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1 | 162c1c1f | Guido Trotter | #
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2 | 162c1c1f | Guido Trotter | #
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3 | 162c1c1f | Guido Trotter | |
4 | 162c1c1f | Guido Trotter | # Copyright (C) 2006, 2007 Google Inc.
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5 | 162c1c1f | Guido Trotter | #
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6 | 162c1c1f | Guido Trotter | # This program is free software; you can redistribute it and/or modify
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7 | 162c1c1f | Guido Trotter | # it under the terms of the GNU General Public License as published by
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8 | 162c1c1f | Guido Trotter | # the Free Software Foundation; either version 2 of the License, or
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9 | 162c1c1f | Guido Trotter | # (at your option) any later version.
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10 | 162c1c1f | Guido Trotter | #
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11 | 162c1c1f | Guido Trotter | # This program is distributed in the hope that it will be useful, but
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12 | 162c1c1f | Guido Trotter | # WITHOUT ANY WARRANTY; without even the implied warranty of
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13 | 162c1c1f | Guido Trotter | # MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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14 | 162c1c1f | Guido Trotter | # General Public License for more details.
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15 | 162c1c1f | Guido Trotter | #
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16 | 162c1c1f | Guido Trotter | # You should have received a copy of the GNU General Public License
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17 | 162c1c1f | Guido Trotter | # along with this program; if not, write to the Free Software
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18 | 162c1c1f | Guido Trotter | # Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
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19 | 162c1c1f | Guido Trotter | # 02110-1301, USA.
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20 | 162c1c1f | Guido Trotter | |
21 | 162c1c1f | Guido Trotter | """Module implementing the Ganeti locking code."""
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22 | 162c1c1f | Guido Trotter | |
23 | 162c1c1f | Guido Trotter | # pylint: disable-msg=W0613,W0201
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24 | 162c1c1f | Guido Trotter | |
25 | 162c1c1f | Guido Trotter | import threading |
26 | 162c1c1f | Guido Trotter | |
27 | 162c1c1f | Guido Trotter | |
28 | 162c1c1f | Guido Trotter | class SharedLock: |
29 | 162c1c1f | Guido Trotter | """Implements a shared lock.
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30 | 162c1c1f | Guido Trotter |
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31 | 162c1c1f | Guido Trotter | Multiple threads can acquire the lock in a shared way, calling
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32 | 162c1c1f | Guido Trotter | acquire_shared(). In order to acquire the lock in an exclusive way threads
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33 | 162c1c1f | Guido Trotter | can call acquire_exclusive().
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34 | 162c1c1f | Guido Trotter |
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35 | 162c1c1f | Guido Trotter | The lock prevents starvation but does not guarantee that threads will acquire
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36 | 162c1c1f | Guido Trotter | the shared lock in the order they queued for it, just that they will
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37 | 162c1c1f | Guido Trotter | eventually do so.
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38 | 162c1c1f | Guido Trotter |
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39 | 162c1c1f | Guido Trotter | """
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40 | 162c1c1f | Guido Trotter | def __init__(self): |
41 | 162c1c1f | Guido Trotter | """Construct a new Shared Lock"""
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42 | 162c1c1f | Guido Trotter | # we have two conditions, c_shr and c_exc, sharing the same lock.
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43 | 162c1c1f | Guido Trotter | self.__lock = threading.Lock()
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44 | 162c1c1f | Guido Trotter | self.__turn_shr = threading.Condition(self.__lock) |
45 | 162c1c1f | Guido Trotter | self.__turn_exc = threading.Condition(self.__lock) |
46 | 162c1c1f | Guido Trotter | |
47 | 162c1c1f | Guido Trotter | # current lock holders
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48 | 162c1c1f | Guido Trotter | self.__shr = set() |
49 | 162c1c1f | Guido Trotter | self.__exc = None |
50 | 162c1c1f | Guido Trotter | |
51 | 162c1c1f | Guido Trotter | # lock waiters
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52 | 162c1c1f | Guido Trotter | self.__nwait_exc = 0 |
53 | 162c1c1f | Guido Trotter | self.__nwait_shr = 0 |
54 | 162c1c1f | Guido Trotter | |
55 | 162c1c1f | Guido Trotter | def __is_sharer(self): |
56 | 162c1c1f | Guido Trotter | """Is the current thread sharing the lock at this time?"""
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57 | 162c1c1f | Guido Trotter | return threading.currentThread() in self.__shr |
58 | 162c1c1f | Guido Trotter | |
59 | 162c1c1f | Guido Trotter | def __is_exclusive(self): |
60 | 162c1c1f | Guido Trotter | """Is the current thread holding the lock exclusively at this time?"""
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61 | 162c1c1f | Guido Trotter | return threading.currentThread() == self.__exc |
62 | 162c1c1f | Guido Trotter | |
63 | 162c1c1f | Guido Trotter | def __is_owned(self, shared=-1): |
64 | 162c1c1f | Guido Trotter | """Is the current thread somehow owning the lock at this time?
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65 | 162c1c1f | Guido Trotter |
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66 | 162c1c1f | Guido Trotter | This is a private version of the function, which presumes you're holding
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67 | 162c1c1f | Guido Trotter | the internal lock.
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68 | 162c1c1f | Guido Trotter |
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69 | 162c1c1f | Guido Trotter | """
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70 | 162c1c1f | Guido Trotter | if shared < 0: |
71 | 162c1c1f | Guido Trotter | return self.__is_sharer() or self.__is_exclusive() |
72 | 162c1c1f | Guido Trotter | elif shared:
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73 | 162c1c1f | Guido Trotter | return self.__is_sharer() |
74 | 162c1c1f | Guido Trotter | else:
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75 | 162c1c1f | Guido Trotter | return self.__is_exclusive() |
76 | 162c1c1f | Guido Trotter | |
77 | 162c1c1f | Guido Trotter | def _is_owned(self, shared=-1): |
78 | 162c1c1f | Guido Trotter | """Is the current thread somehow owning the lock at this time?
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79 | 162c1c1f | Guido Trotter |
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80 | 162c1c1f | Guido Trotter | Args:
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81 | 162c1c1f | Guido Trotter | shared:
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82 | 162c1c1f | Guido Trotter | < 0: check for any type of ownership (default)
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83 | 162c1c1f | Guido Trotter | 0: check for exclusive ownership
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84 | 162c1c1f | Guido Trotter | > 0: check for shared ownership
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85 | 162c1c1f | Guido Trotter |
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86 | 162c1c1f | Guido Trotter | """
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87 | 162c1c1f | Guido Trotter | self.__lock.acquire()
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88 | 162c1c1f | Guido Trotter | try:
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89 | 162c1c1f | Guido Trotter | result = self.__is_owned(shared)
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90 | 162c1c1f | Guido Trotter | finally:
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91 | 162c1c1f | Guido Trotter | self.__lock.release()
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92 | 162c1c1f | Guido Trotter | |
93 | 162c1c1f | Guido Trotter | return result
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94 | 162c1c1f | Guido Trotter | |
95 | 162c1c1f | Guido Trotter | def acquire(self, blocking=1, shared=0): |
96 | 162c1c1f | Guido Trotter | """Acquire a shared lock.
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97 | 162c1c1f | Guido Trotter |
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98 | 162c1c1f | Guido Trotter | Args:
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99 | 162c1c1f | Guido Trotter | shared: whether to acquire in shared mode. By default an exclusive lock
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100 | 162c1c1f | Guido Trotter | will be acquired.
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101 | 162c1c1f | Guido Trotter | blocking: whether to block while trying to acquire or to operate in try-lock mode.
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102 | 162c1c1f | Guido Trotter | this locking mode is not supported yet.
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103 | 162c1c1f | Guido Trotter |
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104 | 162c1c1f | Guido Trotter | """
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105 | 162c1c1f | Guido Trotter | if not blocking: |
106 | 162c1c1f | Guido Trotter | # We don't have non-blocking mode for now
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107 | 162c1c1f | Guido Trotter | raise NotImplementedError |
108 | 162c1c1f | Guido Trotter | |
109 | 162c1c1f | Guido Trotter | self.__lock.acquire()
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110 | 162c1c1f | Guido Trotter | try:
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111 | 162c1c1f | Guido Trotter | # We cannot acquire the lock if we already have it
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112 | 162c1c1f | Guido Trotter | assert not self.__is_owned(), "double acquire() on a non-recursive lock" |
113 | 162c1c1f | Guido Trotter | |
114 | 162c1c1f | Guido Trotter | if shared:
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115 | 162c1c1f | Guido Trotter | self.__nwait_shr += 1 |
116 | 162c1c1f | Guido Trotter | try:
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117 | 162c1c1f | Guido Trotter | # If there is an exclusive holder waiting we have to wait. We'll
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118 | 162c1c1f | Guido Trotter | # only do this once, though, when we start waiting for the lock. Then
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119 | 162c1c1f | Guido Trotter | # we'll just wait while there are no exclusive holders.
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120 | 162c1c1f | Guido Trotter | if self.__nwait_exc > 0: |
121 | 162c1c1f | Guido Trotter | # TODO: if !blocking...
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122 | 162c1c1f | Guido Trotter | self.__turn_shr.wait()
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123 | 162c1c1f | Guido Trotter | |
124 | 162c1c1f | Guido Trotter | while self.__exc is not None: |
125 | 162c1c1f | Guido Trotter | # TODO: if !blocking...
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126 | 162c1c1f | Guido Trotter | self.__turn_shr.wait()
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127 | 162c1c1f | Guido Trotter | |
128 | 162c1c1f | Guido Trotter | self.__shr.add(threading.currentThread())
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129 | 162c1c1f | Guido Trotter | finally:
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130 | 162c1c1f | Guido Trotter | self.__nwait_shr -= 1 |
131 | 162c1c1f | Guido Trotter | |
132 | 162c1c1f | Guido Trotter | else:
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133 | 162c1c1f | Guido Trotter | self.__nwait_exc += 1 |
134 | 162c1c1f | Guido Trotter | try:
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135 | 162c1c1f | Guido Trotter | # This is to save ourselves from a nasty race condition that could
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136 | 162c1c1f | Guido Trotter | # theoretically make the sharers starve.
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137 | 162c1c1f | Guido Trotter | if self.__nwait_shr > 0 or self.__nwait_exc > 1: |
138 | 162c1c1f | Guido Trotter | # TODO: if !blocking...
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139 | 162c1c1f | Guido Trotter | self.__turn_exc.wait()
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140 | 162c1c1f | Guido Trotter | |
141 | 162c1c1f | Guido Trotter | while len(self.__shr) > 0 or self.__exc is not None: |
142 | 162c1c1f | Guido Trotter | # TODO: if !blocking...
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143 | 162c1c1f | Guido Trotter | self.__turn_exc.wait()
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144 | 162c1c1f | Guido Trotter | |
145 | 162c1c1f | Guido Trotter | self.__exc = threading.currentThread()
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146 | 162c1c1f | Guido Trotter | finally:
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147 | 162c1c1f | Guido Trotter | self.__nwait_exc -= 1 |
148 | 162c1c1f | Guido Trotter | |
149 | 162c1c1f | Guido Trotter | finally:
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150 | 162c1c1f | Guido Trotter | self.__lock.release()
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151 | 162c1c1f | Guido Trotter | |
152 | 162c1c1f | Guido Trotter | return True |
153 | 162c1c1f | Guido Trotter | |
154 | 162c1c1f | Guido Trotter | def release(self): |
155 | 162c1c1f | Guido Trotter | """Release a Shared Lock.
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156 | 162c1c1f | Guido Trotter |
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157 | 162c1c1f | Guido Trotter | You must have acquired the lock, either in shared or in exclusive mode,
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158 | 162c1c1f | Guido Trotter | before calling this function.
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159 | 162c1c1f | Guido Trotter |
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160 | 162c1c1f | Guido Trotter | """
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161 | 162c1c1f | Guido Trotter | self.__lock.acquire()
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162 | 162c1c1f | Guido Trotter | try:
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163 | 162c1c1f | Guido Trotter | # Autodetect release type
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164 | 162c1c1f | Guido Trotter | if self.__is_exclusive(): |
165 | 162c1c1f | Guido Trotter | self.__exc = None |
166 | 162c1c1f | Guido Trotter | |
167 | 162c1c1f | Guido Trotter | # An exclusive holder has just had the lock, time to put it in shared
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168 | 162c1c1f | Guido Trotter | # mode if there are shared holders waiting. Otherwise wake up the next
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169 | 162c1c1f | Guido Trotter | # exclusive holder.
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170 | 162c1c1f | Guido Trotter | if self.__nwait_shr > 0: |
171 | 162c1c1f | Guido Trotter | self.__turn_shr.notifyAll()
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172 | 162c1c1f | Guido Trotter | elif self.__nwait_exc > 0: |
173 | 162c1c1f | Guido Trotter | self.__turn_exc.notify()
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174 | 162c1c1f | Guido Trotter | |
175 | 162c1c1f | Guido Trotter | elif self.__is_sharer(): |
176 | 162c1c1f | Guido Trotter | self.__shr.remove(threading.currentThread())
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177 | 162c1c1f | Guido Trotter | |
178 | 162c1c1f | Guido Trotter | # If there are shared holders waiting there *must* be an exclusive holder
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179 | 162c1c1f | Guido Trotter | # waiting as well; otherwise what were they waiting for?
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180 | 162c1c1f | Guido Trotter | assert (self.__nwait_shr == 0 or self.__nwait_exc > 0, |
181 | 162c1c1f | Guido Trotter | "Lock sharers waiting while no exclusive is queueing")
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182 | 162c1c1f | Guido Trotter | |
183 | 162c1c1f | Guido Trotter | # If there are no more shared holders and some exclusive holders are
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184 | 162c1c1f | Guido Trotter | # waiting let's wake one up.
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185 | 162c1c1f | Guido Trotter | if len(self.__shr) == 0 and self.__nwait_exc > 0: |
186 | 162c1c1f | Guido Trotter | self.__turn_exc.notify()
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187 | 162c1c1f | Guido Trotter | |
188 | 162c1c1f | Guido Trotter | else:
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189 | 162c1c1f | Guido Trotter | assert False, "Cannot release non-owned lock" |
190 | 162c1c1f | Guido Trotter | |
191 | 162c1c1f | Guido Trotter | finally:
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192 | 162c1c1f | Guido Trotter | self.__lock.release()
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