RE: [PATCH v10 6/6] x86/split_lock: Enable split lock detection by kernel parameter

From: Luck, Tony
Date: Fri Nov 22 2019 - 12:48:18 EST

> When we use byte ops, we must consider the word as 4 independent
> variables. And in that case the later load might observe the lock-byte
> state from 3, because the modification to the lock byte from 4 is in
> CPU2's store-buffer.

So we absolutely violate this with the optimization for constant arguments
to set_bit(), clear_bit() and change_bit() that are implemented as byte ops.

So is code that does:

set_bit(0, bitmap);

on one CPU. While another is doing:

set_bit(mybit, bitmap);

on another CPU safe? The first operates on just one byte, the second on 8 bytes.