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
|
/* Copyright (C) 2003-2014 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 <lowlevellock.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. */
*once_control = 0;
lll_futex_wake (once_control, INT_MAX, LLL_PRIVATE);
}
/* This is similar to a lock implementation, but we distinguish between three
states: not yet initialized (0), initialization finished (2), and
initialization in progress (__fork_generation | 1). 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. */
int
__pthread_once (once_control, init_routine)
pthread_once_t *once_control;
void (*init_routine) (void);
{
while (1)
{
int oldval, val, newval;
/* We need acquire memory order for this load because if the value
signals that initialization has finished, we need to be see any
data modifications done during initialization. */
val = *once_control;
atomic_read_barrier();
do
{
/* Check if the initialization has already been done. */
if (__glibc_likely ((val & 2) != 0))
return 0;
oldval = val;
/* 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 | 1;
/* We need acquire memory order here for the same reason as for the
load from once_control above. */
val = atomic_compare_and_exchange_val_acq (once_control, newval,
oldval);
}
while (__glibc_unlikely (val != oldval));
/* Check if another thread already runs the initializer. */
if ((oldval & 1) != 0)
{
/* Check whether the initializer execution was interrupted by a
fork. We know that for both values, bit 0 is set and bit 1 is
not. */
if (oldval == newval)
{
/* Same generation, some other thread was faster. Wait. */
lll_futex_wait (once_control, newval, LLL_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_write_barrier();
*once_control = 2;
/* Wake up all other threads. */
lll_futex_wake (once_control, INT_MAX, LLL_PRIVATE);
break;
}
return 0;
}
weak_alias (__pthread_once, pthread_once)
hidden_def (__pthread_once)
|
