about summary refs log tree commit diff
path: root/sysdeps/pthread/aio_misc.c
blob: 5d2b81e479772eae00ac3d9e725c468f5d9b855f (plain) (blame)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
/* Handle general operations.
   Copyright (C) 1997-2015 Free Software Foundation, Inc.
   This file is part of the GNU C Library.
   Contributed by Ulrich Drepper <drepper@cygnus.com>, 1997.

   The GNU C Library is free software; you can redistribute it and/or
   modify it under the terms of the GNU Lesser General Public
   License as published by the Free Software Foundation; either
   version 2.1 of the License, or (at your option) any later version.

   The GNU C Library is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
   Lesser General Public License for more details.

   You should have received a copy of the GNU Lesser General Public
   License along with the GNU C Library; if not, see
   <http://www.gnu.org/licenses/>.  */

#include <aio.h>
#include <assert.h>
#include <errno.h>
#include <limits.h>
#include <pthread.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/param.h>
#include <sys/stat.h>
#include <sys/time.h>
#include <aio_misc.h>

#ifndef aio_create_helper_thread
# define aio_create_helper_thread __aio_create_helper_thread

extern inline int
__aio_create_helper_thread (pthread_t *threadp, void *(*tf) (void *), void *arg)
{
  pthread_attr_t attr;

  /* Make sure the thread is created detached.  */
  pthread_attr_init (&attr);
  pthread_attr_setdetachstate (&attr, PTHREAD_CREATE_DETACHED);

  int ret = pthread_create (threadp, &attr, tf, arg);

  (void) pthread_attr_destroy (&attr);
  return ret;
}
#endif

static void add_request_to_runlist (struct requestlist *newrequest);

/* Pool of request list entries.  */
static struct requestlist **pool;

/* Number of total and allocated pool entries.  */
static size_t pool_max_size;
static size_t pool_size;

/* We implement a two dimensional array but allocate each row separately.
   The macro below determines how many entries should be used per row.
   It should better be a power of two.  */
#define ENTRIES_PER_ROW	32

/* How many rows we allocate at once.  */
#define ROWS_STEP	8

/* List of available entries.  */
static struct requestlist *freelist;

/* List of request waiting to be processed.  */
static struct requestlist *runlist;

/* Structure list of all currently processed requests.  */
static struct requestlist *requests;

/* Number of threads currently running.  */
static int nthreads;

/* Number of threads waiting for work to arrive. */
static int idle_thread_count;


/* These are the values used to optimize the use of AIO.  The user can
   overwrite them by using the `aio_init' function.  */
static struct aioinit optim =
{
  20,	/* int aio_threads;	Maximal number of threads.  */
  64,	/* int aio_num;		Number of expected simultaneous requests. */
  0,
  0,
  0,
  0,
  1,
  0
};


/* Since the list is global we need a mutex protecting it.  */
pthread_mutex_t __aio_requests_mutex = PTHREAD_RECURSIVE_MUTEX_INITIALIZER_NP;

/* When you add a request to the list and there are idle threads present,
   you signal this condition variable. When a thread finishes work, it waits
   on this condition variable for a time before it actually exits. */
pthread_cond_t __aio_new_request_notification = PTHREAD_COND_INITIALIZER;


/* Functions to handle request list pool.  */
static struct requestlist *
get_elem (void)
{
  struct requestlist *result;

  if (freelist == NULL)
    {
      struct requestlist *new_row;
      int cnt;

      assert (sizeof (struct aiocb) == sizeof (struct aiocb64));

      if (pool_size + 1 >= pool_max_size)
	{
	  size_t new_max_size = pool_max_size + ROWS_STEP;
	  struct requestlist **new_tab;

	  new_tab = (struct requestlist **)
	    realloc (pool, new_max_size * sizeof (struct requestlist *));

	  if (new_tab == NULL)
	    return NULL;

	  pool_max_size = new_max_size;
	  pool = new_tab;
	}

      /* Allocate the new row.  */
      cnt = pool_size == 0 ? optim.aio_num : ENTRIES_PER_ROW;
      new_row = (struct requestlist *) calloc (cnt,
					       sizeof (struct requestlist));
      if (new_row == NULL)
	return NULL;

      pool[pool_size++] = new_row;

      /* Put all the new entries in the freelist.  */
      do
	{
	  new_row->next_prio = freelist;
	  freelist = new_row++;
	}
      while (--cnt > 0);
    }

  result = freelist;
  freelist = freelist->next_prio;

  return result;
}


void
internal_function
__aio_free_request (struct requestlist *elem)
{
  elem->running = no;
  elem->next_prio = freelist;
  freelist = elem;
}


struct requestlist *
internal_function
__aio_find_req (aiocb_union *elem)
{
  struct requestlist *runp = requests;
  int fildes = elem->aiocb.aio_fildes;

  while (runp != NULL && runp->aiocbp->aiocb.aio_fildes < fildes)
    runp = runp->next_fd;

  if (runp != NULL)
    {
      if (runp->aiocbp->aiocb.aio_fildes != fildes)
	runp = NULL;
      else
	while (runp != NULL && runp->aiocbp != elem)
	  runp = runp->next_prio;
    }

  return runp;
}


struct requestlist *
internal_function
__aio_find_req_fd (int fildes)
{
  struct requestlist *runp = requests;

  while (runp != NULL && runp->aiocbp->aiocb.aio_fildes < fildes)
    runp = runp->next_fd;

  return (runp != NULL && runp->aiocbp->aiocb.aio_fildes == fildes
	  ? runp : NULL);
}


void
internal_function
__aio_remove_request (struct requestlist *last, struct requestlist *req,
		      int all)
{
  assert (req->running == yes || req->running == queued
	  || req->running == done);

  if (last != NULL)
    last->next_prio = all ? NULL : req->next_prio;
  else
    {
      if (all || req->next_prio == NULL)
	{
	  if (req->last_fd != NULL)
	    req->last_fd->next_fd = req->next_fd;
	  else
	    requests = req->next_fd;
	  if (req->next_fd != NULL)
	    req->next_fd->last_fd = req->last_fd;
	}
      else
	{
	  if (req->last_fd != NULL)
	    req->last_fd->next_fd = req->next_prio;
	  else
	    requests = req->next_prio;

	  if (req->next_fd != NULL)
	    req->next_fd->last_fd = req->next_prio;

	  req->next_prio->last_fd = req->last_fd;
	  req->next_prio->next_fd = req->next_fd;

	  /* Mark this entry as runnable.  */
	  req->next_prio->running = yes;
	}

      if (req->running == yes)
	{
	  struct requestlist *runp = runlist;

	  last = NULL;
	  while (runp != NULL)
	    {
	      if (runp == req)
		{
		  if (last == NULL)
		    runlist = runp->next_run;
		  else
		    last->next_run = runp->next_run;
		  break;
		}
	      last = runp;
	      runp = runp->next_run;
	    }
	}
    }
}


/* The thread handler.  */
static void *handle_fildes_io (void *arg);


/* User optimization.  */
void
__aio_init (const struct aioinit *init)
{
  /* Get the mutex.  */
  pthread_mutex_lock (&__aio_requests_mutex);

  /* Only allow writing new values if the table is not yet allocated.  */
  if (pool == NULL)
    {
      optim.aio_threads = init->aio_threads < 1 ? 1 : init->aio_threads;
      assert (powerof2 (ENTRIES_PER_ROW));
      optim.aio_num = (init->aio_num < ENTRIES_PER_ROW
		       ? ENTRIES_PER_ROW
		       : init->aio_num & ~(ENTRIES_PER_ROW - 1));
    }

  if (init->aio_idle_time != 0)
    optim.aio_idle_time = init->aio_idle_time;

  /* Release the mutex.  */
  pthread_mutex_unlock (&__aio_requests_mutex);
}
weak_alias (__aio_init, aio_init)


/* The main function of the async I/O handling.  It enqueues requests
   and if necessary starts and handles threads.  */
struct requestlist *
internal_function
__aio_enqueue_request (aiocb_union *aiocbp, int operation)
{
  int result = 0;
  int policy, prio;
  struct sched_param param;
  struct requestlist *last, *runp, *newp;
  int running = no;

  if (operation == LIO_SYNC || operation == LIO_DSYNC)
    aiocbp->aiocb.aio_reqprio = 0;
  else if (aiocbp->aiocb.aio_reqprio < 0
#ifdef AIO_PRIO_DELTA_MAX
	   || aiocbp->aiocb.aio_reqprio > AIO_PRIO_DELTA_MAX
#endif
	   )
    {
      /* Invalid priority value.  */
      __set_errno (EINVAL);
      aiocbp->aiocb.__error_code = EINVAL;
      aiocbp->aiocb.__return_value = -1;
      return NULL;
    }

  /* Compute priority for this request.  */
  pthread_getschedparam (pthread_self (), &policy, &param);
  prio = param.sched_priority - aiocbp->aiocb.aio_reqprio;

  /* Get the mutex.  */
  pthread_mutex_lock (&__aio_requests_mutex);

  last = NULL;
  runp = requests;
  /* First look whether the current file descriptor is currently
     worked with.  */
  while (runp != NULL
	 && runp->aiocbp->aiocb.aio_fildes < aiocbp->aiocb.aio_fildes)
    {
      last = runp;
      runp = runp->next_fd;
    }

  /* Get a new element for the waiting list.  */
  newp = get_elem ();
  if (newp == NULL)
    {
      pthread_mutex_unlock (&__aio_requests_mutex);
      __set_errno (EAGAIN);
      return NULL;
    }
  newp->aiocbp = aiocbp;
#ifdef BROKEN_THREAD_SIGNALS
  newp->caller_pid = (aiocbp->aiocb.aio_sigevent.sigev_notify == SIGEV_SIGNAL
		      ? getpid () : 0);
#endif
  newp->waiting = NULL;

  aiocbp->aiocb.__abs_prio = prio;
  aiocbp->aiocb.__policy = policy;
  aiocbp->aiocb.aio_lio_opcode = operation;
  aiocbp->aiocb.__error_code = EINPROGRESS;
  aiocbp->aiocb.__return_value = 0;

  if (runp != NULL
      && runp->aiocbp->aiocb.aio_fildes == aiocbp->aiocb.aio_fildes)
    {
      /* The current file descriptor is worked on.  It makes no sense
	 to start another thread since this new thread would fight
	 with the running thread for the resources.  But we also cannot
	 say that the thread processing this desriptor shall immediately
	 after finishing the current job process this request if there
	 are other threads in the running queue which have a higher
	 priority.  */

      /* Simply enqueue it after the running one according to the
	 priority.  */
      last = NULL;
      while (runp->next_prio != NULL
	     && runp->next_prio->aiocbp->aiocb.__abs_prio >= prio)
	{
	  last = runp;
	  runp = runp->next_prio;
	}

      newp->next_prio = runp->next_prio;
      runp->next_prio = newp;

      running = queued;
    }
  else
    {
      running = yes;
      /* Enqueue this request for a new descriptor.  */
      if (last == NULL)
	{
	  newp->last_fd = NULL;
	  newp->next_fd = requests;
	  if (requests != NULL)
	    requests->last_fd = newp;
	  requests = newp;
	}
      else
	{
	  newp->next_fd = last->next_fd;
	  newp->last_fd = last;
	  last->next_fd = newp;
	  if (newp->next_fd != NULL)
	    newp->next_fd->last_fd = newp;
	}

      newp->next_prio = NULL;
      last = NULL;
    }

  if (running == yes)
    {
      /* We try to create a new thread for this file descriptor.  The
	 function which gets called will handle all available requests
	 for this descriptor and when all are processed it will
	 terminate.

	 If no new thread can be created or if the specified limit of
	 threads for AIO is reached we queue the request.  */

      /* See if we need to and are able to create a thread.  */
      if (nthreads < optim.aio_threads && idle_thread_count == 0)
	{
	  pthread_t thid;

	  running = newp->running = allocated;

	  /* Now try to start a thread.  */
	  result = aio_create_helper_thread (&thid, handle_fildes_io, newp);
	  if (result == 0)
	    /* We managed to enqueue the request.  All errors which can
	       happen now can be recognized by calls to `aio_return' and
	       `aio_error'.  */
	    ++nthreads;
	  else
	    {
	      /* Reset the running flag.  The new request is not running.  */
	      running = newp->running = yes;

	      if (nthreads == 0)
		{
		  /* We cannot create a thread in the moment and there is
		     also no thread running.  This is a problem.  `errno' is
		     set to EAGAIN if this is only a temporary problem.  */
		  __aio_remove_request (last, newp, 0);
		}
	      else
		result = 0;
	    }
	}
    }

  /* Enqueue the request in the run queue if it is not yet running.  */
  if (running == yes && result == 0)
    {
      add_request_to_runlist (newp);

      /* If there is a thread waiting for work, then let it know that we
	 have just given it something to do. */
      if (idle_thread_count > 0)
	pthread_cond_signal (&__aio_new_request_notification);
    }

  if (result == 0)
    newp->running = running;
  else
    {
      /* Something went wrong.  */
      __aio_free_request (newp);
      aiocbp->aiocb.__error_code = result;
      __set_errno (result);
      newp = NULL;
    }

  /* Release the mutex.  */
  pthread_mutex_unlock (&__aio_requests_mutex);

  return newp;
}


static void *
handle_fildes_io (void *arg)
{
  pthread_t self = pthread_self ();
  struct sched_param param;
  struct requestlist *runp = (struct requestlist *) arg;
  aiocb_union *aiocbp;
  int policy;
  int fildes;

  pthread_getschedparam (self, &policy, &param);

  do
    {
      /* If runp is NULL, then we were created to service the work queue
	 in general, not to handle any particular request. In that case we
	 skip the "do work" stuff on the first pass, and go directly to the
	 "get work off the work queue" part of this loop, which is near the
	 end. */
      if (runp == NULL)
	pthread_mutex_lock (&__aio_requests_mutex);
      else
	{
	  /* Hopefully this request is marked as running.  */
	  assert (runp->running == allocated);

	  /* Update our variables.  */
	  aiocbp = runp->aiocbp;
	  fildes = aiocbp->aiocb.aio_fildes;

	  /* Change the priority to the requested value (if necessary).  */
	  if (aiocbp->aiocb.__abs_prio != param.sched_priority
	      || aiocbp->aiocb.__policy != policy)
	    {
	      param.sched_priority = aiocbp->aiocb.__abs_prio;
	      policy = aiocbp->aiocb.__policy;
	      pthread_setschedparam (self, policy, &param);
	    }

	  /* Process request pointed to by RUNP.  We must not be disturbed
	     by signals.  */
	  if ((aiocbp->aiocb.aio_lio_opcode & 127) == LIO_READ)
	    {
	      if (sizeof (off_t) != sizeof (off64_t)
		  && aiocbp->aiocb.aio_lio_opcode & 128)
		aiocbp->aiocb.__return_value =
		  TEMP_FAILURE_RETRY (__pread64 (fildes, (void *)
						 aiocbp->aiocb64.aio_buf,
						 aiocbp->aiocb64.aio_nbytes,
						 aiocbp->aiocb64.aio_offset));
	      else
		aiocbp->aiocb.__return_value =
		  TEMP_FAILURE_RETRY (pread (fildes,
					     (void *) aiocbp->aiocb.aio_buf,
					     aiocbp->aiocb.aio_nbytes,
					     aiocbp->aiocb.aio_offset));

	      if (aiocbp->aiocb.__return_value == -1 && errno == ESPIPE)
		/* The Linux kernel is different from others.  It returns
		   ESPIPE if using pread on a socket.  Other platforms
		   simply ignore the offset parameter and behave like
		   read.  */
		aiocbp->aiocb.__return_value =
		  TEMP_FAILURE_RETRY (read (fildes,
					    (void *) aiocbp->aiocb64.aio_buf,
					    aiocbp->aiocb64.aio_nbytes));
	    }
	  else if ((aiocbp->aiocb.aio_lio_opcode & 127) == LIO_WRITE)
	    {
	      if (sizeof (off_t) != sizeof (off64_t)
		  && aiocbp->aiocb.aio_lio_opcode & 128)
		aiocbp->aiocb.__return_value =
		  TEMP_FAILURE_RETRY (__pwrite64 (fildes, (const void *)
						  aiocbp->aiocb64.aio_buf,
						  aiocbp->aiocb64.aio_nbytes,
						  aiocbp->aiocb64.aio_offset));
	      else
		aiocbp->aiocb.__return_value =
		  TEMP_FAILURE_RETRY (__libc_pwrite (fildes, (const void *)
					      aiocbp->aiocb.aio_buf,
					      aiocbp->aiocb.aio_nbytes,
					      aiocbp->aiocb.aio_offset));

	      if (aiocbp->aiocb.__return_value == -1 && errno == ESPIPE)
		/* The Linux kernel is different from others.  It returns
		   ESPIPE if using pwrite on a socket.  Other platforms
		   simply ignore the offset parameter and behave like
		   write.  */
		aiocbp->aiocb.__return_value =
		  TEMP_FAILURE_RETRY (write (fildes,
					     (void *) aiocbp->aiocb64.aio_buf,
					     aiocbp->aiocb64.aio_nbytes));
	    }
	  else if (aiocbp->aiocb.aio_lio_opcode == LIO_DSYNC)
	    aiocbp->aiocb.__return_value =
	      TEMP_FAILURE_RETRY (fdatasync (fildes));
	  else if (aiocbp->aiocb.aio_lio_opcode == LIO_SYNC)
	    aiocbp->aiocb.__return_value =
	      TEMP_FAILURE_RETRY (fsync (fildes));
	  else
	    {
	      /* This is an invalid opcode.  */
	      aiocbp->aiocb.__return_value = -1;
	      __set_errno (EINVAL);
	    }

	  /* Get the mutex.  */
	  pthread_mutex_lock (&__aio_requests_mutex);

	  /* In theory we would need here a write memory barrier since the
	     callers test using aio_error() whether the request finished
	     and once this value != EINPROGRESS the field __return_value
	     must be committed to memory.

	     But since the pthread_mutex_lock call involves write memory
	     barriers as well it is not necessary.  */

	  if (aiocbp->aiocb.__return_value == -1)
	    aiocbp->aiocb.__error_code = errno;
	  else
	    aiocbp->aiocb.__error_code = 0;

	  /* Send the signal to notify about finished processing of the
	     request.  */
	  __aio_notify (runp);

	  /* For debugging purposes we reset the running flag of the
	     finished request.  */
	  assert (runp->running == allocated);
	  runp->running = done;

	  /* Now dequeue the current request.  */
	  __aio_remove_request (NULL, runp, 0);
	  if (runp->next_prio != NULL)
	    add_request_to_runlist (runp->next_prio);

	  /* Free the old element.  */
	  __aio_free_request (runp);
	}

      runp = runlist;

      /* If the runlist is empty, then we sleep for a while, waiting for
	 something to arrive in it. */
      if (runp == NULL && optim.aio_idle_time >= 0)
	{
	  struct timeval now;
	  struct timespec wakeup_time;

	  ++idle_thread_count;
	  __gettimeofday (&now, NULL);
	  wakeup_time.tv_sec = now.tv_sec + optim.aio_idle_time;
	  wakeup_time.tv_nsec = now.tv_usec * 1000;
	  if (wakeup_time.tv_nsec >= 1000000000)
	    {
	      wakeup_time.tv_nsec -= 1000000000;
	      ++wakeup_time.tv_sec;
	    }
	  pthread_cond_timedwait (&__aio_new_request_notification,
				  &__aio_requests_mutex,
				  &wakeup_time);
	  --idle_thread_count;
	  runp = runlist;
	}

      if (runp == NULL)
	--nthreads;
      else
	{
	  assert (runp->running == yes);
	  runp->running = allocated;
	  runlist = runp->next_run;

	  /* If we have a request to process, and there's still another in
	     the run list, then we need to either wake up or create a new
	     thread to service the request that is still in the run list. */
	  if (runlist != NULL)
	    {
	      /* There are at least two items in the work queue to work on.
		 If there are other idle threads, then we should wake them
		 up for these other work elements; otherwise, we should try
		 to create a new thread. */
	      if (idle_thread_count > 0)
		pthread_cond_signal (&__aio_new_request_notification);
	      else if (nthreads < optim.aio_threads)
		{
		  pthread_t thid;
		  pthread_attr_t attr;

		  /* Make sure the thread is created detached.  */
		  pthread_attr_init (&attr);
		  pthread_attr_setdetachstate (&attr, PTHREAD_CREATE_DETACHED);

		  /* Now try to start a thread. If we fail, no big deal,
		     because we know that there is at least one thread (us)
		     that is working on AIO operations. */
		  if (pthread_create (&thid, &attr, handle_fildes_io, NULL)
		      == 0)
		    ++nthreads;
		}
	    }
	}

      /* Release the mutex.  */
      pthread_mutex_unlock (&__aio_requests_mutex);
    }
  while (runp != NULL);

  return NULL;
}


/* Free allocated resources.  */
libc_freeres_fn (free_res)
{
  size_t row;

  for (row = 0; row < pool_max_size; ++row)
    free (pool[row]);

  free (pool);
}


/* Add newrequest to the runlist. The __abs_prio flag of newrequest must
   be correctly set to do this. Also, you had better set newrequest's
   "running" flag to "yes" before you release your lock or you'll throw an
   assertion. */
static void
add_request_to_runlist (struct requestlist *newrequest)
{
  int prio = newrequest->aiocbp->aiocb.__abs_prio;
  struct requestlist *runp;

  if (runlist == NULL || runlist->aiocbp->aiocb.__abs_prio < prio)
    {
      newrequest->next_run = runlist;
      runlist = newrequest;
    }
  else
    {
      runp = runlist;

      while (runp->next_run != NULL
	     && runp->next_run->aiocbp->aiocb.__abs_prio >= prio)
	runp = runp->next_run;

      newrequest->next_run = runp->next_run;
      runp->next_run = newrequest;
    }
}