4 # Copyright (C) 2006, 2007 Google Inc.
6 # This program is free software; you can redistribute it and/or modify
7 # it under the terms of the GNU General Public License as published by
8 # the Free Software Foundation; either version 2 of the License, or
9 # (at your option) any later version.
11 # This program is distributed in the hope that it will be useful, but
12 # WITHOUT ANY WARRANTY; without even the implied warranty of
13 # MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
14 # General Public License for more details.
16 # You should have received a copy of the GNU General Public License
17 # along with this program; if not, write to the Free Software
18 # Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
21 """Module implementing the Ganeti locking code."""
23 # pylint: disable-msg=W0613,W0201
26 # Wouldn't it be better to define LockingError in the locking module?
27 # Well, for now that's how the rest of the code does it...
28 from ganeti import errors
32 """Implements a shared lock.
34 Multiple threads can acquire the lock in a shared way, calling
35 acquire_shared(). In order to acquire the lock in an exclusive way threads
36 can call acquire_exclusive().
38 The lock prevents starvation but does not guarantee that threads will acquire
39 the shared lock in the order they queued for it, just that they will
44 """Construct a new SharedLock"""
45 # we have two conditions, c_shr and c_exc, sharing the same lock.
46 self.__lock = threading.Lock()
47 self.__turn_shr = threading.Condition(self.__lock)
48 self.__turn_exc = threading.Condition(self.__lock)
50 # current lock holders
58 # is this lock in the deleted state?
59 self.__deleted = False
61 def __is_sharer(self):
62 """Is the current thread sharing the lock at this time?"""
63 return threading.currentThread() in self.__shr
65 def __is_exclusive(self):
66 """Is the current thread holding the lock exclusively at this time?"""
67 return threading.currentThread() == self.__exc
69 def __is_owned(self, shared=-1):
70 """Is the current thread somehow owning the lock at this time?
72 This is a private version of the function, which presumes you're holding
77 return self.__is_sharer() or self.__is_exclusive()
79 return self.__is_sharer()
81 return self.__is_exclusive()
83 def _is_owned(self, shared=-1):
84 """Is the current thread somehow owning the lock at this time?
88 < 0: check for any type of ownership (default)
89 0: check for exclusive ownership
90 > 0: check for shared ownership
95 result = self.__is_owned(shared)
102 """Wait on the given condition, and raise an exception if the current lock
103 is declared deleted in the meantime.
106 c: condition to wait on
111 raise errors.LockError('deleted lock')
113 def __exclusive_acquire(self):
114 """Acquire the lock exclusively.
116 This is a private function that presumes you are already holding the
117 internal lock. It's defined separately to avoid code duplication between
118 acquire() and delete()
121 self.__nwait_exc += 1
123 # This is to save ourselves from a nasty race condition that could
124 # theoretically make the sharers starve.
125 if self.__nwait_shr > 0 or self.__nwait_exc > 1:
126 self.__wait(self.__turn_exc)
128 while len(self.__shr) > 0 or self.__exc is not None:
129 self.__wait(self.__turn_exc)
131 self.__exc = threading.currentThread()
133 self.__nwait_exc -= 1
136 def acquire(self, blocking=1, shared=0):
137 """Acquire a shared lock.
140 shared: whether to acquire in shared mode. By default an exclusive lock
142 blocking: whether to block while trying to acquire or to operate in try-lock mode.
143 this locking mode is not supported yet.
147 # We don't have non-blocking mode for now
148 raise NotImplementedError
150 self.__lock.acquire()
153 raise errors.LockError('deleted lock')
155 # We cannot acquire the lock if we already have it
156 assert not self.__is_owned(), "double acquire() on a non-recursive lock"
159 self.__nwait_shr += 1
161 # If there is an exclusive holder waiting we have to wait. We'll
162 # only do this once, though, when we start waiting for the lock. Then
163 # we'll just wait while there are no exclusive holders.
164 if self.__nwait_exc > 0:
165 # TODO: if !blocking...
166 self.__wait(self.__turn_shr)
168 while self.__exc is not None:
169 # TODO: if !blocking...
170 self.__wait(self.__turn_shr)
172 self.__shr.add(threading.currentThread())
174 self.__nwait_shr -= 1
177 # TODO: if !blocking...
178 # (or modify __exclusive_acquire for non-blocking mode)
179 self.__exclusive_acquire()
182 self.__lock.release()
187 """Release a Shared Lock.
189 You must have acquired the lock, either in shared or in exclusive mode,
190 before calling this function.
193 self.__lock.acquire()
195 # Autodetect release type
196 if self.__is_exclusive():
199 # An exclusive holder has just had the lock, time to put it in shared
200 # mode if there are shared holders waiting. Otherwise wake up the next
202 if self.__nwait_shr > 0:
203 self.__turn_shr.notifyAll()
204 elif self.__nwait_exc > 0:
205 self.__turn_exc.notify()
207 elif self.__is_sharer():
208 self.__shr.remove(threading.currentThread())
210 # If there are shared holders waiting there *must* be an exclusive holder
211 # waiting as well; otherwise what were they waiting for?
212 assert (self.__nwait_shr == 0 or self.__nwait_exc > 0,
213 "Lock sharers waiting while no exclusive is queueing")
215 # If there are no more shared holders and some exclusive holders are
216 # waiting let's wake one up.
217 if len(self.__shr) == 0 and self.__nwait_exc > 0:
218 self.__turn_exc.notify()
221 assert False, "Cannot release non-owned lock"
224 self.__lock.release()
226 def delete(self, blocking=1):
227 """Delete a Shared Lock.
229 This operation will declare the lock for removal. First the lock will be
230 acquired in exclusive mode if you don't already own it, then the lock
231 will be put in a state where any future and pending acquire() fail.
234 blocking: whether to block while trying to acquire or to operate in
235 try-lock mode. this locking mode is not supported yet unless
236 you are already holding exclusively the lock.
239 self.__lock.acquire()
241 assert not self.__is_sharer(), "cannot delete() a lock while sharing it"
244 raise errors.LockError('deleted lock')
246 if not self.__is_exclusive():
248 # We don't have non-blocking mode for now
249 raise NotImplementedError
250 self.__exclusive_acquire()
252 self.__deleted = True
254 # Wake up everybody, they will fail acquiring the lock and
255 # raise an exception instead.
256 self.__turn_exc.notifyAll()
257 self.__turn_shr.notifyAll()
260 self.__lock.release()
264 """Implements a set of locks.
266 This abstraction implements a set of shared locks for the same resource type,
267 distinguished by name. The user can lock a subset of the resources and the
268 LockSet will take care of acquiring the locks always in the same order, thus
271 All the locks needed in the same set must be acquired together, though.
274 def __init__(self, members=None):
275 """Constructs a new LockSet.
278 members: initial members of the set
281 # Used internally to guarantee coherency.
282 self.__lock = SharedLock()
284 # The lockdict indexes the relationship name -> lock
285 # The order-of-locking is implied by the alphabetical order of names
288 if members is not None:
290 self.__lockdict[name] = SharedLock()
292 # The owner dict contains the set of locks each thread owns. For
293 # performance each thread can access its own key without a global lock on
294 # this structure. It is paramount though that *no* other type of access is
295 # done to this structure (eg. no looping over its keys). *_owner helper
296 # function are defined to guarantee access is correct, but in general never
297 # do anything different than __owners[threading.currentThread()], or there
302 """Is the current thread a current level owner?"""
303 return threading.currentThread() in self.__owners
305 def _add_owned(self, name):
306 """Note the current thread owns the given lock"""
308 self.__owners[threading.currentThread()].add(name)
310 self.__owners[threading.currentThread()] = set([name])
312 def _del_owned(self, name):
313 """Note the current thread owns the given lock"""
314 self.__owners[threading.currentThread()].remove(name)
316 if not self.__owners[threading.currentThread()]:
317 del self.__owners[threading.currentThread()]
319 def _list_owned(self):
320 """Get the set of resource names owned by the current thread"""
322 return self.__owners[threading.currentThread()].copy()
327 """Return the current set of names.
329 Only call this function while holding __lock and don't iterate on the
330 result after releasing the lock.
333 return set(self.__lockdict.keys())
336 """Return a copy of the current set of elements.
338 Used only for debugging purposes.
340 self.__lock.acquire(shared=1)
342 result = self.__names()
344 self.__lock.release()
347 def acquire(self, names, blocking=1, shared=0):
348 """Acquire a set of resource locks.
351 names: the names of the locks which shall be acquired.
352 (special lock names, or instance/node names)
353 shared: whether to acquire in shared mode. By default an exclusive lock
355 blocking: whether to block while trying to acquire or to operate in try-lock mode.
356 this locking mode is not supported yet.
359 True: when all the locks are successfully acquired
362 errors.LockError: when any lock we try to acquire has been deleted
363 before we succeed. In this case none of the locks requested will be
368 # We don't have non-blocking mode for now
369 raise NotImplementedError
371 # Check we don't already own locks at this level
372 assert not self._is_owned(), "Cannot acquire locks in the same set twice"
374 # Support passing in a single resource to acquire rather than many
375 if isinstance(names, basestring):
381 # First we look the locks up on __lockdict. We have no way of being sure
382 # they will still be there after, but this makes it a lot faster should
383 # just one of them be the already wrong
386 lock = self.__lockdict[lname] # raises KeyError if the lock is not there
387 acquire_list.append((lname, lock))
389 raise errors.LockError('non-existing lock in set (%s)' % lname)
391 # Now acquire_list contains a sorted list of resources and locks we want.
392 # In order to get them we loop on this (private) list and acquire() them.
393 # We gave no real guarantee they will still exist till this is done but
394 # .acquire() itself is safe and will alert us if the lock gets deleted.
396 for (lname, lock) in acquire_list:
397 lock.acquire(shared=shared) # raises LockError if the lock is deleted
399 # now the lock cannot be deleted, we have it!
400 self._add_owned(lname)
402 # We shouldn't have problems adding the lock to the owners list, but
403 # if we did we'll try to release this lock and re-raise exception.
404 # Of course something is going to be really wrong, after this.
408 except (errors.LockError):
410 for lname in self._list_owned():
411 self.__lockdict[lname].release()
412 self._del_owned(lname)
413 raise errors.LockError('non-existing lock in set (%s)' % name_fail)
417 def release(self, names=None):
418 """Release a set of resource locks, at the same level.
420 You must have acquired the locks, either in shared or in exclusive mode,
421 before releasing them.
424 names: the names of the locks which shall be released.
425 (defaults to all the locks acquired at that level).
429 assert self._is_owned(), "release() on lock set while not owner"
431 # Support passing in a single resource to release rather than many
432 if isinstance(names, basestring):
436 names = self._list_owned()
439 assert self._list_owned().issuperset(names), (
440 "release() on unheld resources %s" %
441 names.difference(self._list_owned()))
443 for lockname in names:
444 # If we are sure the lock doesn't leave __lockdict without being
445 # exclusively held we can do this...
446 self.__lockdict[lockname].release()
447 self._del_owned(lockname)
449 def add(self, names, acquired=0, shared=0):
450 """Add a new set of elements to the set
453 names: names of the new elements to add
454 acquired: pre-acquire the new resource?
455 shared: is the pre-acquisition shared?
458 # Support passing in a single resource to add rather than many
459 if isinstance(names, basestring):
462 # Acquire the internal lock in an exclusive way, so there cannot be a
464 self.__lock.acquire()
466 invalid_names = self.__names().intersection(names)
468 # This must be an explicit raise, not an assert, because assert is
469 # turned off when using optimization, and this can happen because of
470 # concurrency even if the user doesn't want it.
471 raise errors.LockError("duplicate add() (%s)" % invalid_names)
473 for lockname in names:
477 lock.acquire(shared=shared)
478 # now the lock cannot be deleted, we have it!
480 self._add_owned(lockname)
482 # We shouldn't have problems adding the lock to the owners list,
483 # but if we did we'll try to release this lock and re-raise
484 # exception. Of course something is going to be really wrong,
485 # after this. On the other hand the lock hasn't been added to the
486 # __lockdict yet so no other threads should be pending on it. This
487 # release is just a safety measure.
491 self.__lockdict[lockname] = lock
494 self.__lock.release()
498 def remove(self, names, blocking=1):
499 """Remove elements from the lock set.
501 You can either not hold anything in the lockset or already hold a superset
502 of the elements you want to delete, exclusively.
505 names: names of the resource to remove.
506 blocking: whether to block while trying to acquire or to operate in
507 try-lock mode. this locking mode is not supported yet unless
508 you are already holding exclusively the locks.
511 A list of lock which we failed to delete. The list is always empty if we
512 were holding all the locks exclusively.
515 if not blocking and not self._is_owned():
516 # We don't have non-blocking mode for now
517 raise NotImplementedError
519 # Support passing in a single resource to remove rather than many
520 if isinstance(names, basestring):
523 # If we own any subset of this lock it must be a superset of what we want
524 # to delete. The ownership must also be exclusive, but that will be checked
525 # by the lock itself.
526 assert not self._is_owned() or self._list_owned().issuperset(names), (
527 "remove() on acquired lockset while not owning all elements")
532 # Calling delete() acquires the lock exclusively if we don't already own
533 # it, and causes all pending and subsequent lock acquires to fail. It's
534 # fine to call it out of order because delete() also implies release(),
535 # and the assertion above guarantees that if we either already hold
536 # everything we want to delete, or we hold none.
538 self.__lockdict[lname].delete()
539 except (KeyError, errors.LockError):
540 delete_failed.append(lname)
541 # This cannot happen if we were already holding it, verify:
542 assert not self._is_owned(), "remove failed while holding lockset"
544 # If no LockError was raised we are the ones who deleted the lock.
545 # This means we can safely remove it from lockdict, as any further or
546 # pending delete() or acquire() will fail (and nobody can have the lock
547 # since before our call to delete()).
549 # This is done in an else clause because if the exception was thrown
550 # it's the job of the one who actually deleted it.
551 del self.__lockdict[lname]
552 # And let's remove it from our private list if we owned it.
554 self._del_owned(lname)