<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/133270>133270</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [libc] allocator initialization re-entrancy
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            libc
      </td>
    </tr>

    <tr>
      <th>Assignees</th>
      <td>
      </td>
    </tr>

    <tr>
      <th>Reporter</th>
      <td>
          SchrodingerZhu
      </td>
    </tr>
</table>

<pre>
    I create this meta issue because in https://github.com/microsoft/snmalloc/pull/762, we investigated several edge cases where layering allocator on top of system libc leads to deadlocks (found by me) and memory leaks (credits @mjp41).

These issues are actually quite interesting. Even though they are hard to trigger in glibc in practice because it is unlikely to create too many atexit hooks and it is likely that allocator (and its TLS) is initialized already by libc's internal allocation.

However, given that LLVM-libc has been specifically designed to support overlay usage, it seems to be more relevant to these edge cases.

To mitigate these problems, in libc, we should:

- Add some static buffers for atexit hook.
- Handle re-entrance to our best efforts.
- Explore other solutions:
  A common practice by BSDs is to expose `_malloc_thread_cleanup` as a weak symbol that can be overwritten by custom allocators. This largely avoids complexity to maintain the correctness of cleanup. I really think it is a good thing to have for LLVM-libc.


</pre>
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