ViewVC Help
View File | Revision Log | Show Annotations | Download File
/cvs/libev/ev_iouring.c
Revision: 1.11
Committed: Fri Dec 27 21:56:29 2019 UTC (4 years, 6 months ago) by root
Content type: text/plain
Branch: MAIN
Changes since 1.10: +7 -2 lines
Log Message:
*** empty log message ***

File Contents

# 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 root 1.7 * (FIXME: newer kernels can use 2 mmaps only, need to look into this).
53 root 1.1 * f) hmm, it's practiclaly undebuggable (gdb can't access the memory, and
54 root 1.7 * the bizarre way structure offsets are communicated makes it hard to
55 root 1.1 * just print the ring buffer heads, even *iff* the memory were visible
56     * in gdb. but then, that's also ok, really.
57     * g) well, you cannot specify a timeout when waiting for events. no,
58     * seriously, the interface doesn't support a timeout. never seen _that_
59     * before. sure, you can use a timerfd, but that's another syscall
60     * you could have avoided. overall, this bizarre omission smells
61     * like a ยต-optimisation by the io_uring author for his personal
62     * applications, to the detriment of everybody else who just wants
63     * an event loop. but, umm, ok, if that's all, it could be worse.
64 root 1.7 * (FIXME: jens mentioned timeout commands, need to investigate)
65 root 1.1 * h) there is a hardcoded limit of 4096 outstanding events. okay,
66     * at least there is no arbitrary low system-wide limit...
67 root 1.7 * (FIXME: apparently, this was increased to 32768 in later kernels(
68 root 1.1 * i) unlike linux aio, you *can* register more then the limit
69     * of fd events, and the kernel will "gracefully" signal an
70     * overflow, after which you could destroy and recreate the kernel
71     * state, a bit bigger, or fall back to e.g. poll. thats not
72     * totally insane, but kind of questions the point a high
73     * performance I/O framework when it doesn't really work
74     * under stress.
75 root 1.7 * (FIXME: iouring should no longer drop events, need to investigate)
76 root 1.1 * j) but, oh my! is has exactly the same bugs as the linux aio backend,
77     * where some undocumented poll combinations just fail.
78     * so we need epoll AGAIN as a fallback. AGAIN! epoll!! and of course,
79     * this is completely undocumented, have I mantioned this already?
80     * k) overall, the *API* itself is, I dare to say, not a total trainwreck.
81     * the big isuess with it are the bugs requiring epoll, which might
82     * or might not get fixed (do I hold my breath?).
83     */
84    
85 root 1.9 /* TODO: use internal TIMEOUT */
86     /* TODO: take advantage of single mmap, NODROP etc. */
87     /* TODO: resize cq/sq size independently */
88    
89 root 1.1 #include <sys/timerfd.h>
90     #include <sys/mman.h>
91     #include <poll.h>
92    
93     #define IOURING_INIT_ENTRIES 32
94    
95     /*****************************************************************************/
96     /* syscall wrapdadoop - this section has the raw api/abi definitions */
97    
98     #include <linux/fs.h>
99     #include <linux/types.h>
100    
101     /* mostly directly taken from the kernel or documentation */
102    
103     struct io_uring_sqe
104     {
105     __u8 opcode;
106     __u8 flags;
107     __u16 ioprio;
108     __s32 fd;
109 root 1.8 union {
110     __u64 off;
111     __u64 addr2;
112     };
113 root 1.1 __u64 addr;
114     __u32 len;
115     union {
116     __kernel_rwf_t rw_flags;
117     __u32 fsync_flags;
118     __u16 poll_events;
119     __u32 sync_range_flags;
120     __u32 msg_flags;
121 root 1.8 __u32 timeout_flags;
122     __u32 accept_flags;
123     __u32 cancel_flags;
124     __u32 open_flags;
125     __u32 statx_flags;
126 root 1.1 };
127     __u64 user_data;
128     union {
129     __u16 buf_index;
130     __u64 __pad2[3];
131     };
132     };
133    
134     struct io_uring_cqe
135     {
136     __u64 user_data;
137     __s32 res;
138     __u32 flags;
139     };
140    
141     struct io_sqring_offsets
142     {
143     __u32 head;
144     __u32 tail;
145     __u32 ring_mask;
146     __u32 ring_entries;
147     __u32 flags;
148     __u32 dropped;
149     __u32 array;
150     __u32 resv1;
151     __u64 resv2;
152     };
153    
154     struct io_cqring_offsets
155     {
156     __u32 head;
157     __u32 tail;
158     __u32 ring_mask;
159     __u32 ring_entries;
160     __u32 overflow;
161     __u32 cqes;
162     __u64 resv[2];
163     };
164    
165     struct io_uring_params
166     {
167     __u32 sq_entries;
168     __u32 cq_entries;
169     __u32 flags;
170     __u32 sq_thread_cpu;
171     __u32 sq_thread_idle;
172 root 1.8 __u32 features;
173     __u32 resv[4];
174 root 1.1 struct io_sqring_offsets sq_off;
175     struct io_cqring_offsets cq_off;
176     };
177    
178     #define IORING_OP_POLL_ADD 6
179     #define IORING_OP_POLL_REMOVE 7
180    
181     #define IORING_ENTER_GETEVENTS 0x01
182    
183     #define IORING_OFF_SQ_RING 0x00000000ULL
184     #define IORING_OFF_CQ_RING 0x08000000ULL
185     #define IORING_OFF_SQES 0x10000000ULL
186    
187 root 1.8 #define IORING_FEAT_SINGLE_MMAP 0x1
188     #define IORING_FEAT_NODROP 0x2
189     #define IORING_FEAT_SUBMIT_STABLE 0x4
190    
191 root 1.1 inline_size
192     int
193     evsys_io_uring_setup (unsigned entries, struct io_uring_params *params)
194     {
195     return ev_syscall2 (SYS_io_uring_setup, entries, params);
196     }
197    
198     inline_size
199     int
200     evsys_io_uring_enter (int fd, unsigned to_submit, unsigned min_complete, unsigned flags, const sigset_t *sig, size_t sigsz)
201     {
202     return ev_syscall6 (SYS_io_uring_enter, fd, to_submit, min_complete, flags, sig, sigsz);
203     }
204    
205     /*****************************************************************************/
206     /* actual backed implementation */
207    
208     /* we hope that volatile will make the compiler access this variables only once */
209     #define EV_SQ_VAR(name) *(volatile unsigned *)((char *)iouring_sq_ring + iouring_sq_ ## name)
210     #define EV_CQ_VAR(name) *(volatile unsigned *)((char *)iouring_cq_ring + iouring_cq_ ## name)
211    
212     /* the index array */
213     #define EV_SQ_ARRAY ((unsigned *)((char *)iouring_sq_ring + iouring_sq_array))
214    
215     /* the submit/completion queue entries */
216     #define EV_SQES ((struct io_uring_sqe *) iouring_sqes)
217     #define EV_CQES ((struct io_uring_cqe *)((char *)iouring_cq_ring + iouring_cq_cqes))
218    
219     static
220     struct io_uring_sqe *
221     iouring_sqe_get (EV_P)
222     {
223     unsigned tail = EV_SQ_VAR (tail);
224    
225     if (tail + 1 - EV_SQ_VAR (head) > EV_SQ_VAR (ring_entries))
226     {
227     /* queue full, flush */
228     evsys_io_uring_enter (iouring_fd, iouring_to_submit, 0, 0, 0, 0);
229     iouring_to_submit = 0;
230     }
231    
232     assert (("libev: io_uring queue full after flush", tail + 1 - EV_SQ_VAR (head) <= EV_SQ_VAR (ring_entries)));
233    
234     return EV_SQES + (tail & EV_SQ_VAR (ring_mask));
235     }
236    
237     inline_size
238     struct io_uring_sqe *
239     iouring_sqe_submit (EV_P_ struct io_uring_sqe *sqe)
240     {
241     unsigned idx = sqe - EV_SQES;
242    
243     EV_SQ_ARRAY [idx] = idx;
244     ECB_MEMORY_FENCE_RELEASE;
245     ++EV_SQ_VAR (tail);
246     /*ECB_MEMORY_FENCE_RELEASE; /* for the time being we assume this is not needed */
247     ++iouring_to_submit;
248     }
249    
250     /*****************************************************************************/
251    
252     /* when the timerfd expires we simply note the fact,
253     * as the purpose of the timerfd is to wake us up, nothing else.
254     * the next iteration should re-set it.
255     */
256     static void
257     iouring_tfd_cb (EV_P_ struct ev_io *w, int revents)
258     {
259     iouring_tfd_to = EV_TSTAMP_HUGE;
260     }
261    
262     /* called for full and partial cleanup */
263     ecb_cold
264     static int
265     iouring_internal_destroy (EV_P)
266     {
267     close (iouring_tfd);
268     close (iouring_fd);
269    
270     if (iouring_sq_ring != MAP_FAILED) munmap (iouring_sq_ring, iouring_sq_ring_size);
271     if (iouring_cq_ring != MAP_FAILED) munmap (iouring_cq_ring, iouring_cq_ring_size);
272     if (iouring_sqes != MAP_FAILED) munmap (iouring_sqes , iouring_sqes_size );
273    
274 root 1.8 if (ev_is_active (&iouring_tfd_w))
275     {
276     ev_ref (EV_A);
277     ev_io_stop (EV_A_ &iouring_tfd_w);
278     }
279 root 1.1 }
280    
281     ecb_cold
282     static int
283     iouring_internal_init (EV_P)
284     {
285     struct io_uring_params params = { 0 };
286    
287     iouring_to_submit = 0;
288    
289     iouring_tfd = -1;
290     iouring_sq_ring = MAP_FAILED;
291     iouring_cq_ring = MAP_FAILED;
292     iouring_sqes = MAP_FAILED;
293    
294     for (;;)
295     {
296     iouring_fd = evsys_io_uring_setup (iouring_entries, &params);
297    
298     if (iouring_fd >= 0)
299     break; /* yippie */
300    
301     if (errno != EINVAL)
302     return -1; /* we failed */
303    
304 root 1.11 #if TODO
305     if ((~params.features) & (IORING_FEAT_NODROP | IORING_FEATURE_SINGLE_MMAP))
306     return -1; /* we require the above features */
307     #endif
308    
309 root 1.1 /* EINVAL: lots of possible reasons, but maybe
310     * it is because we hit the unqueryable hardcoded size limit
311     */
312    
313     /* we hit the limit already, give up */
314     if (iouring_max_entries)
315     return -1;
316    
317     /* first time we hit EINVAL? assume we hit the limit, so go back and retry */
318     iouring_entries >>= 1;
319     iouring_max_entries = iouring_entries;
320     }
321    
322     iouring_sq_ring_size = params.sq_off.array + params.sq_entries * sizeof (unsigned);
323     iouring_cq_ring_size = params.cq_off.cqes + params.cq_entries * sizeof (struct io_uring_cqe);
324     iouring_sqes_size = params.sq_entries * sizeof (struct io_uring_sqe);
325    
326     iouring_sq_ring = mmap (0, iouring_sq_ring_size, PROT_READ | PROT_WRITE,
327     MAP_SHARED | MAP_POPULATE, iouring_fd, IORING_OFF_SQ_RING);
328     iouring_cq_ring = mmap (0, iouring_cq_ring_size, PROT_READ | PROT_WRITE,
329     MAP_SHARED | MAP_POPULATE, iouring_fd, IORING_OFF_CQ_RING);
330     iouring_sqes = mmap (0, iouring_sqes_size, PROT_READ | PROT_WRITE,
331     MAP_SHARED | MAP_POPULATE, iouring_fd, IORING_OFF_SQES);
332    
333     if (iouring_sq_ring == MAP_FAILED || iouring_cq_ring == MAP_FAILED || iouring_sqes == MAP_FAILED)
334     return -1;
335    
336     iouring_sq_head = params.sq_off.head;
337     iouring_sq_tail = params.sq_off.tail;
338     iouring_sq_ring_mask = params.sq_off.ring_mask;
339     iouring_sq_ring_entries = params.sq_off.ring_entries;
340     iouring_sq_flags = params.sq_off.flags;
341     iouring_sq_dropped = params.sq_off.dropped;
342     iouring_sq_array = params.sq_off.array;
343    
344     iouring_cq_head = params.cq_off.head;
345     iouring_cq_tail = params.cq_off.tail;
346     iouring_cq_ring_mask = params.cq_off.ring_mask;
347     iouring_cq_ring_entries = params.cq_off.ring_entries;
348     iouring_cq_overflow = params.cq_off.overflow;
349     iouring_cq_cqes = params.cq_off.cqes;
350    
351     iouring_tfd = timerfd_create (CLOCK_MONOTONIC, TFD_CLOEXEC);
352    
353     if (iouring_tfd < 0)
354     return iouring_tfd;
355    
356     iouring_tfd_to = EV_TSTAMP_HUGE;
357    
358     return 0;
359     }
360    
361     ecb_cold
362     static void
363     iouring_fork (EV_P)
364     {
365     iouring_internal_destroy (EV_A);
366    
367     while (iouring_internal_init (EV_A) < 0)
368     ev_syserr ("(libev) io_uring_setup");
369    
370 root 1.8 fd_rearm_all (EV_A);
371 root 1.1
372     ev_io_stop (EV_A_ &iouring_tfd_w);
373     ev_io_set (EV_A_ &iouring_tfd_w, iouring_tfd, EV_READ);
374     ev_io_start (EV_A_ &iouring_tfd_w);
375     }
376    
377     /*****************************************************************************/
378    
379     static void
380     iouring_modify (EV_P_ int fd, int oev, int nev)
381     {
382     if (oev)
383     {
384     /* we assume the sqe's are all "properly" initialised */
385     struct io_uring_sqe *sqe = iouring_sqe_get (EV_A);
386     sqe->opcode = IORING_OP_POLL_REMOVE;
387     sqe->fd = fd;
388     sqe->user_data = -1;
389     iouring_sqe_submit (EV_A_ sqe);
390 root 1.2
391     /* increment generation counter to avoid handling old events */
392     ++anfds [fd].egen;
393 root 1.1 }
394    
395     if (nev)
396     {
397     struct io_uring_sqe *sqe = iouring_sqe_get (EV_A);
398     sqe->opcode = IORING_OP_POLL_ADD;
399     sqe->fd = fd;
400 root 1.2 sqe->user_data = (uint32_t)fd | ((__u64)(uint32_t)anfds [fd].egen << 32);
401 root 1.1 sqe->poll_events =
402     (nev & EV_READ ? POLLIN : 0)
403     | (nev & EV_WRITE ? POLLOUT : 0);
404     iouring_sqe_submit (EV_A_ sqe);
405     }
406     }
407    
408     inline_size
409     void
410     iouring_tfd_update (EV_P_ ev_tstamp timeout)
411     {
412     ev_tstamp tfd_to = mn_now + timeout;
413    
414     /* we assume there will be many iterations per timer change, so
415     * we only re-set the timerfd when we have to because its expiry
416     * is too late.
417     */
418     if (ecb_expect_false (tfd_to < iouring_tfd_to))
419     {
420     struct itimerspec its;
421    
422     iouring_tfd_to = tfd_to;
423     EV_TS_SET (its.it_interval, 0.);
424     EV_TS_SET (its.it_value, tfd_to);
425    
426     if (timerfd_settime (iouring_tfd, TFD_TIMER_ABSTIME, &its, 0) < 0)
427     assert (("libev: iouring timerfd_settime failed", 0));
428     }
429     }
430    
431     inline_size
432     void
433     iouring_process_cqe (EV_P_ struct io_uring_cqe *cqe)
434     {
435     int fd = cqe->user_data & 0xffffffffU;
436     uint32_t gen = cqe->user_data >> 32;
437     int res = cqe->res;
438    
439     /* ignore fd removal events, if there are any. TODO: verify */
440 root 1.10 /* TODO: yes, this triggers */
441 root 1.1 if (cqe->user_data == (__u64)-1)
442 root 1.10 return;
443 root 1.1
444     assert (("libev: io_uring fd must be in-bounds", fd >= 0 && fd < anfdmax));
445    
446     /* documentation lies, of course. the result value is NOT like
447     * normal syscalls, but like linux raw syscalls, i.e. negative
448     * error numbers. fortunate, as otherwise there would be no way
449     * to get error codes at all. still, why not document this?
450     */
451    
452     /* ignore event if generation doesn't match */
453     /* this should actually be very rare */
454 root 1.2 if (ecb_expect_false (gen != (uint32_t)anfds [fd].egen))
455 root 1.1 return;
456    
457     if (ecb_expect_false (res < 0))
458     {
459 root 1.11 /*TODO: EINVAL handling (was something failed with this fd)*/
460     /*TODO: EBUSY happens when?*/
461 root 1.1
462 root 1.8 if (res == -EBADF)
463 root 1.1 {
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     /* feed events, we do not expect or handle POLLNVAL */
477     fd_event (
478     EV_A_
479     fd,
480     (res & (POLLOUT | POLLERR | POLLHUP) ? EV_WRITE : 0)
481     | (res & (POLLIN | POLLERR | POLLHUP) ? EV_READ : 0)
482     );
483    
484     /* io_uring is oneshot, so we need to re-arm the fd next iteration */
485     /* this also means we usually have to do at least one syscall per iteration */
486     anfds [fd].events = 0;
487     fd_change (EV_A_ fd, EV_ANFD_REIFY);
488     }
489    
490     /* called when the event queue overflows */
491     ecb_cold
492     static void
493     iouring_overflow (EV_P)
494     {
495     /* we have two options, resize the queue (by tearing down
496     * everything and recreating it, or living with it
497     * and polling.
498 root 1.10 * we implement this by resizing the queue, and, if that fails,
499 root 1.1 * we just recreate the state on every failure, which
500     * kind of is a very inefficient poll.
501     * one danger is, due to the bios toward lower fds,
502     * we will only really get events for those, so
503     * maybe we need a poll() fallback, after all.
504     */
505     /*EV_CQ_VAR (overflow) = 0;*/ /* need to do this if we keep the state and poll manually */
506    
507     fd_rearm_all (EV_A);
508    
509     /* we double the size until we hit the hard-to-probe maximum */
510     if (!iouring_max_entries)
511     {
512     iouring_entries <<= 1;
513     iouring_fork (EV_A);
514     }
515     else
516     {
517     /* we hit the kernel limit, we should fall back to something else.
518     * we can either poll() a few times and hope for the best,
519     * poll always, or switch to epoll.
520 root 1.10 * TODO: is this necessary with newer kernels?
521 root 1.1 */
522    
523     iouring_internal_destroy (EV_A);
524    
525 root 1.10 /* this should make it so that on return, we don't call any uring functions */
526 root 1.1 iouring_to_submit = 0;
527    
528     for (;;)
529     {
530     backend = epoll_init (EV_A_ 0);
531    
532     if (backend)
533     break;
534    
535     ev_syserr ("(libev) iouring switch to epoll");
536     }
537     }
538     }
539    
540     /* handle any events in the completion queue, return true if there were any */
541     static int
542     iouring_handle_cq (EV_P)
543     {
544     unsigned head, tail, mask;
545    
546     head = EV_CQ_VAR (head);
547     ECB_MEMORY_FENCE_ACQUIRE;
548     tail = EV_CQ_VAR (tail);
549    
550     if (head == tail)
551     return 0;
552    
553     /* it can only overflow if we have events, yes, yes? */
554     if (ecb_expect_false (EV_CQ_VAR (overflow)))
555     {
556     iouring_overflow (EV_A);
557     return 1;
558     }
559    
560     mask = EV_CQ_VAR (ring_mask);
561    
562     do
563     iouring_process_cqe (EV_A_ &EV_CQES [head++ & mask]);
564     while (head != tail);
565    
566     EV_CQ_VAR (head) = head;
567     ECB_MEMORY_FENCE_RELEASE;
568    
569     return 1;
570     }
571    
572     static void
573     iouring_poll (EV_P_ ev_tstamp timeout)
574     {
575     /* if we have events, no need for extra syscalls, but we might have to queue events */
576     if (iouring_handle_cq (EV_A))
577 root 1.3 timeout = EV_TS_CONST (0.);
578 root 1.1 else
579     /* no events, so maybe wait for some */
580     iouring_tfd_update (EV_A_ timeout);
581    
582 root 1.6 /* only enter the kernel if we have something to submit, or we need to wait */
583 root 1.1 if (timeout || iouring_to_submit)
584     {
585     int res;
586    
587     EV_RELEASE_CB;
588    
589     res = evsys_io_uring_enter (iouring_fd, iouring_to_submit, 1,
590 root 1.3 timeout > EV_TS_CONST (0.) ? IORING_ENTER_GETEVENTS : 0, 0, 0);
591 root 1.1 iouring_to_submit = 0;
592    
593     EV_ACQUIRE_CB;
594    
595     if (ecb_expect_false (res < 0))
596     if (errno == EINTR)
597     /* ignore */;
598     else
599     ev_syserr ("(libev) iouring setup");
600     else
601     iouring_handle_cq (EV_A);
602     }
603     }
604    
605     inline_size
606     int
607     iouring_init (EV_P_ int flags)
608     {
609     iouring_entries = IOURING_INIT_ENTRIES;
610     iouring_max_entries = 0;
611    
612     if (iouring_internal_init (EV_A) < 0)
613     {
614     iouring_internal_destroy (EV_A);
615     return 0;
616     }
617    
618 root 1.6 ev_io_init (&iouring_tfd_w, iouring_tfd_cb, iouring_tfd, EV_READ);
619 root 1.8 ev_set_priority (&iouring_tfd_w, EV_MINPRI);
620 root 1.1 ev_io_start (EV_A_ &iouring_tfd_w);
621     ev_unref (EV_A); /* watcher should not keep loop alive */
622    
623     backend_modify = iouring_modify;
624     backend_poll = iouring_poll;
625    
626     return EVBACKEND_IOURING;
627     }
628    
629     inline_size
630     void
631     iouring_destroy (EV_P)
632     {
633     iouring_internal_destroy (EV_A);
634     }
635