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bpo-30341: Add an explaining comment in _PyTrash_thread_destroy_chain() #1545

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Merged
merged 3 commits into from
May 13, 2017
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bpo-30341: Add an explaining comment in _PyTrash_thread_destroy_chain() #1545

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dd8db39
bpo-30341: Add an explaining comment in _PyTrash_thread_destroy_chain()
zhangyangyu May 11, 2017
6da8579
delete first sentence and add Antoine's example
zhangyangyu May 11, 2017
19c3757
move increase and decrese outside of the loop
zhangyangyu May 11, 2017
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2 changes: 1 addition & 1 deletion Include/object.h
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Original file line number Diff line number Diff line change
Expand Up @@ -1029,7 +1029,7 @@ without deallocating anything (and so unbounded call-stack depth is avoided).
When the call stack finishes unwinding again, code generated by the END macro
notices this, and calls another routine to deallocate all the objects that
may have been added to the list of deferred deallocations. In effect, a
chain of N deallocations is broken into N / PyTrash_UNWIND_LEVEL pieces,
chain of N deallocations is broken into (N-1)/(PyTrash_UNWIND_LEVEL-1) pieces,
with the call stack never exceeding a depth of PyTrash_UNWIND_LEVEL.
*/

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17 changes: 15 additions & 2 deletions Objects/object.c
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Expand Up @@ -2093,6 +2093,19 @@ void
_PyTrash_thread_destroy_chain(void)
{
PyThreadState *tstate = PyThreadState_GET();
/* We need to increase trash_delete_nesting here, otherwise,
_PyTrash_thread_destroy_chain will be called recursively
and then possibly crash. An example that may crash without
increase:
N = 500000 # need to be large enough
ob = object()
tups = [(ob,) for i in range(N)]
for i in range(49):
tups = [(tup,) for tup in tups]
del tups
*/
assert(tstate->trash_delete_nesting == 0);
++tstate->trash_delete_nesting;
while (tstate->trash_delete_later) {
PyObject *op = tstate->trash_delete_later;
destructor dealloc = Py_TYPE(op)->tp_dealloc;
Expand All @@ -2107,10 +2120,10 @@ _PyTrash_thread_destroy_chain(void)
* up distorting allocation statistics.
*/
assert(op->ob_refcnt == 0);
++tstate->trash_delete_nesting;
(*dealloc)(op);
--tstate->trash_delete_nesting;
assert(tstate->trash_delete_nesting == 1);
}
--tstate->trash_delete_nesting;
}

#ifndef Py_TRACE_REFS
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