"What The Hardware Does" is not What Your Program Does: Uninitialized Memory

[Amanieu]

@RalfJung

Oh, so the following can actually happen when using jemalloc :

• Allocate memory, deallocate it. MADV_FREE gets called.
• Newly allocated memory reuses that storage. The new memory gets read.
• The kernel decides to free the page.
• Memory gets read again, giving a different result.
• Memory gets written, now allocating a page again and persisting the result.

Is that correct?

Pretty much. This is essentially a data race, which you can somewhat get around by using volatile loads. However the "cute data structure" will definitely not work in such conditions.