Issue #12352: Fix a deadlock in multiprocessing.Heap when a block is freed by

the garbage collector while the Heap lock is held.
parent afa44a80
......@@ -101,6 +101,8 @@ class Heap(object):
self._stop_to_block = {}
self._allocated_blocks = set()
self._arenas = []
# list of pending blocks to free - see free() comment below
self._pending_free_blocks = []
@staticmethod
def _roundup(n, alignment):
......@@ -175,11 +177,35 @@ class Heap(object):
return start, stop
def _free_pending_blocks(self):
# Free all the blocks in the pending list - called with the lock held.
while True:
try:
block = self._pending_free_blocks.pop()
except IndexError:
break
self._allocated_blocks.remove(block)
self._free(block)
def free(self, block):
# free a block returned by malloc()
# Since free() can be called asynchronously by the GC, it could happen
# that it's called while self._lock is held: in that case,
# self._lock.acquire() would deadlock (issue #12352). To avoid that, a
# trylock is used instead, and if the lock can't be acquired
# immediately, the block is added to a list of blocks to be freed
# synchronously sometimes later from malloc() or free(), by calling
# _free_pending_blocks() (appending and retrieving from a list is not
# strictly thread-safe but under cPython it's atomic thanks to the GIL).
assert os.getpid() == self._lastpid
self._lock.acquire()
if not self._lock.acquire(False):
# can't acquire the lock right now, add the block to the list of
# pending blocks to free
self._pending_free_blocks.append(block)
else:
# we hold the lock
try:
self._free_pending_blocks()
self._allocated_blocks.remove(block)
self._free(block)
finally:
......@@ -191,6 +217,7 @@ class Heap(object):
if os.getpid() != self._lastpid:
self.__init__() # reinitialize after fork
self._lock.acquire()
self._free_pending_blocks()
try:
size = self._roundup(max(size,1), self._alignment)
(arena, start, stop) = self._malloc(size)
......
......@@ -1580,6 +1580,8 @@ class _TestHeap(BaseTestCase):
# verify the state of the heap
all = []
occupied = 0
heap._lock.acquire()
self.addCleanup(heap._lock.release)
for L in list(heap._len_to_seq.values()):
for arena, start, stop in L:
all.append((heap._arenas.index(arena), start, stop,
......@@ -1597,6 +1599,28 @@ class _TestHeap(BaseTestCase):
self.assertTrue((arena != narena and nstart == 0) or
(stop == nstart))
def test_free_from_gc(self):
# Check that freeing of blocks by the garbage collector doesn't deadlock
# (issue #12352).
# Make sure the GC is enabled, and set lower collection thresholds to
# make collections more frequent (and increase the probability of
# deadlock).
if not gc.isenabled():
gc.enable()
self.addCleanup(gc.disable)
thresholds = gc.get_threshold()
self.addCleanup(gc.set_threshold, *thresholds)
gc.set_threshold(10)
# perform numerous block allocations, with cyclic references to make
# sure objects are collected asynchronously by the gc
for i in range(5000):
a = multiprocessing.heap.BufferWrapper(1)
b = multiprocessing.heap.BufferWrapper(1)
# circular references
a.buddy = b
b.buddy = a
#
#
#
......
......@@ -107,6 +107,9 @@ Core and Builtins
Library
-------
- Issue #12352: Fix a deadlock in multiprocessing.Heap when a block is freed by
the garbage collector while the Heap lock is held.
- Issue #985064: Make plistlib more resilient to faulty input plists.
Patch by Mher Movsisyan.
......
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