From: Tom de Vries <tdevries@suse.de>
To: gdb-patches@sourceware.org
Subject: [PATCH] [gdb/dap] Fix exit race
Date: Tue, 13 Feb 2024 16:01:41 +0100 [thread overview]
Message-ID: <20240213150141.28034-1-tdevries@suse.de> (raw)
When running test-case gdb.dap/eof.exp, we're likely to get a coredump due to
a segfault in new_threadstate.
At the point of the core dump, the gdb main thread looks like:
...
(gdb) bt
#0 0x0000fffee30d2280 in __pthread_kill_implementation () from /lib64/libc.so.6
#1 0x0000fffee3085800 [PAC] in raise () from /lib64/libc.so.6
#2 0x00000000007b03e8 [PAC] in handle_fatal_signal (sig=11)
at gdb/event-top.c:926
#3 0x00000000007b0470 in handle_sigsegv (sig=11)
at gdb/event-top.c:976
#4 <signal handler called>
#5 0x0000fffee3a4db14 in new_threadstate () from /lib64/libpython3.12.so.1.0
#6 0x0000fffee3ab0548 [PAC] in PyGILState_Ensure () from /lib64/libpython3.12.so.1.0
#7 0x0000000000a6d034 [PAC] in gdbpy_gil::gdbpy_gil (this=0xffffcb279738)
at gdb/python/python-internal.h:787
#8 0x0000000000ab87ac in gdbpy_event::~gdbpy_event (this=0xfffea8001ee0,
__in_chrg=<optimized out>) at gdb/python/python.c:1051
#9 0x0000000000ab9460 in std::_Function_base::_Base_manager<...>::_M_destroy
(__victim=...) at /usr/include/c++/13/bits/std_function.h:175
#10 0x0000000000ab92dc in std::_Function_base::_Base_manager<...>::_M_manager
(__dest=..., __source=..., __op=std::__destroy_functor)
at /usr/include/c++/13/bits/std_function.h:203
#11 0x0000000000ab8f14 in std::_Function_handler<...>::_M_manager(...) (...)
at /usr/include/c++/13/bits/std_function.h:282
#12 0x000000000042dd9c in std::_Function_base::~_Function_base (this=0xfffea8001c10,
__in_chrg=<optimized out>) at /usr/include/c++/13/bits/std_function.h:244
#13 0x000000000042e654 in std::function<void ()>::~function() (this=0xfffea8001c10,
__in_chrg=<optimized out>) at /usr/include/c++/13/bits/std_function.h:334
#14 0x0000000000b68e60 in std::_Destroy<std::function<void ()> >(...) (...)
at /usr/include/c++/13/bits/stl_construct.h:151
#15 0x0000000000b68cd0 in std::_Destroy_aux<false>::__destroy<...>(...) (...)
at /usr/include/c++/13/bits/stl_construct.h:163
#16 0x0000000000b689d8 in std::_Destroy<...>(...) (...)
at /usr/include/c++/13/bits/stl_construct.h:196
#17 0x0000000000b68414 in std::_Destroy<...>(...) (...)
at /usr/include/c++/13/bits/alloc_traits.h:948
#18 std::vector<...>::~vector() (this=0x2a183c8 <runnables>)
at /usr/include/c++/13/bits/stl_vector.h:732
#19 0x0000fffee3088370 in __run_exit_handlers () from /lib64/libc.so.6
#20 0x0000fffee3088450 [PAC] in exit () from /lib64/libc.so.6
#21 0x0000000000c95600 [PAC] in quit_force (exit_arg=0x0, from_tty=0)
at gdb/top.c:1822
#22 0x0000000000609140 in quit_command (args=0x0, from_tty=0)
at gdb/cli/cli-cmds.c:508
#23 0x0000000000c926a4 in quit_cover () at gdb/top.c:300
#24 0x00000000007b09d4 in async_disconnect (arg=0x0)
at gdb/event-top.c:1230
#25 0x0000000000548acc in invoke_async_signal_handlers ()
at gdb/async-event.c:234
#26 0x000000000157d2d4 in gdb_do_one_event (mstimeout=-1)
at gdbsupport/event-loop.cc:199
#27 0x0000000000943a84 in start_event_loop () at gdb/main.c:401
#28 0x0000000000943bfc in captured_command_loop () at gdb/main.c:465
#29 0x000000000094567c in captured_main (data=0xffffcb279d08)
at gdb/main.c:1335
#30 0x0000000000945700 in gdb_main (args=0xffffcb279d08)
at gdb/main.c:1354
#31 0x0000000000423ab4 in main (argc=14, argv=0xffffcb279e98)
at gdb/gdb.c:39
...
The direct cause of the segfault is calling PyGILState_Ensure after
calling Py_Finalize.
AFAICT the problem is a race between the gdb main thread and DAP's JSON writer
thread.
On one side, we have the following events:
- DAP's JSON reader thread reads an EOF, and lets DAP's main thread known
by writing None into read_queue
- DAP's main thread lets DAP's JSON writer thread known by writing None into
write_queue
- DAP's JSON writer thread sees the None in its queue, and calls
send_gdb("quit")
- a corresponding gdbpy_event is deposited in the runnables vector, to be
run by the gdb main thread
On the other side, we have the following events:
- the gdb main thread receives a SIGHUP
- the corresponding handler calls quit_force, which calls do_final_cleanups
- one of the final cleanups is finalize_python, which calls Py_Finalize
- quit_force calls exit, which triggers the exit handlers
- one of the exit handlers is the destructor of the runnables vector
- destruction of the vector triggers destruction of the remaining element
- the remaining element is a gdbpy_event, and the destructor (indirectly)
calls PyGILState_Ensure
It's good to note that both events (EOF and SIGHUP) are caused by this line in
the test-case:
...
catch "close -i $gdb_spawn_id"
...
where "expect close" closes the stdin and stdout file descriptors, which
causes the SIGHUP to be send.
So, for the system I'm running this on, the send_gdb("quit") is actually not
needed.
I'm not sure if we support any systems where it's actually needed.
Fix this by removing the send_gdb("quit").
Tested on aarch64-linux.
PR dap/31306
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31306
---
gdb/python/lib/gdb/dap/io.py | 1 -
1 file changed, 1 deletion(-)
diff --git a/gdb/python/lib/gdb/dap/io.py b/gdb/python/lib/gdb/dap/io.py
index 5149edae977..4edd504c727 100644
--- a/gdb/python/lib/gdb/dap/io.py
+++ b/gdb/python/lib/gdb/dap/io.py
@@ -68,7 +68,6 @@ def start_json_writer(stream, queue):
# This is an exit request. The stream is already
# flushed, so all that's left to do is request an
# exit.
- send_gdb("quit")
break
obj["seq"] = seq
seq = seq + 1
base-commit: a16034bf6417dc2259fef43fd5bcc2dd1dac562f
--
2.35.3
next reply other threads:[~2024-02-13 15:01 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-02-13 15:01 Tom de Vries [this message]
2024-02-13 18:10 ` Tom Tromey
2024-02-14 15:39 ` Tom de Vries
2024-02-14 15:46 ` Tom Tromey
2024-02-23 17:01 ` Tom Tromey
2024-02-23 21:08 ` Tom Tromey
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