Re: [PATCH v3 0/9] s390: Improve this_cpu operations

[David Laight <david.laight.linux@gmail.com>]

On Wed, 20 May 2026 17:23:37 -0700
Yang Shi <yang@os.amperecomputing.com> wrote:

> On 5/20/26 3:34 PM, David Laight wrote:
...
> >
> > But I'm sure I remember that some cpu don't like having the same
> > physical address at different virtual addresses (and not just those
> > with VIVT caches like some sparc cpu).  
> 
> Yeah, VIVT cache doesn't like it due to cache alias. But the mapping is 
> really percpu, so the mapping to the physical address belonging to 
> another CPU should never pollute the current CPU's cache if I don't miss 
> something.
>
> > I'm sure code can end up accessing the current cpu's percpu data
> > using the same address that other cpu use - there are definitely
> > places where it needs that address.  
> 
> No, it is not. In the percpu page table approach, we use different 
> address for this_cpu_*() and per_cpu_ptr() which is mainly used to 
> initialize percpu data for all CPUs.

You missed something.

Look, for example, at kernel/locking/osq_lock.c
The code uses this_cpu_ptr() and then both dereferences the pointer
and writes it to places that other cpu will use.
It also uses per_cpu_ptr() to get an address it can use for the per-cpu
data of another cpu.
(That code all assumes preemption is disabled.)

-- David

> Thanks,
> Yang