Bug report

In #138664 we added a critical section on the code object in PyCode_Addr2Line to fix crashes involving profiling.

There are two problems with using a lock here:

1. Code objects are shared across threads and PyCode_Addr2Line is a frequently function, leading to contention during profiling.

2. This introduces a stop-the-world safe point in profiling code, which can cause bugs if the profiling code isn't expecting it. For example, PyTorch's profiler isn't robust to a stop-the-world event (or GIL release) happening during profiling, and this leads to crashes. The problem is essentially:

• PyTorch assumes that after PyEval_SetProfileAllThreads(NULL, NULL); no threads are still in the PyTorch profile code. This is incorrect due to the PyCode_Addr2Line() critical section suspending the thread state in the middle of a profile
• PyTorch frees a bunch of profiler state, but thread suspended on PyCode_Addr2Line() resumes and tries reading that state leading to a crash.

This is a bug in PyTorch, so I'm going to fix it there as well, but the critical section in PyCode_Addr2Line() makes it much more likely to encounter the bug.

Proposed fix to CPython

1. No critical section in PyCode_Addr2Line. Use atomic reads on co->_co_monitoring and co->_co_monitoring->lines
2. Use atomic stores to initialize co->_co_monitoring and co->_co_monitoring->lines and reorder the stores so that the contents are initialized before the pointer is stored.

cc @kumaraditya303

Linked PRs

• gh-148037: remove critical section from PyCode_Addr2Line #148103
• [3.14] gh-148037: remove critical section from PyCode_Addr2Line (GH… #148353