GeezerVison

As reported in #127716, on no-gil build, several threads calling memoryview.release() on views of the same buffer will cause problem.

Root cause

Let's take a look at memoryview.release()

static void _memory_release(PyMemoryViewObject *self)
{
    assert(get_exports(self) == 0);
    if (self->flags & _Py_MEMORYVIEW_RELEASED)   // (1) test
        return;

    self->flags |= _Py_MEMORYVIEW_RELEASED;      // (2) set
    assert(self->mbuf->exports > 0);
    if (--self->mbuf->exports == 0) {            // (3) non-atomic decrement
        mbuf_release(self->mbuf);
    }
}

Here "test the flag" (1) and "set the flag" (2) are two separate, unsynchronized operations. When mbuf->exports == 1, two threads race like this:

Both threads get past the guard, so both run the decrement. As a result, mbuf->exports drops below 0.

Fix

To fix this, we have to make self->flags's test-and-set a single atomic operation, so that exactly one thread wins the transition and performs the decrement. Here I'm using _Py_atomic_or_uint32. It is used in both release entry points:

• _memory_release() — This code path belongs to the case "Debug build with ASan" in the original issue.
• mbuf_release() — This code path belongs to the case "Plain debug build (no ASan)"

Testing

• The reproducer from the original issue no longer aborts (with and without ASan).
• python -m test test_memoryview passes (with and without GIL).
• In the default (with-GIL) build, the atomic ops degrade to the original plain ones, so behavior and performance are unchanged.

• Issue: make memoryview thread safe in free-threading #127716