_entries;
152     __u32 cq_entries;
153     __u32 flags;
154     __u32 sq_thread_cpu;
155     __u32 sq_thread_idle;
156     __u32 resv[5];
157     struct io_sqring_offsets sq_off;
158     struct io_cqring_offsets cq_off;
159     };
160    
161     #define IORING_OP_POLL_ADD 6
162     #define IORING_OP_POLL_REMOVE 7
163    
164     #define IORING_ENTER_GETEVENTS 0x01
165    
166     #define IORING_OFF_SQ_RING 0x00000000ULL
167     #define IORING_OFF_CQ_RING 0x08000000ULL
168     #define IORING_OFF_SQES 0x10000000ULL
169    
170     inline_size
171     int
172     evsys_io_uring_setup (unsigned entries, struct io_uring_params *params)
173     {
174     return ev_syscall2 (SYS_io_uring_setup, entries, params);
175     }
176    
177     inline_size
178     int
179     evsys_io_uring_enter (int fd, unsigned to_submit, unsigned min_complete, unsigned flags, const sigset_t *sig, size_t sigsz)
180     {
181     return ev_syscall6 (SYS_io_uring_enter, fd, to_submit, min_complete, flags, sig, sigsz);
182     }
183    
184     /*****************************************************************************/
185     /* actual backed implementation */
186    
187     /* we hope that volatile will make the compiler access this variables only once */
188     #define EV_SQ_VAR(name) *(volatile unsigned *)((char *)iouring_sq_ring + iouring_sq_ ## name)
189     #define EV_CQ_VAR(name) *(volatile unsigned *)((char *)iouring_cq_ring + iouring_cq_ ## name)
190    
191     /* the index array */
192     #define EV_SQ_ARRAY ((unsigned *)((char *)iouring_sq_ring + iouring_sq_array))
193    
194     /* the submit/completion queue entries */
195     #define EV_SQES ((struct io_uring_sqe *) iouring_sqes)
196     #define EV_CQES ((struct io_uring_cqe *)((char *)iouring_cq_ring + iouring_cq_cqes))
197    
198     static
199     struct io_uring_sqe *
200     iouring_sqe_get (EV_P)
201     {
202     unsigned tail = EV_SQ_VAR (tail);
203    
204     if (tail + 1 - EV_SQ_VAR (head) > EV_SQ_VAR (ring_entries))
205     {
206     /* queue full, flush */
207     evsys_io_uring_enter (iouring_fd, iouring_to_submit, 0, 0, 0, 0);
208     iouring_to_submit = 0;
209     }
210    
211     assert (("libev: io_uring queue full after flush", tail + 1 - EV_SQ_VAR (head) <= EV_SQ_VAR (ring_entries)));
212    
213     return EV_SQES + (tail & EV_SQ_VAR (ring_mask));
214     }
215    
216     inline_size
217     struct io_uring_sqe *
218     iouring_sqe_submit (EV_P_ struct io_uring_sqe *sqe)
219     {
220     unsigned idx = sqe - EV_SQES;
221    
222     EV_SQ_ARRAY [idx] = idx;
223     ECB_MEMORY_FENCE_RELEASE;
224     ++EV_SQ_VAR (tail);
225     /*ECB_MEMORY_FENCE_RELEASE; /* for the time being we assume this is not needed */
226     ++iouring_to_submit;
227     }
228    
229     /*****************************************************************************/
230    
231     /* when the timerfd expires we simply note the fact,
232     * as the purpose of the timerfd is to wake us up, nothing else.
233     * the next iteration should re-set it.
234     */
235     static void
236     iouring_tfd_cb (EV_P_ struct ev_io *w, int revents)
237     {
238     iouring_tfd_to = EV_TSTAMP_HUGE;
239     }
240    
241     static void
242     iouring_epoll_cb (EV_P_ struct ev_io *w, int revents)
243     {
244     epoll_poll (EV_A_ 0);
245     }
246    
247     /* called for full and partial cleanup */
248     ecb_cold
249     static int
250     iouring_internal_destroy (EV_P)
251     {
252     close (iouring_tfd);
253     close (iouring_fd);
254    
255     if (iouring_sq_ring != MAP_FAILED) munmap (iouring_sq_ring, iouring_sq_ring_size);
256     if (iouring_cq_ring != MAP_FAILED) munmap (iouring_cq_ring, iouring_cq_ring_size);
257     if (iouring_sqes != MAP_FAILED) munmap (iouring_sqes , iouring_sqes_size );
258    
259     if (ev_is_active (&iouring_epoll_w)) ev_ref (EV_A); ev_io_stop (EV_A_ &iouring_epoll_w);
260     if (ev_is_active (&iouring_tfd_w )) ev_ref (EV_A); ev_io_stop (EV_A_ &iouring_tfd_w );
261     }
262    
263     ecb_cold
264     static int
265     iouring_internal_init (EV_P)
266     {
267     struct io_uring_params params = { 0 };
268    
269     iouring_to_submit = 0;
270    
271     iouring_tfd = -1;
272     iouring_sq_ring = MAP_FAILED;
273     iouring_cq_ring = MAP_FAILED;
274     iouring_sqes = MAP_FAILED;
275    
276     for (;;)
277     {
278     iouring_fd = evsys_io_uring_setup (iouring_entries, &params);
279    
280     if (iouring_fd >= 0)
281     break; /* yippie */
282    
283     if (errno != EINVAL)
284     return -1; /* we failed */
285    
286     /* EINVAL: lots of possible reasons, but maybe
287     * it is because we hit the unqueryable hardcoded size limit
288     */
289    
290     /* we hit the limit already, give up */
291     if (iouring_max_entries)
292     return -1;
293    
294     /* first time we hit EINVAL? assume we hit the limit, so go back and retry */
295     iouring_entries >>= 1;
296     iouring_max_entries = iouring_entries;
297     }
298    
299     iouring_sq_ring_size = params.sq_off.array + params.sq_entries * sizeof (unsigned);
300     iouring_cq_ring_size = params.cq_off.cqes + params.cq_entries * sizeof (struct io_uring_cqe);
301     iouring_sqes_size = params.sq_entries * sizeof (struct io_uring_sqe);
302    
303     iouring_sq_ring = mmap (0, iouring_sq_ring_size, PROT_READ | PROT_WRITE,
304     MAP_SHARED | MAP_POPULATE, iouring_fd, IORING_OFF_SQ_RING);
305     iouring_cq_ring = mmap (0, iouring_cq_ring_size, PROT_READ | PROT_WRITE,
306     MAP_SHARED | MAP_POPULATE, iouring_fd, IORING_OFF_CQ_RING);
307     iouring_sqes = mmap (0, iouring_sqes_size, PROT_READ | PROT_WRITE,
308     MAP_SHARED | MAP_POPULATE, iouring_fd, IORING_OFF_SQES);
309    
310     if (iouring_sq_ring == MAP_FAILED || iouring_cq_ring == MAP_FAILED || iouring_sqes == MAP_FAILED)
311     return -1;
312    
313     iouring_sq_head = params.sq_off.head;
314     iouring_sq_tail = params.sq_off.tail;
315     iouring_sq_ring_mask = params.sq_off.ring_mask;
316     iouring_sq_ring_entries = params.sq_off.ring_entries;
317     iouring_sq_flags = params.sq_off.flags;
318     iouring_sq_dropped = params.sq_off.dropped;
319     iouring_sq_array = params.sq_off.array;
320    
321     iouring_cq_head = params.cq_off.head;
322     iouring_cq_tail = params.cq_off.tail;
323     iouring_cq_ring_mask = params.cq_off.ring_mask;
324     iouring_cq_ring_entries = params.cq_off.ring_entries;
325     iouring_cq_overflow = params.cq_off.overflow;
326     iouring_cq_cqes = params.cq_off.cqes;
327    
328     iouring_tfd = timerfd_create (CLOCK_MONOTONIC, TFD_CLOEXEC);
329    
330     if (iouring_tfd < 0)
331     return iouring_tfd;
332    
333     iouring_tfd_to = EV_TSTAMP_HUGE;
334    
335     return 0;
336     }
337    
338     ecb_cold
339     static void
340     iouring_fork (EV_P)
341     {
342     iouring_internal_destroy (EV_A);
343    
344     while (iouring_internal_init (EV_A) < 0)
345     ev_syserr ("(libev) io_uring_setup");
346    
347     /* forking epoll should also effectively unregister all fds from the backend */
348     epoll_fork (EV_A);
349     /* epoll_fork already did this. hopefully */
350     /*fd_rearm_all (EV_A);*/
351    
352     ev_io_stop (EV_A_ &iouring_epoll_w);
353     ev_io_set (EV_A_ &iouring_epoll_w, backend_fd, EV_READ);
354     ev_io_start (EV_A_ &iouring_epoll_w);
355    
356     ev_io_stop (EV_A_ &iouring_tfd_w);
357     ev_io_set (EV_A_ &iouring_tfd_w, iouring_tfd, EV_READ);
358     ev_io_start (EV_A_ &iouring_tfd_w);
359     }
360    
361     /*****************************************************************************/
362    
363     static void
364     iouring_modify (EV_P_ int fd, int oev, int nev)
365     {
366     fprintf (stderr,"modify %d (%d, %d) %d\n", fd, oev,nev, anfds[fd].eflags);//D
367     if (ecb_expect_false (anfds [fd].eflags))
368     {
369     /* we handed this fd over to epoll, so undo this first */
370     /* we do it manually because the optimisations on epoll_modify won't do us any good */
371     epoll_ctl (iouring_fd, EPOLL_CTL_DEL, fd, 0);
372     anfds [fd].eflags = 0;
373     oev = 0;
374     }
375    
376     if (oev)
377     {
378     /* we assume the sqe's are all "properly" initialised */
379     struct io_uring_sqe *sqe = iouring_sqe_get (EV_A);
380     sqe->opcode = IORING_OP_POLL_REMOVE;
381     sqe->fd = fd;
382     sqe->user_data = -1;
383     iouring_sqe_submit (EV_A_ sqe);
384 root 1.2
385     /* increment generation counter to avoid handling old events */
386     ++anfds [fd].egen;
387 root 1.1 }
388    
389     if (nev)
390     {
391     struct io_uring_sqe *sqe = iouring_sqe_get (EV_A);
392     sqe->opcode = IORING_OP_POLL_ADD;
393     sqe->fd = fd;
394 root 1.2 sqe->user_data = (uint32_t)fd | ((__u64)(uint32_t)anfds [fd].egen << 32);
395 root 1.1 sqe->poll_events =
396     (nev & EV_READ ? POLLIN : 0)
397     | (nev & EV_WRITE ? POLLOUT : 0);
398     iouring_sqe_submit (EV_A_ sqe);
399     }
400     }
401    
402     inline_size
403     void
404     iouring_tfd_update (EV_P_ ev_tstamp timeout)
405     {
406     ev_tstamp tfd_to = mn_now + timeout;
407    
408     /* we assume there will be many iterations per timer change, so
409     * we only re-set the timerfd when we have to because its expiry
410     * is too late.
411     */
412     if (ecb_expect_false (tfd_to < iouring_tfd_to))
413     {
414     struct itimerspec its;
415    
416     iouring_tfd_to = tfd_to;
417     EV_TS_SET (its.it_interval, 0.);
418     EV_TS_SET (its.it_value, tfd_to);
419    
420     if (timerfd_settime (iouring_tfd, TFD_TIMER_ABSTIME, &its, 0) < 0)
421     assert (("libev: iouring timerfd_settime failed", 0));
422     }
423     }
424    
425     inline_size
426     void
427     iouring_process_cqe (EV_P_ struct io_uring_cqe *cqe)
428     {
429     int fd = cqe->user_data & 0xffffffffU;
430     uint32_t gen = cqe->user_data >> 32;
431     int res = cqe->res;
432    
433     /* ignore fd removal events, if there are any. TODO: verify */
434     if (cqe->user_data == (__u64)-1)
435     abort ();//D
436    
437     assert (("libev: io_uring fd must be in-bounds", fd >= 0 && fd < anfdmax));
438    
439     /* documentation lies, of course. the result value is NOT like
440     * normal syscalls, but like linux raw syscalls, i.e. negative
441     * error numbers. fortunate, as otherwise there would be no way
442     * to get error codes at all. still, why not document this?
443     */
444    
445     /* ignore event if generation doesn't match */
446     /* this should actually be very rare */
447 root 1.2 if (ecb_expect_false (gen != (uint32_t)anfds [fd].egen))
448 root 1.1 return;
449    
450     if (ecb_expect_false (res < 0))
451     {
452     if (res == -EINVAL)
453     {
454     /* we assume this error code means the fd/poll combination is buggy
455     * and fall back to epoll.
456     * this error code might also indicate a bug, but the kernel doesn't
457     * distinguish between those two conditions, so... sigh...
458     */
459    
460     epoll_modify (EV_A_ fd, 0, anfds [fd].events);
461     }
462     else if (res == -EBADF)
463     {
464     assert (("libev: event loop rejected bad fd", res != -EBADF));
465     fd_kill (EV_A_ fd);
466     }
467     else
468     {
469     errno = -res;
470     ev_syserr ("(libev) IORING_OP_POLL_ADD");
471     }
472    
473     return;
474     }
475    
476     fprintf (stderr, "fd %d event, rearm\n", fd);//D
477    
478     /* feed events, we do not expect or handle POLLNVAL */
479     fd_event (
480     EV_A_
481     fd,
482     (res & (POLLOUT | POLLERR | POLLHUP) ? EV_WRITE : 0)
483     | (res & (POLLIN | POLLERR | POLLHUP) ? EV_READ : 0)
484     );
485    
486     /* io_uring is oneshot, so we need to re-arm the fd next iteration */
487     /* this also means we usually have to do at least one syscall per iteration */
488     anfds [fd].events = 0;
489     fd_change (EV_A_ fd, EV_ANFD_REIFY);
490     }
491    
492     /* called when the event queue overflows */
493     ecb_cold
494     static void
495     iouring_overflow (EV_P)
496     {
497     /* we have two options, resize the queue (by tearing down
498     * everything and recreating it, or living with it
499     * and polling.
500     * we implement this by resizing tghe queue, and, if that fails,
501     * we just recreate the state on every failure, which
502     * kind of is a very inefficient poll.
503     * one danger is, due to the bios toward lower fds,
504     * we will only really get events for those, so
505     * maybe we need a poll() fallback, after all.
506     */
507     /*EV_CQ_VAR (overflow) = 0;*/ /* need to do this if we keep the state and poll manually */
508    
509     fd_rearm_all (EV_A);
510    
511     /* we double the size until we hit the hard-to-probe maximum */
512     if (!iouring_max_entries)
513     {
514     iouring_entries <<= 1;
515     iouring_fork (EV_A);
516     }
517     else
518     {
519     /* we hit the kernel limit, we should fall back to something else.
520     * we can either poll() a few times and hope for the best,
521     * poll always, or switch to epoll.
522     * since we use epoll anyways, go epoll.
523     */
524    
525     iouring_internal_destroy (EV_A);
526    
527     /* this should make it so that on return, we don'T call any uring functions */
528     iouring_to_submit = 0;
529    
530     for (;;)
531     {
532     backend = epoll_init (EV_A_ 0);
533    
534     if (backend)
535     break;
536    
537     ev_syserr ("(libev) iouring switch to epoll");
538     }
539     }
540     }
541    
542     /* handle any events in the completion queue, return true if there were any */
543     static int
544     iouring_handle_cq (EV_P)
545     {
546     unsigned head, tail, mask;
547    
548     head = EV_CQ_VAR (head);
549     ECB_MEMORY_FENCE_ACQUIRE;
550     tail = EV_CQ_VAR (tail);
551    
552     if (head == tail)
553     return 0;
554    
555     /* it can only overflow if we have events, yes, yes? */
556     if (ecb_expect_false (EV_CQ_VAR (overflow)))
557     {
558     iouring_overflow (EV_A);
559     return 1;
560     }
561    
562     mask = EV_CQ_VAR (ring_mask);
563    
564     do
565     iouring_process_cqe (EV_A_ &EV_CQES [head++ & mask]);
566     while (head != tail);
567    
568     EV_CQ_VAR (head) = head;
569     ECB_MEMORY_FENCE_RELEASE;
570    
571     return 1;
572     }
573    
574     static void
575     iouring_poll (EV_P_ ev_tstamp timeout)
576     {
577     /* if we have events, no need for extra syscalls, but we might have to queue events */
578     if (iouring_handle_cq (EV_A))
579 root 1.3 timeout = EV_TS_CONST (0.);
580 root 1.1 else
581     /* no events, so maybe wait for some */
582     iouring_tfd_update (EV_A_ timeout);
583    
584     /* only enter the kernel if we have somethign to submit, or we need to wait */
585     if (timeout || iouring_to_submit)
586     {
587     int res;
588    
589     EV_RELEASE_CB;
590    
591     res = evsys_io_uring_enter (iouring_fd, iouring_to_submit, 1,
592 root 1.3 timeout > EV_TS_CONST (0.) ? IORING_ENTER_GETEVENTS : 0, 0, 0);
593 root 1.1 iouring_to_submit = 0;
594    
595     EV_ACQUIRE_CB;
596    
597     if (ecb_expect_false (res < 0))
598     if (errno == EINTR)
599     /* ignore */;
600     else
601     ev_syserr ("(libev) iouring setup");
602     else
603     iouring_handle_cq (EV_A);
604     }
605     }
606    
607     inline_size
608     int
609     iouring_init (EV_P_ int flags)
610     {
611     if (!epoll_init (EV_A_ 0))
612     return 0;
613    
614 root 1.4 ev_io_init (&iouring_epoll_w, iouring_epoll_cb, backend_fd, EV_READ);
615 root 1.1 ev_set_priority (&iouring_epoll_w, EV_MAXPRI);
616    
617     ev_io_init (&iouring_tfd_w, iouring_tfd_cb, iouring_tfd, EV_READ);
618     ev_set_priority (&iouring_tfd_w, EV_MAXPRI);
619    
620     iouring_entries = IOURING_INIT_ENTRIES;
621     iouring_max_entries = 0;
622    
623     if (iouring_internal_init (EV_A) < 0)
624     {
625     iouring_internal_destroy (EV_A);
626     return 0;
627     }
628    
629     ev_io_start (EV_A_ &iouring_epoll_w);
630     ev_unref (EV_A); /* watcher should not keep loop alive */
631    
632     ev_io_start (EV_A_ &iouring_tfd_w);
633     ev_unref (EV_A); /* watcher should not keep loop alive */
634    
635     backend_modify = iouring_modify;
636     backend_poll = iouring_poll;
637    
638     return EVBACKEND_IOURING;
639     }
640    
641     inline_size
642     void
643     iouring_destroy (EV_P)
644     {
645     iouring_internal_destroy (EV_A);
646     epoll_destroy (EV_A);
647     }
648