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Guido van Rossum 1994-05-16 09:35:22 +00:00
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69
Demo/threads/bug.py Normal file
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# The following self-contained little program usually freezes with most
# threads reporting
#
# Unhandled exception in thread:
# Traceback (innermost last):
# File "importbug.py", line 6
# x = whrandom.randint(1,3)
# AttributeError: randint
#
# Here's the program; it doesn't use anything from the attached module:
import thread
def task():
global N
import whrandom
x = whrandom.randint(1,3)
a.acquire()
N = N - 1
if N == 0: done.release()
a.release()
a = thread.allocate_lock()
done = thread.allocate_lock()
N = 10
done.acquire()
for i in range(N):
thread.start_new_thread(task, ())
done.acquire()
print 'done'
# Sticking an acquire/release pair around the 'import' statement makes the
# problem go away.
#
# I believe that what happens is:
#
# 1) The first thread to hit the import atomically reaches, and executes
# most of, get_module. In particular, it finds Lib/whrandom.pyc,
# installs its name in sys.modules, and executes
#
# v = eval_code(co, d, d, d, (object *)NULL);
#
# to initialize the module.
#
# 2) eval_code "ticker"-slices the 1st thread out, and gives another thread
# a chance. When this 2nd thread hits the same 'import', import_module
# finds 'whrandom' in sys.modules, so just proceeds.
#
# 3) But the 1st thread is still "in the middle" of executing whrandom.pyc.
# So the 2nd thread has a good chance of trying to look up 'randint'
# before the 1st thread has placed it in whrandom's dict.
#
# 4) The more threads there are, the more likely that at least one of them
# will do this before the 1st thread finishes the import work.
#
# If that's right, a perhaps not-too-bad workaround would be to introduce a
# static "you can't interrupt this thread" flag in ceval.c, check it before
# giving up interpreter_lock, and have IMPORT_NAME set it & restore (plain
# clearing would not work) it around its call to import_module. To its
# credit, there's something wonderfully perverse about fixing a race via an
# unprotected static <grin>.
#
# as-with-most-other-things-(pseudo-)parallel-programming's-more-fun-
# in-python-too!-ly y'rs - tim
#
# Tim Peters tim@ksr.com
# not speaking for Kendall Square Research Corp

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Demo/threads/sync.py Normal file
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# Defines classes that provide synchronization objects. Note that use of
# this module requires that your Python support threads.
#
# condition() # a POSIX-like condition-variable object
# barrier(n) # an n-thread barrier
# event() # an event object
#
# CONDITIONS
#
# A condition object is created via
# import this_module
# your_condition_object = this_module.condition()
#
# Methods:
# .acquire()
# acquire the lock associated with the condition
# .release()
# release the lock associated with the condition
# .wait()
# block the thread until such time as some other thread does a
# .signal or .broadcast on the same condition, and release the
# lock associated with the condition. The lock associated with
# the condition MUST be in the acquired state at the time
# .wait is invoked.
# .signal()
# wake up exactly one thread (if any) that previously did a .wait
# on the condition; that thread will awaken with the lock associated
# with the condition in the acquired state. If no threads are
# .wait'ing, this is a nop. If more than one thread is .wait'ing on
# the condition, any of them may be awakened.
# .broadcast()
# wake up all threads (if any) that are .wait'ing on the condition;
# the threads are woken up serially, each with the lock in the
# acquired state, so should .release() as soon as possible. If no
# threads are .wait'ing, this is a nop.
#
# Note that if a thread does a .wait *while* a signal/broadcast is
# in progress, it's guaranteeed to block until a subsequenct
# signal/broadcast.
#
# Secret feature: `broadcast' actually takes an integer argument,
# and will wake up exactly that many waiting threads (or the total
# number waiting, if that's less). Use of this is dubious, though,
# and probably won't be supported if this form of condition is
# reimplemented in C.
#
# DIFFERENCES FROM POSIX
#
# + A separate mutex is not needed to guard condition data. Instead, a
# condition object can (must) be .acquire'ed and .release'ed directly.
# This eliminates a common error in using POSIX conditions.
#
# + Because of implementation difficulties, a POSIX `signal' wakes up
# _at least_ one .wait'ing thread. Race conditions make it difficult
# to stop that. This implementation guarantees to wake up only one,
# but you probably shouldn't rely on that.
#
# PROTOCOL
#
# Condition objects are used to block threads until "some condition" is
# true. E.g., a thread may wish to wait until a producer pumps out data
# for it to consume, or a server may wish to wait until someone requests
# its services, or perhaps a whole bunch of threads want to wait until a
# preceding pass over the data is complete. Early models for conditions
# relied on some other thread figuring out when a blocked thread's
# condition was true, and made the other thread responsible both for
# waking up the blocked thread and guaranteeing that it woke up with all
# data in a correct state. This proved to be very delicate in practice,
# and gave conditions a bad name in some circles.
#
# The POSIX model addresses these problems by making a thread responsible
# for ensuring that its own state is correct when it wakes, and relies
# on a rigid protocol to make this easy; so long as you stick to the
# protocol, POSIX conditions are easy to "get right":
#
# A) The thread that's waiting for some arbitrarily-complex condition
# (ACC) to become true does:
#
# condition.acquire()
# while not (code to evaluate the ACC):
# condition.wait()
# # That blocks the thread, *and* releases the lock. When a
# # condition.signal() happens, it will wake up some thread that
# # did a .wait, *and* acquire the lock again before .wait
# # returns.
# #
# # Because the lock is acquired at this point, the state used
# # in evaluating the ACC is frozen, so it's safe to go back &
# # reevaluate the ACC.
#
# # At this point, ACC is true, and the thread has the condition
# # locked.
# # So code here can safely muck with the shared state that
# # went into evaluating the ACC -- if it wants to.
# # When done mucking with the shared state, do
# condition.release()
#
# B) Threads that are mucking with shared state that may affect the
# ACC do:
#
# condition.acquire()
# # muck with shared state
# condition.release()
# if it's possible that ACC is true now:
# condition.signal() # or .broadcast()
#
# Note: You may prefer to put the "if" clause before the release().
# That's fine, but do note that anyone waiting on the signal will
# stay blocked until the release() is done (since acquiring the
# condition is part of what .wait() does before it returns).
#
# TRICK OF THE TRADE
#
# With simpler forms of conditions, it can be impossible to know when
# a thread that's supposed to do a .wait has actually done it. But
# because this form of condition releases a lock as _part_ of doing a
# wait, the state of that lock can be used to guarantee it.
#
# E.g., suppose thread A spawns thread B and later wants to wait for B to
# complete:
#
# In A: In B:
#
# B_done = condition() ... do work ...
# B_done.acquire() B_done.acquire(); B_done.release()
# spawn B B_done.signal()
# ... some time later ... ... and B exits ...
# B_done.wait()
#
# Because B_done was in the acquire'd state at the time B was spawned,
# B's attempt to acquire B_done can't succeed until A has done its
# B_done.wait() (which releases B_done). So B's B_done.signal() is
# guaranteed to be seen by the .wait(). Without the lock trick, B
# may signal before A .waits, and then A would wait forever.
#
# BARRIERS
#
# A barrier object is created via
# import this_module
# your_barrier = this_module.barrier(num_threads)
#
# Methods:
# .enter()
# the thread blocks until num_threads threads in all have done
# .enter(). Then the num_threads threads that .enter'ed resume,
# and the barrier resets to capture the next num_threads threads
# that .enter it.
#
# EVENTS
#
# An event object is created via
# import this_module
# your_event = this_module.event()
#
# An event has two states, `posted' and `cleared'. An event is
# created in the cleared state.
#
# Methods:
#
# .post()
# Put the event in the posted state, and resume all threads
# .wait'ing on the event (if any).
#
# .clear()
# Put the event in the cleared state.
#
# .is_posted()
# Returns 0 if the event is in the cleared state, or 1 if the event
# is in the posted state.
#
# .wait()
# If the event is in the posted state, returns immediately.
# If the event is in the cleared state, blocks the calling thread
# until the event is .post'ed by another thread.
#
# Note that an event, once posted, remains posted until explicitly
# cleared. Relative to conditions, this is both the strength & weakness
# of events. It's a strength because the .post'ing thread doesn't have to
# worry about whether the threads it's trying to communicate with have
# already done a .wait (a condition .signal is seen only by threads that
# do a .wait _prior_ to the .signal; a .signal does not persist). But
# it's a weakness because .clear'ing an event is error-prone: it's easy
# to mistakenly .clear an event before all the threads you intended to
# see the event get around to .wait'ing on it. But so long as you don't
# need to .clear an event, events are easy to use safely.
#
# Tim Peters tim@ksr.com
# not speaking for Kendall Square Research Corp
import thread
class condition:
def __init__(self):
# the lock actually used by .acquire() and .release()
self.mutex = thread.allocate_lock()
# lock used to block threads until a signal
self.checkout = thread.allocate_lock()
self.checkout.acquire()
# internal critical-section lock, & the data it protects
self.idlock = thread.allocate_lock()
self.id = 0
self.waiting = 0 # num waiters subject to current release
self.pending = 0 # num waiters awaiting next signal
self.torelease = 0 # num waiters to release
self.releasing = 0 # 1 iff release is in progress
def acquire(self):
self.mutex.acquire()
def release(self):
self.mutex.release()
def wait(self):
mutex, checkout, idlock = self.mutex, self.checkout, self.idlock
if not mutex.locked():
raise ValueError, \
"condition must be .acquire'd when .wait() invoked"
idlock.acquire()
myid = self.id
self.pending = self.pending + 1
idlock.release()
mutex.release()
while 1:
checkout.acquire(); idlock.acquire()
if myid < self.id:
break
checkout.release(); idlock.release()
self.waiting = self.waiting - 1
self.torelease = self.torelease - 1
if self.torelease:
checkout.release()
else:
self.releasing = 0
if self.waiting == self.pending == 0:
self.id = 0
idlock.release()
mutex.acquire()
def signal(self):
self.broadcast(1)
def broadcast(self, num = -1):
if num < -1:
raise ValueError, '.broadcast called with num ' + `num`
if num == 0:
return
self.idlock.acquire()
if self.pending:
self.waiting = self.waiting + self.pending
self.pending = 0
self.id = self.id + 1
if num == -1:
self.torelease = self.waiting
else:
self.torelease = min( self.waiting,
self.torelease + num )
if self.torelease and not self.releasing:
self.releasing = 1
self.checkout.release()
self.idlock.release()
class barrier:
def __init__(self, n):
self.n = n
self.togo = n
self.full = condition()
def enter(self):
full = self.full
full.acquire()
self.togo = self.togo - 1
if self.togo:
full.wait()
else:
self.togo = self.n
full.broadcast()
full.release()
class event:
def __init__(self):
self.state = 0
self.posted = condition()
def post(self):
self.posted.acquire()
self.state = 1
self.posted.broadcast()
self.posted.release()
def clear(self):
self.posted.acquire()
self.state = 0
self.posted.release()
def is_posted(self):
self.posted.acquire()
answer = self.state
self.posted.release()
return answer
def wait(self):
self.posted.acquire()
while not self.state:
self.posted.wait()
self.posted.release()
# The rest of the file is a test case, that runs a number of parallelized
# quicksorts in parallel. If it works, you'll get about 600 lines of
# tracing output, with a line like
# test passed! 209 threads created in all
# as the last line. The content and order of preceding lines will
# vary across runs.
def _new_thread(func, *args):
global TID
tid.acquire(); id = TID = TID+1; tid.release()
io.acquire(); alive.append(id); \
print 'starting thread', id, '--', len(alive), 'alive'; \
io.release()
thread.start_new_thread( func, (id,) + args )
def _qsort(tid, a, l, r, finished):
# sort a[l:r]; post finished when done
io.acquire(); print 'thread', tid, 'qsort', l, r; io.release()
if r-l > 1:
pivot = a[l]
j = l+1 # make a[l:j] <= pivot, and a[j:r] > pivot
for i in range(j, r):
if a[i] <= pivot:
a[j], a[i] = a[i], a[j]
j = j + 1
a[l], a[j-1] = a[j-1], pivot
l_subarray_sorted = event()
r_subarray_sorted = event()
_new_thread(_qsort, a, l, j-1, l_subarray_sorted)
_new_thread(_qsort, a, j, r, r_subarray_sorted)
l_subarray_sorted.wait()
r_subarray_sorted.wait()
io.acquire(); print 'thread', tid, 'qsort done'; \
alive.remove(tid); io.release()
finished.post()
def _randarray(tid, a, finished):
io.acquire(); print 'thread', tid, 'randomizing array'; \
io.release()
for i in range(1, len(a)):
wh.acquire(); j = randint(0,i); wh.release()
a[i], a[j] = a[j], a[i]
io.acquire(); print 'thread', tid, 'randomizing done'; \
alive.remove(tid); io.release()
finished.post()
def _check_sort(a):
if a != range(len(a)):
raise ValueError, ('a not sorted', a)
def _run_one_sort(tid, a, bar, done):
# randomize a, and quicksort it
# for variety, all the threads running this enter a barrier
# at the end, and post `done' after the barrier exits
io.acquire(); print 'thread', tid, 'randomizing', a; \
io.release()
finished = event()
_new_thread(_randarray, a, finished)
finished.wait()
io.acquire(); print 'thread', tid, 'sorting', a; io.release()
finished.clear()
_new_thread(_qsort, a, 0, len(a), finished)
finished.wait()
_check_sort(a)
io.acquire(); print 'thread', tid, 'entering barrier'; \
io.release()
bar.enter()
io.acquire(); print 'thread', tid, 'leaving barrier'; \
io.release()
io.acquire(); alive.remove(tid); io.release()
bar.enter() # make sure they've all removed themselves from alive
## before 'done' is posted
bar.enter() # just to be cruel
done.post()
def test():
global TID, tid, io, wh, randint, alive
import whrandom
randint = whrandom.randint
TID = 0 # thread ID (1, 2, ...)
tid = thread.allocate_lock() # for changing TID
io = thread.allocate_lock() # for printing, and 'alive'
wh = thread.allocate_lock() # for calls to whrandom
alive = [] # IDs of active threads
NSORTS = 5
arrays = []
for i in range(NSORTS):
arrays.append( range( (i+1)*10 ) )
bar = barrier(NSORTS)
finished = event()
for i in range(NSORTS):
_new_thread(_run_one_sort, arrays[i], bar, finished)
finished.wait()
print 'all threads done, and checking results ...'
if alive:
raise ValueError, ('threads still alive at end', alive)
for i in range(NSORTS):
a = arrays[i]
if len(a) != (i+1)*10:
raise ValueError, ('length of array', i, 'screwed up')
_check_sort(a)
print 'test passed!', TID, 'threads created in all'
if __name__ == '__main__':
test()
# end of module