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

[Peter Zijlstra]

On Fri, Nov 22, 2019 at 05:48:14PM +0000, Luck, Tony wrote:
> > 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.

It is safe if all you care about is the consistency of that one bit.