summaryrefslogtreecommitdiff
path: root/nptl/pthread_rwlock_rdlock.c
blob: 004a386fd5ca22ea61475c199604f02902e675e6 (plain)
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
/* Copyright (C) 2003-2015 Free Software Foundation, Inc.
   This file is part of the GNU C Library.
   Contributed by Martin Schwidefsky <schwidefsky@de.ibm.com>, 2003.

   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 <errno.h>
#include <sysdep.h>
#include <lowlevellock.h>
#include <pthread.h>
#include <pthreadP.h>
#include <stap-probe.h>
#include <elide.h>
#include <stdbool.h>


/* Acquire read lock for RWLOCK.  Slow path.  */
static int __attribute__((noinline))
__pthread_rwlock_rdlock_slow (pthread_rwlock_t *rwlock)
{
  int result = 0;
  bool wake = false;

  /* Lock is taken in caller.  */

  while (1)
    {
      /* Make sure we are not holding the rwlock as a writer.  This is
	 a deadlock situation we recognize and report.  */
      if (__builtin_expect (rwlock->__data.__writer
			    == THREAD_GETMEM (THREAD_SELF, tid), 0))
	{
	  result = EDEADLK;
	  break;
	}

      /* Remember that we are a reader.  */
      if (__glibc_unlikely (++rwlock->__data.__nr_readers_queued == 0))
	{
	  /* Overflow on number of queued readers.  */
	  --rwlock->__data.__nr_readers_queued;
	  result = EAGAIN;
	  break;
	}

      int waitval = rwlock->__data.__readers_wakeup;

      /* Free the lock.  */
      lll_unlock (rwlock->__data.__lock, rwlock->__data.__shared);

      /* Wait for the writer to finish.  */
      lll_futex_wait (&rwlock->__data.__readers_wakeup, waitval,
		      rwlock->__data.__shared);

      /* Get the lock.  */
      lll_lock (rwlock->__data.__lock, rwlock->__data.__shared);

      --rwlock->__data.__nr_readers_queued;

      /* Get the rwlock if there is no writer...  */
      if (rwlock->__data.__writer == 0
	  /* ...and if either no writer is waiting or we prefer readers.  */
	  && (!rwlock->__data.__nr_writers_queued
	      || PTHREAD_RWLOCK_PREFER_READER_P (rwlock)))
	{
	  /* Increment the reader counter.  Avoid overflow.  */
	  if (__glibc_unlikely (++rwlock->__data.__nr_readers == 0))
	    {
	      /* Overflow on number of readers.	 */
	      --rwlock->__data.__nr_readers;
	      result = EAGAIN;
	    }
	  else
	    {
	      LIBC_PROBE (rdlock_acquire_read, 1, rwlock);
	      /* See pthread_rwlock_rdlock.  */
	      if (rwlock->__data.__nr_readers == 1
		  && rwlock->__data.__nr_readers_queued > 0
		  && rwlock->__data.__nr_writers_queued > 0)
		{
		  ++rwlock->__data.__readers_wakeup;
		  wake = true;
		}
	    }

	  break;
	}
    }

  /* We are done, free the lock.  */
  lll_unlock (rwlock->__data.__lock, rwlock->__data.__shared);

  if (wake)
    lll_futex_wake (&rwlock->__data.__readers_wakeup, INT_MAX,
		    rwlock->__data.__shared);

  return result;
}


/* Fast path of acquiring read lock on RWLOCK.  */

int
__pthread_rwlock_rdlock (pthread_rwlock_t *rwlock)
{
  int result = 0;
  bool wake = false;

  LIBC_PROBE (rdlock_entry, 1, rwlock);

  if (ELIDE_LOCK (rwlock->__data.__rwelision,
		  rwlock->__data.__lock == 0
		  && rwlock->__data.__writer == 0
		  && rwlock->__data.__nr_readers == 0))
    return 0;

  /* Make sure we are alone.  */
  lll_lock (rwlock->__data.__lock, rwlock->__data.__shared);

  /* Get the rwlock if there is no writer...  */
  if (rwlock->__data.__writer == 0
      /* ...and if either no writer is waiting or we prefer readers.  */
      && (!rwlock->__data.__nr_writers_queued
	  || PTHREAD_RWLOCK_PREFER_READER_P (rwlock)))
    {
      /* Increment the reader counter.  Avoid overflow.  */
      if (__glibc_unlikely (++rwlock->__data.__nr_readers == 0))
	{
	  /* Overflow on number of readers.	 */
	  --rwlock->__data.__nr_readers;
	  result = EAGAIN;
	}
      else
	{
	  LIBC_PROBE (rdlock_acquire_read, 1, rwlock);
	  /* If we are the first reader, and there are blocked readers and
	     writers (which we don't prefer, see above), then it can be the
	     case that we stole the lock from a writer that was already woken
	     to acquire it.  That means that we need to take over the writer's
	     responsibility to wake all readers (see pthread_rwlock_unlock).
	     Thus, wake all readers in this case.  */
	  if (rwlock->__data.__nr_readers == 1
	      && rwlock->__data.__nr_readers_queued > 0
	      && rwlock->__data.__nr_writers_queued > 0)
	    {
	      ++rwlock->__data.__readers_wakeup;
	      wake = true;
	    }
	}

      /* We are done, free the lock.  */
      lll_unlock (rwlock->__data.__lock, rwlock->__data.__shared);

      if (wake)
	lll_futex_wake (&rwlock->__data.__readers_wakeup, INT_MAX,
			rwlock->__data.__shared);

      return result;
    }

  return __pthread_rwlock_rdlock_slow (rwlock);
}

weak_alias (__pthread_rwlock_rdlock, pthread_rwlock_rdlock)
hidden_def (__pthread_rwlock_rdlock)