Submitted By: Xi Ruoyao Date: 2020-10-06 Initial Package Version: 9.2 Origin: Upstream GIT Repository Description: Fix segfault on startup with libpython3.9.so linked From c47bae859a5af0d95224d90000df0e529f7c5aa0 Mon Sep 17 00:00:00 2001 From: Kevin Buettner Date: Wed, 27 May 2020 20:05:40 -0700 Subject: [PATCH] Fix Python3.9 related runtime problems Python3.9b1 is now available on Rawhide. GDB w/ Python 3.9 support can be built using the configure switch -with-python=/usr/bin/python3.9. Attempting to run gdb/Python3.9 segfaults on startup: #0 0x00007ffff7b0582c in PyEval_ReleaseLock () from /lib64/libpython3.9.so.1.0 #1 0x000000000069ccbf in do_start_initialization () at worktree-test1/gdb/python/python.c:1789 #2 _initialize_python () at worktree-test1/gdb/python/python.c:1877 #3 0x00000000007afb0a in initialize_all_files () at init.c:237 ... Consulting the the documentation... https://docs.python.org/3/c-api/init.html ...we find that PyEval_ReleaseLock() has been deprecated since version 3.2. It recommends using PyEval_SaveThread or PyEval_ReleaseThread() instead. In do_start_initialization, in gdb/python/python.c, we can replace the calls to PyThreadState_Swap() and PyEval_ReleaseLock() with a single call to PyEval_SaveThread. (Thanks to Keith Seitz for working this out.) With that in place, GDB gets a little bit further. It still dies on startup, but the backtrace is different: #0 0x00007ffff7b04306 in PyOS_InterruptOccurred () from /lib64/libpython3.9.so.1.0 #1 0x0000000000576e86 in check_quit_flag () at worktree-test1/gdb/extension.c:776 #2 0x0000000000576f8a in set_active_ext_lang (now_active=now_active@entry=0x983c00 ) at worktree-test1/gdb/extension.c:705 #3 0x000000000069d399 in gdbpy_enter::gdbpy_enter (this=0x7fffffffd2d0, gdbarch=0x0, language=0x0) at worktree-test1/gdb/python/python.c:211 #4 0x0000000000686e00 in python_new_inferior (inf=0xddeb10) at worktree-test1/gdb/python/py-inferior.c:251 #5 0x00000000005d9fb9 in std::function::operator()(inferior*) const (__args#0=, this=0xccad20) at /usr/include/c++/10/bits/std_function.h:617 #6 gdb::observers::observable::notify (args#0=0xddeb10, this=) at worktree-test1/gdb/../gdbsupport/observable.h:106 #7 add_inferior_silent (pid=0) at worktree-test1/gdb/inferior.c:113 #8 0x00000000005dbcb8 in initialize_inferiors () at worktree-test1/gdb/inferior.c:947 ... We checked with some Python Developers and were told that we should acquire the GIL prior to calling any Python C API function. We definitely don't have the GIL for calls of PyOS_InterruptOccurred(). I moved class_gdbpy_gil earlier in the file and use it in gdbpy_check_quit_flag() to acquire (and automatically release) the GIL. With those changes in place, I was able to run to a GDB prompt. But, when trying to quit, it segfaulted again due to due to some other problems with gdbpy_check_quit_flag(): Thread 1 "gdb" received signal SIGSEGV, Segmentation fault. 0x00007ffff7bbab0c in new_threadstate () from /lib64/libpython3.9.so.1.0 (top-gdb) bt 8 #0 0x00007ffff7bbab0c in new_threadstate () from /lib64/libpython3.9.so.1.0 #1 0x00007ffff7afa5ea in PyGILState_Ensure.cold () from /lib64/libpython3.9.so.1.0 #2 0x000000000069b58c in gdbpy_gil::gdbpy_gil (this=) at worktree-test1/gdb/python/python.c:278 #3 gdbpy_check_quit_flag (extlang=) at worktree-test1/gdb/python/python.c:278 #4 0x0000000000576e96 in check_quit_flag () at worktree-test1/gdb/extension.c:776 #5 0x000000000057700c in restore_active_ext_lang (previous=0xe9c050) at worktree-test1/gdb/extension.c:729 #6 0x000000000088913a in do_my_cleanups ( pmy_chain=0xc31870 , old_chain=0xae5720 ) at worktree-test1/gdbsupport/cleanups.cc:131 #7 do_final_cleanups () at worktree-test1/gdbsupport/cleanups.cc:143 In this case, we're trying to call a Python C API function after Py_Finalize() has been called from finalize_python(). I made finalize_python set gdb_python_initialized to false and then cause check_quit_flag() to return early when it's false. With these changes in place, GDB seems to be working again with Python3.9b1. I think it likely that there are other problems lurking. I wouldn't be surprised to find that there are other calls into Python where we don't first make sure that we have the GIL. Further changes may well be needed. I see no regressions testing on Rawhide using a GDB built with the default Python version (3.8.3) versus one built using Python 3.9b1. I've also tested on Fedora 28, 29, 30, 31, and 32 (all x86_64) using the default (though updated) system installed versions of Python on those OSes. This means that I've tested against Python versions 2.7.15, 2.7.17, 2.7.18, 3.7.7, 3.8.2, and 3.8.3. In each case GDB still builds without problem and shows no regressions after applying this patch. gdb/ChangeLog: 2020-MM-DD Kevin Buettner Keith Seitz * python/python.c (do_start_initialization): For Python 3.9 and later, call PyEval_SaveThread instead of PyEval_ReleaseLock. (class gdbpy_gil): Move to earlier in file. (finalize_python): Set gdb_python_initialized. (gdbpy_check_quit_flag): Acquire GIL via gdbpy_gil. Return early when not initialized. --- gdb/ChangeLog | 10 ++++++++ gdb/python/python.c | 56 ++++++++++++++++++++++++--------------------- 2 files changed, 40 insertions(+), 26 deletions(-) diff --git a/gdb/python/python.c b/gdb/python/python.c index 67f362b852d..4bdd2201abc 100644 --- a/gdb/python/python.c +++ b/gdb/python/python.c @@ -238,6 +238,30 @@ gdbpy_enter::~gdbpy_enter () PyGILState_Release (m_state); } +/* A helper class to save and restore the GIL, but without touching + the other globals that are handled by gdbpy_enter. */ + +class gdbpy_gil +{ +public: + + gdbpy_gil () + : m_state (PyGILState_Ensure ()) + { + } + + ~gdbpy_gil () + { + PyGILState_Release (m_state); + } + + DISABLE_COPY_AND_ASSIGN (gdbpy_gil); + +private: + + PyGILState_STATE m_state; +}; + /* Set the quit flag. */ static void @@ -251,6 +275,10 @@ gdbpy_set_quit_flag (const struct extension_language_defn *extlang) static int gdbpy_check_quit_flag (const struct extension_language_defn *extlang) { + if (!gdb_python_initialized) + return 0; + + gdbpy_gil gil; return PyOS_InterruptOccurred (); } @@ -943,30 +971,6 @@ gdbpy_source_script (const struct extension_language_defn *extlang, /* Posting and handling events. */ -/* A helper class to save and restore the GIL, but without touching - the other globals that are handled by gdbpy_enter. */ - -class gdbpy_gil -{ -public: - - gdbpy_gil () - : m_state (PyGILState_Ensure ()) - { - } - - ~gdbpy_gil () - { - PyGILState_Release (m_state); - } - - DISABLE_COPY_AND_ASSIGN (gdbpy_gil); - -private: - - PyGILState_STATE m_state; -}; - /* A single event. */ struct gdbpy_event { @@ -1616,6 +1620,7 @@ finalize_python (void *ignore) Py_Finalize (); + gdb_python_initialized = false; restore_active_ext_lang (previous_active); } @@ -1785,8 +1790,7 @@ do_start_initialization () return false; /* Release the GIL while gdb runs. */ - PyThreadState_Swap (NULL); - PyEval_ReleaseLock (); + PyEval_SaveThread (); make_final_cleanup (finalize_python, NULL); -- 2.18.4