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Relevant thread: http://mail.python.org/pipermail/python-dev/2012-December/123225.html
PyOS_StdioReadline features numerous calls that require the GIL to be held. Ideally, the GIL drop-take should be moved closer to the actual underlying read system call.

So, could you propose a patch?

My quick and dirty fix is simple:
 _PyOS_ReadlineTState = PyThreadState_GET();
 /* CCP change, cannot release the GIL here because PyOS_StdioReadline uses
 * the regular MALLOC
 */
 /*
 Py_BEGIN_ALLOW_THREADS
 */
#ifdef WITH_THREAD
 PyThread_acquire_lock(_PyOS_ReadlineLock, 1);
#endif
 /* This is needed to handle the unlikely case that the
 * interpreter is in interactive mode *and* stdin/out are not
 * a tty. This can happen, for example if python is run like
 * this: python -i < test1.py
 */
 if (!isatty (fileno (sys_stdin)) || !isatty (fileno (sys_stdout)))
 rv = PyOS_StdioReadline (sys_stdin, sys_stdout, prompt);
 else
 rv = (*PyOS_ReadlineFunctionPointer)(sys_stdin, sys_stdout,
 prompt);
 /*
 Py_END_ALLOW_THREADS
 */
#ifdef WITH_THREAD
 PyThread_release_lock(_PyOS_ReadlineLock);
#endif
Basically, we just comment out the lock release since we don't need it. The reason we found this was that we were using GIL a custom mallocator which should have been run with the GIL but wasn ́t.

I just found the readline/GIL issue while working on #18203. I created #18205 but then I found this issue. I just closed #18205 as a duplicate. Here is a patch for Python 3.4.
--
Copy of the initial message (msg191089):
The callback PyOS_ReadlineFunctionPointer (used to read a line from the standard input) must return a buffer allocated by PyMem_Malloc(), but PyOS_Readline() releases the GIL before calling PyOS_ReadlineFunctionPointer.
Simplified extract of PyOS_Readline():
 Py_BEGIN_ALLOW_THREADS
 if (!isatty (fileno (sys_stdin)) || !isatty (fileno (sys_stdout)))
 rv = PyOS_StdioReadline (sys_stdin, sys_stdout, prompt);
 else
 rv = (*PyOS_ReadlineFunctionPointer)(sys_stdin, sys_stdout,
 prompt);
 Py_END_ALLOW_THREADS
tok_nextc() calls PyOS_Readline() and calls PyMem_FREE() to release its result.
PyOS_ReadlineFunctionPointer should allocate memory using malloc(), not using PyMem_Malloc(). But PyOS_Readline() should copy the line into a buffer allocated by PyMem_Malloc() to keep backward compatibility.
See also issue #18203 and #3329.

See the following thread on python-dev, the root problem is that PyMem_Malloc() cannot be called with the GIL held. This is a bug in my opinion, and it should be fixed.
http://mail.python.org/pipermail/python-dev/2013-June/126822.html 

Updated patch for the final API of #3329. Update also the documentation. PyOS_ReadlineFunctionPointer must now use PyMem_RawMalloc() or PyMem_RawRealloc(), instead of PyMem_Malloc() or PyMem_Realloc().

New changeset 98dbe677dfe7 by Victor Stinner in branch 'default':
Close #16742: Fix misuse of memory allocations in PyOS_Readline()
http://hg.python.org/cpython/rev/98dbe677dfe7 

New changeset 6c9050ad1afc by Victor Stinner in branch 'default':
Issue #16742: My fix on PyOS_StdioReadline() was incomplete, PyMem_FREE() was
http://hg.python.org/cpython/rev/6c9050ad1afc 

Perhaps in debug builds the memory apis should verify consistency and matching useage.

> Kristján Valur Jónsson added the comment:
>
> Perhaps in debug builds the memory apis should verify consistency and
matching useage.
Python does check usage of apis in debug mode. Memory allocation failure
are almost never checked. See my pyfailmalloc module for that.