summary refs log tree commit diff
path: root/nptl/pthread_once.c
blob: beaeb79ef8afe76af809d01e45851300a391f627 (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
/* Copyright (C) 2003-2016 Free Software Foundation, Inc.
   This file is part of the GNU C Library.
   Contributed by Jakub Jelinek <jakub@redhat.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 "pthreadP.h"
#include <futex-internal.h>
#include <atomic.h>


unsigned long int __fork_generation attribute_hidden;


static void
clear_once_control (void *arg)
{
  pthread_once_t *once_control = (pthread_once_t *) arg;

  /* Reset to the uninitialized state here.  We don't need a stronger memory
     order because we do not need to make any other of our writes visible to
     other threads that see this value: This function will be called if we
     get interrupted (see __pthread_once), so all we need to relay to other
     threads is the state being reset again.  */
  atomic_store_relaxed (once_control, 0);
  futex_wake ((unsigned int *) once_control, INT_MAX, FUTEX_PRIVATE);
}


/* This is similar to a lock implementation, but we distinguish between three
   states: not yet initialized (0), initialization in progress
   (__fork_generation | __PTHREAD_ONCE_INPROGRESS), and initialization
   finished (__PTHREAD_ONCE_DONE); __fork_generation does not use the bits
   that are used for __PTHREAD_ONCE_INPROGRESS and __PTHREAD_ONCE_DONE (which
   is what __PTHREAD_ONCE_FORK_GEN_INCR is used for).  If in the first state,
   threads will try to run the initialization by moving to the second state;
   the first thread to do so via a CAS on once_control runs init_routine,
   other threads block.
   When forking the process, some threads can be interrupted during the second
   state; they won't be present in the forked child, so we need to restart
   initialization in the child.  To distinguish an in-progress initialization
   from an interrupted initialization (in which case we need to reclaim the
   lock), we look at the fork generation that's part of the second state: We
   can reclaim iff it differs from the current fork generation.
   XXX: This algorithm has an ABA issue on the fork generation: If an
   initialization is interrupted, we then fork 2^30 times (30 bits of
   once_control are used for the fork generation), and try to initialize
   again, we can deadlock because we can't distinguish the in-progress and
   interrupted cases anymore.
   XXX: We split out this slow path because current compilers do not generate
   as efficient code when the fast path in __pthread_once below is not in a
   separate function.  */
static int
__attribute__ ((noinline))
__pthread_once_slow (pthread_once_t *once_control, void (*init_routine) (void))
{
  while (1)
    {
      int val, newval;

      /* We need acquire memory order for this load because if the value
         signals that initialization has finished, we need to see any
         data modifications done during initialization.  */
      val = atomic_load_acquire (once_control);
      do
	{
	  /* Check if the initialization has already been done.  */
	  if (__glibc_likely ((val & __PTHREAD_ONCE_DONE) != 0))
	    return 0;

	  /* We try to set the state to in-progress and having the current
	     fork generation.  We don't need atomic accesses for the fork
	     generation because it's immutable in a particular process, and
	     forked child processes start with a single thread that modified
	     the generation.  */
	  newval = __fork_generation | __PTHREAD_ONCE_INPROGRESS;
	  /* We need acquire memory order here for the same reason as for the
	     load from once_control above.  */
	}
      while (__glibc_unlikely (!atomic_compare_exchange_weak_acquire (
	  once_control, &val, newval)));

      /* Check if another thread already runs the initializer.	*/
      if ((val & __PTHREAD_ONCE_INPROGRESS) != 0)
	{
	  /* Check whether the initializer execution was interrupted by a
	     fork.  We know that for both values, __PTHREAD_ONCE_INPROGRESS
	     is set and __PTHREAD_ONCE_DONE is not.  */
	  if (val == newval)
	    {
	      /* Same generation, some other thread was faster.  Wait and
		 retry.  */
	      futex_wait_simple ((unsigned int *) once_control,
				 (unsigned int) newval, FUTEX_PRIVATE);
	      continue;
	    }
	}

      /* This thread is the first here.  Do the initialization.
	 Register a cleanup handler so that in case the thread gets
	 interrupted the initialization can be restarted.  */
      pthread_cleanup_push (clear_once_control, once_control);

      init_routine ();

      pthread_cleanup_pop (0);


      /* Mark *once_control as having finished the initialization.  We need
         release memory order here because we need to synchronize with other
         threads that want to use the initialized data.  */
      atomic_store_release (once_control, __PTHREAD_ONCE_DONE);

      /* Wake up all other threads.  */
      futex_wake ((unsigned int *) once_control, INT_MAX, FUTEX_PRIVATE);
      break;
    }

  return 0;
}

int
__pthread_once (pthread_once_t *once_control, void (*init_routine) (void))
{
  /* Fast path.  See __pthread_once_slow.  */
  int val;
  val = atomic_load_acquire (once_control);
  if (__glibc_likely ((val & __PTHREAD_ONCE_DONE) != 0))
    return 0;
  else
    return __pthread_once_slow (once_control, init_routine);
}
weak_alias (__pthread_once, pthread_once)
hidden_def (__pthread_once)