libev/ev_linuxaio.c

/*
 * libev linux aio fd activity backend
 *
 * Copyright (c) 2019 Marc Alexander Lehmann <libev@schmorp.de>
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without modifica-
 * tion, are permitted provided that the following conditions are met:
 *
 *   1.  Redistributions of source code must retain the above copyright notice,
 *       this list of conditions and the following disclaimer.
 *
 *   2.  Redistributions in binary form must reproduce the above copyright
 *       notice, this list of conditions and the following disclaimer in the
 *       documentation and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR IMPLIED
 * WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MER-
 * CHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO
 * EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPE-
 * CIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
 * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS;
 * OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
 * WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTH-
 * ERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED
 * OF THE POSSIBILITY OF SUCH DAMAGE.
 *
 * Alternatively, the contents of this file may be used under the terms of
 * the GNU General Public License ("GPL") version 2 or any later version,
 * in which case the provisions of the GPL are applicable instead of
 * the above. If you wish to allow the use of your version of this file
 * only under the terms of the GPL and not to allow others to use your
 * version of this file under the BSD license, indicate your decision
 * by deleting the provisions above and replace them with the notice
 * and other provisions required by the GPL. If you do not delete the
 * provisions above, a recipient may use your version of this file under
 * either the BSD or the GPL.
 */

#include <sys/time.h> /* actually linux/time.h, but we must assume they are compatible */
#include <poll.h>
#include <linux/aio_abi.h>

/* we try to fill 4kn pages exactly.
 * the ring buffer header is 32 bytes, every io event is 32 bytes.
 * the kernel takes the io event number, doubles it, adds 2, adds the ring buffer
 * so the calculation below will use "exactly" 8kB for the ring buffer
 */
#define EV_LINUXAIO_DEPTH (256 / 2 - 2 - 1) /* max. number of io events per batch */

/*****************************************************************************/
/* syscall wrapdadoop */

#include <sys/syscall.h> /* no glibc wrappers */

/* aio_abi.h is not versioned in any way, so we cannot test for its existance */
#define IOCB_CMD_POLL 5

/* taken from linux/fs/aio.c */
#define AIO_RING_MAGIC                  0xa10a10a1
#define AIO_RING_INCOMPAT_FEATURES      0
struct aio_ring
{
  unsigned id;    /* kernel internal index number */
  unsigned nr;    /* number of io_events */
  unsigned head;  /* Written to by userland or by kernel. */
  unsigned tail;

  unsigned magic;
  unsigned compat_features;
  unsigned incompat_features;
  unsigned header_length;  /* size of aio_ring */

  struct io_event io_events[0];
};

static int
ev_io_setup (unsigned nr_events, aio_context_t *ctx_idp)
{
  return syscall (SYS_io_setup, nr_events, ctx_idp);
}

static int
ev_io_destroy (aio_context_t ctx_id)
{
  return syscall (SYS_io_destroy, ctx_id);
}

static int
ev_io_submit (aio_context_t ctx_id, long nr, struct iocb *cbp[])
{
  return syscall (SYS_io_submit, ctx_id, nr, cbp);
}

static int
ev_io_cancel (aio_context_t ctx_id, struct iocb *cbp, struct io_event *result)
{
  return syscall (SYS_io_cancel, ctx_id, cbp, result);
}

static int
ev_io_getevents (aio_context_t ctx_id, long min_nr, long nr, struct io_event *events, struct timespec *timeout)
{
  return syscall (SYS_io_getevents, ctx_id, min_nr, nr, events, timeout);
}

typedef void (*ev_io_cb) (long nr, struct io_event *events);

/*****************************************************************************/
/* actual backed implementation */

/* two iocbs for every fd, one for read, one for write */
typedef struct aniocb
{
  struct iocb io;
  /*int inuse;*/
} *ANIOCBP;

inline_size
void
linuxaio_array_needsize_iocbp (ANIOCBP *base, int count)
{
  /* TODO: quite the overhead to allocate every iocb separately */
  while (count--)
    {
      *base = (ANIOCBP)ev_malloc (sizeof (**base));
      /* TODO: full zero initialize required? */
      memset (*base, 0, sizeof (**base));
      /* would be nice to initialize fd/data as well */
      (*base)->io.aio_lio_opcode = IOCB_CMD_POLL;
      ++base;
    }
}

static void
linuxaio_free_iocbp (EV_P)
{
  while (linuxaio_iocbpmax--)
    ev_free (linuxaio_iocbps [linuxaio_iocbpmax]);

  /* next resize will completely reallocate the array */
  linuxaio_iocbpmax = 0;
  linuxaio_submitcnt = 0; /* all pointers invalidated */
}

static void
linuxaio_modify (EV_P_ int fd, int oev, int nev)
{
  array_needsize (ANIOCBP, linuxaio_iocbps, linuxaio_iocbpmax, fd + 1, linuxaio_array_needsize_iocbp);
  struct aniocb *iocb = linuxaio_iocbps [fd];

  if (iocb->io.aio_buf)
    ev_io_cancel (linuxaio_ctx, &iocb->io, (struct io_event *)0); /* always returns an error relevant kernels */

  if (nev)
    {
      iocb->io.aio_data       = fd;
      iocb->io.aio_fildes     = fd;
      iocb->io.aio_buf        =
          (nev & EV_READ ? POLLIN : 0)
          | (nev & EV_WRITE ? POLLOUT : 0);

      /* queue iocb up for io_submit */
      /* this assumes we only ever get one call per fd per loop iteration */
      ++linuxaio_submitcnt;
      array_needsize (struct iocb *, linuxaio_submits, linuxaio_submitmax, linuxaio_submitcnt, array_needsize_noinit);
      linuxaio_submits [linuxaio_submitcnt - 1] = &iocb->io;
    }
}

static void
linuxaio_parse_events (EV_P_ struct io_event *ev, int nr)
{
  while (nr)
    {
      int fd  = ev->data;
      int res = ev->res;

      assert (("libev: iocb fd must be in-bounds", fd >= 0 && fd < anfdmax));

      /* linux aio is oneshot: rearm fd */
      linuxaio_iocbps [fd]->io.aio_buf = 0;
      anfds [fd].events = 0;
      fd_change (EV_A_ fd, 0);

      /* feed events, we do not expect or handle POLLNVAL */
      if (ecb_expect_false (res & POLLNVAL))
        fd_kill (EV_A_ fd);
      else
        fd_event (
          EV_A_
          fd,
          (res & (POLLOUT | POLLERR | POLLHUP) ? EV_WRITE : 0)
          | (res & (POLLIN | POLLERR | POLLHUP) ? EV_READ : 0)
        );

      --nr;
      ++ev;
    }
}

/* get any events from ringbuffer, return true if any were handled */
static int
linuxaio_get_events_from_ring (EV_P)
{
  struct aio_ring *ring = (struct aio_ring *)linuxaio_ctx;

  ECB_MEMORY_FENCE_ACQUIRE;

  unsigned head = ring->head;
  unsigned tail = *(volatile unsigned *)&ring->tail;

  if (ring->magic != AIO_RING_MAGIC
      || ring->incompat_features != AIO_RING_INCOMPAT_FEATURES
      || ring->header_length != sizeof (struct aio_ring) /* TODO: or use it to find io_event[0]? */
      || head == tail)
    return 0;

  /* parse all available events, but only once, to avoid starvation */
  if (tail > head) /* normal case around */
    linuxaio_parse_events (EV_A_ ring->io_events + head, tail - head);
  else
    {
      /* wrapped around */
      linuxaio_parse_events (EV_A_ ring->io_events + head, ring->nr - head);
      linuxaio_parse_events (EV_A_ ring->io_events, tail);
    }

  ring->head = tail;

  return 1;
}

/* read at least one event from kernel, or timeout */
inline_size
void
linuxaio_get_events (EV_P_ ev_tstamp timeout)
{
  struct timespec ts;
  struct io_event ioev;
  int res;

  if (linuxaio_get_events_from_ring (EV_A))
    return;

  /* no events, so wait for at least one, then poll ring buffer again */
  /* this degraded to one event per loop iteration */
  /* if the ring buffer changes layout, but so be it */

  ts.tv_sec  = (long)timeout;
  ts.tv_nsec = (long)((timeout - ts.tv_sec) * 1e9);

  res = ev_io_getevents (linuxaio_ctx, 1, 1, &ioev, &ts);

  if (res < 0)
    ev_syserr ("(libev) io_getevents");
  else if (res)
    {
      /* at least one event received, handle it and any remaining ones in the ring buffer */
      linuxaio_parse_events (EV_A_ &ioev, 1);
      linuxaio_get_events_from_ring (EV_A);
    }
}

static void
linuxaio_poll (EV_P_ ev_tstamp timeout)
{
  int submitted;

  /* first phase: submit new iocbs */

  /* io_submit might return less than the requested number of iocbs */
  /* this is, afaics, only because of errors, but we go by the book and use a loop, */
  /* which allows us to pinpoint the errornous iocb */
  for (submitted = 0; submitted < linuxaio_submitcnt; )
    {
      int res = ev_io_submit (linuxaio_ctx, linuxaio_submitcnt - submitted, linuxaio_submits + submitted);

      if (res < 0)
        if (errno == EAGAIN)
          {
            /* This happens when the ring buffer is full, at least. I assume this means
             * that the event was queued synchronously during io_submit, and thus
             * the buffer overflowd.
             * In this case, we just try next loop iteration.
             */
            memmove (linuxaio_submits, linuxaio_submits + submitted, (linuxaio_submitcnt - submitted) * sizeof (*linuxaio_submits));
            linuxaio_submitcnt -= submitted;
            timeout = 0;
            break;
          }
        else
          /* TODO: we get EAGAIN when the ring buffer is full for some reason */
          /* TODO: should we always just try next time? */
          ev_syserr ("(libev) io_submit");

      submitted += res;
    }

  linuxaio_submitcnt = 0;

  /* second phase: fetch and parse events */

  linuxaio_get_events (EV_A_ timeout);
}

inline_size
int
linuxaio_init (EV_P_ int flags)
{
  /* would be great to have a nice test for IOCB_CMD_POLL instead */
  /* also: test some semi-common fd types, such as files and ttys in recommended_backends */
  if (ev_linux_version () < 0x041200) /* 4.18 introduced IOCB_CMD_POLL */
    return 0;

  linuxaio_ctx = 0;
  if (ev_io_setup (EV_LINUXAIO_DEPTH, &linuxaio_ctx) < 0)
    return 0;

  backend_modify  = linuxaio_modify;
  backend_poll    = linuxaio_poll;

  linuxaio_iocbpmax = 0;
  linuxaio_iocbps = 0;

  linuxaio_submits = 0;
  linuxaio_submitmax = 0;
  linuxaio_submitcnt = 0;

  return EVBACKEND_LINUXAIO;
}

inline_size
void
linuxaio_destroy (EV_P)
{
  linuxaio_free_iocbp (EV_A);
  ev_io_destroy (linuxaio_ctx);
}

inline_size
void
linuxaio_fork (EV_P)
{
  /* TODO: verify and test */
  linuxaio_destroy (EV_A);

  linuxaio_ctx = 0;
  while (ev_io_setup (EV_LINUXAIO_DEPTH, &linuxaio_ctx) < 0)
    ev_syserr ("(libev) io_setup");

  fd_rearm_all (EV_A);
}

Revision:	1.5
Committed:	Fri Jun 21 03:41:40 2019 UTC (4 years, 10 months ago) by root
Content type:	text/plain
Branch:	MAIN
Changes since 1.4:	+2 -2 lines
Log Message:	* empty log message *
#	User	Rev	Content
1	root	1.1	/*
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			#include <sys/time.h> /* actually linux/time.h, but we must assume they are compatible */
41	root	1.2	#include <poll.h>
42	root	1.1	#include <linux/aio_abi.h>
43
44			/* we try to fill 4kn pages exactly.
45			* the ring buffer header is 32 bytes, every io event is 32 bytes.
46			* the kernel takes the io event number, doubles it, adds 2, adds the ring buffer
47			* so the calculation below will use "exactly" 8kB for the ring buffer
48			*/
49			#define EV_LINUXAIO_DEPTH (256 / 2 - 2 - 1) /* max. number of io events per batch */
50
51			/*****************************************************************************/
52			/* syscall wrapdadoop */
53
54			#include <sys/syscall.h> /* no glibc wrappers */
55
56	root	1.5	/* aio_abi.h is not versioned in any way, so we cannot test for its existance */
57	root	1.1	#define IOCB_CMD_POLL 5
58
59			/* taken from linux/fs/aio.c */
60			#define AIO_RING_MAGIC 0xa10a10a1
61			#define AIO_RING_INCOMPAT_FEATURES 0
62			struct aio_ring
63			{
64			unsigned id; /* kernel internal index number */
65			unsigned nr; /* number of io_events */
66			unsigned head; /* Written to by userland or by kernel. */
67			unsigned tail;
68
69			unsigned magic;
70			unsigned compat_features;
71			unsigned incompat_features;
72			unsigned header_length; /* size of aio_ring */
73
74			struct io_event io_events[0];
75			};
76
77			static int
78			ev_io_setup (unsigned nr_events, aio_context_t *ctx_idp)
79			{
80			return syscall (SYS_io_setup, nr_events, ctx_idp);
81			}
82
83			static int
84			ev_io_destroy (aio_context_t ctx_id)
85			{
86			return syscall (SYS_io_destroy, ctx_id);
87			}
88
89			static int
90			ev_io_submit (aio_context_t ctx_id, long nr, struct iocb *cbp[])
91			{
92			return syscall (SYS_io_submit, ctx_id, nr, cbp);
93			}
94
95			static int
96			ev_io_cancel (aio_context_t ctx_id, struct iocb cbp, struct io_event result)
97			{
98			return syscall (SYS_io_cancel, ctx_id, cbp, result);
99			}
100
101			static int
102			ev_io_getevents (aio_context_t ctx_id, long min_nr, long nr, struct io_event events, struct timespec timeout)
103			{
104			return syscall (SYS_io_getevents, ctx_id, min_nr, nr, events, timeout);
105			}
106
107			typedef void (ev_io_cb) (long nr, struct io_event events);
108
109			/*****************************************************************************/
110			/* actual backed implementation */
111
112			/* two iocbs for every fd, one for read, one for write */
113			typedef struct aniocb
114			{
115			struct iocb io;
116			/int inuse;/
117			} *ANIOCBP;
118
119			inline_size
120			void
121			linuxaio_array_needsize_iocbp (ANIOCBP *base, int count)
122			{
123			/* TODO: quite the overhead to allocate every iocb separately */
124			while (count--)
125			{
126			base = (ANIOCBP)ev_malloc (sizeof (*base));
127	root	1.2	/* TODO: full zero initialize required? */
128	root	1.1	memset (base, 0, sizeof (*base));
129			/* would be nice to initialize fd/data as well */
130			(*base)->io.aio_lio_opcode = IOCB_CMD_POLL;
131			++base;
132			}
133			}
134
135			static void
136			linuxaio_free_iocbp (EV_P)
137			{
138			while (linuxaio_iocbpmax--)
139			ev_free (linuxaio_iocbps [linuxaio_iocbpmax]);
140
141	root	1.2	/* next resize will completely reallocate the array */
142	root	1.1	linuxaio_iocbpmax = 0;
143	root	1.2	linuxaio_submitcnt = 0; /* all pointers invalidated */
144	root	1.1	}
145
146			static void
147			linuxaio_modify (EV_P_ int fd, int oev, int nev)
148			{
149			array_needsize (ANIOCBP, linuxaio_iocbps, linuxaio_iocbpmax, fd + 1, linuxaio_array_needsize_iocbp);
150			struct aniocb *iocb = linuxaio_iocbps [fd];
151
152			if (iocb->io.aio_buf)
153	root	1.4	ev_io_cancel (linuxaio_ctx, &iocb->io, (struct io_event )0); / always returns an error relevant kernels */
154	root	1.1
155			if (nev)
156			{
157			iocb->io.aio_data = fd;
158			iocb->io.aio_fildes = fd;
159			iocb->io.aio_buf =
160			(nev & EV_READ ? POLLIN : 0)
161			\| (nev & EV_WRITE ? POLLOUT : 0);
162
163			/* queue iocb up for io_submit */
164			/* this assumes we only ever get one call per fd per loop iteration */
165			++linuxaio_submitcnt;
166			array_needsize (struct iocb *, linuxaio_submits, linuxaio_submitmax, linuxaio_submitcnt, array_needsize_noinit);
167			linuxaio_submits [linuxaio_submitcnt - 1] = &iocb->io;
168			}
169			}
170
171			static void
172			linuxaio_parse_events (EV_P_ struct io_event *ev, int nr)
173			{
174			while (nr)
175			{
176			int fd = ev->data;
177			int res = ev->res;
178
179	root	1.2	assert (("libev: iocb fd must be in-bounds", fd >= 0 && fd < anfdmax));
180	root	1.1
181			/* linux aio is oneshot: rearm fd */
182			linuxaio_iocbps [fd]->io.aio_buf = 0;
183			anfds [fd].events = 0;
184			fd_change (EV_A_ fd, 0);
185
186			/* feed events, we do not expect or handle POLLNVAL */
187			if (ecb_expect_false (res & POLLNVAL))
188			fd_kill (EV_A_ fd);
189			else
190			fd_event (
191			EV_A_
192			fd,
193			(res & (POLLOUT \| POLLERR \| POLLHUP) ? EV_WRITE : 0)
194			\| (res & (POLLIN \| POLLERR \| POLLHUP) ? EV_READ : 0)
195			);
196
197			--nr;
198			++ev;
199			}
200			}
201
202			/* get any events from ringbuffer, return true if any were handled */
203			static int
204			linuxaio_get_events_from_ring (EV_P)
205			{
206			struct aio_ring ring = (struct aio_ring )linuxaio_ctx;
207
208			ECB_MEMORY_FENCE_ACQUIRE;
209
210			unsigned head = ring->head;
211			unsigned tail = (volatile unsigned )&ring->tail;
212
213			if (ring->magic != AIO_RING_MAGIC
214			\|\| ring->incompat_features != AIO_RING_INCOMPAT_FEATURES
215			\|\| ring->header_length != sizeof (struct aio_ring) /* TODO: or use it to find io_event[0]? */
216			\|\| head == tail)
217			return 0;
218
219			/* parse all available events, but only once, to avoid starvation */
220			if (tail > head) /* normal case around */
221			linuxaio_parse_events (EV_A_ ring->io_events + head, tail - head);
222			else
223			{
224			/* wrapped around */
225			linuxaio_parse_events (EV_A_ ring->io_events + head, ring->nr - head);
226			linuxaio_parse_events (EV_A_ ring->io_events, tail);
227			}
228
229			ring->head = tail;
230
231			return 1;
232			}
233
234			/* read at least one event from kernel, or timeout */
235			inline_size
236			void
237			linuxaio_get_events (EV_P_ ev_tstamp timeout)
238			{
239			struct timespec ts;
240			struct io_event ioev;
241			int res;
242
243			if (linuxaio_get_events_from_ring (EV_A))
244			return;
245
246			/* no events, so wait for at least one, then poll ring buffer again */
247			/* this degraded to one event per loop iteration */
248			/* if the ring buffer changes layout, but so be it */
249
250			ts.tv_sec = (long)timeout;
251			ts.tv_nsec = (long)((timeout - ts.tv_sec) * 1e9);
252
253			res = ev_io_getevents (linuxaio_ctx, 1, 1, &ioev, &ts);
254
255			if (res < 0)
256			ev_syserr ("(libev) io_getevents");
257			else if (res)
258			{
259			/* at least one event received, handle it and any remaining ones in the ring buffer */
260			linuxaio_parse_events (EV_A_ &ioev, 1);
261			linuxaio_get_events_from_ring (EV_A);
262			}
263			}
264
265			static void
266			linuxaio_poll (EV_P_ ev_tstamp timeout)
267			{
268			int submitted;
269
270			/* first phase: submit new iocbs */
271
272			/* io_submit might return less than the requested number of iocbs */
273			/* this is, afaics, only because of errors, but we go by the book and use a loop, */
274			/* which allows us to pinpoint the errornous iocb */
275			for (submitted = 0; submitted < linuxaio_submitcnt; )
276			{
277			int res = ev_io_submit (linuxaio_ctx, linuxaio_submitcnt - submitted, linuxaio_submits + submitted);
278
279			if (res < 0)
280			if (errno == EAGAIN)
281			{
282			/* This happens when the ring buffer is full, at least. I assume this means
283			* that the event was queued synchronously during io_submit, and thus
284			* the buffer overflowd.
285			* In this case, we just try next loop iteration.
286			*/
287	root	1.5	memmove (linuxaio_submits, linuxaio_submits + submitted, (linuxaio_submitcnt - submitted) * sizeof (*linuxaio_submits));
288	root	1.1	linuxaio_submitcnt -= submitted;
289			timeout = 0;
290			break;
291			}
292			else
293			/* TODO: we get EAGAIN when the ring buffer is full for some reason */
294			/* TODO: should we always just try next time? */
295			ev_syserr ("(libev) io_submit");
296
297			submitted += res;
298			}
299
300			linuxaio_submitcnt = 0;
301
302			/* second phase: fetch and parse events */
303
304			linuxaio_get_events (EV_A_ timeout);
305			}
306
307			inline_size
308			int
309			linuxaio_init (EV_P_ int flags)
310			{
311			/* would be great to have a nice test for IOCB_CMD_POLL instead */
312	root	1.2	/* also: test some semi-common fd types, such as files and ttys in recommended_backends */
313	root	1.1	if (ev_linux_version () < 0x041200) /* 4.18 introduced IOCB_CMD_POLL */
314			return 0;
315
316	root	1.3	linuxaio_ctx = 0;
317	root	1.1	if (ev_io_setup (EV_LINUXAIO_DEPTH, &linuxaio_ctx) < 0)
318			return 0;
319
320			backend_modify = linuxaio_modify;
321			backend_poll = linuxaio_poll;
322
323			linuxaio_iocbpmax = 0;
324			linuxaio_iocbps = 0;
325
326			linuxaio_submits = 0;
327			linuxaio_submitmax = 0;
328			linuxaio_submitcnt = 0;
329
330			return EVBACKEND_LINUXAIO;
331			}
332
333			inline_size
334			void
335			linuxaio_destroy (EV_P)
336			{
337			linuxaio_free_iocbp (EV_A);
338			ev_io_destroy (linuxaio_ctx);
339			}
340
341			inline_size
342			void
343			linuxaio_fork (EV_P)
344			{
345	root	1.2	/* TODO: verify and test */
346			linuxaio_destroy (EV_A);
347
348	root	1.3	linuxaio_ctx = 0;
349	root	1.2	while (ev_io_setup (EV_LINUXAIO_DEPTH, &linuxaio_ctx) < 0)
350			ev_syserr ("(libev) io_setup");
351
352			fd_rearm_all (EV_A);
353	root	1.1	}
354