The read-side code in question is hand-written assembly code, isn't it? And on the hardware side, observing pc will probably result in an actual memory barrier

The read-side code in question is hand-written assembly code, isn't it? And on the hardware side, observing pc will probably result in an actual memory barrier happening anyway (even if no signal is sent internally the kernel must still suspend the thread to capture register state, and then it needs a barrier to communicate that result back to the asking thread).