Re: [PATCH 3/5] mm/vmstat: use cmpxchg loop in cpu_vm_stats_fold

[Marcelo Tosatti]

On Thu, Feb 02, 2023 at 03:38:58PM +0100, Christoph Lameter wrote:
> On Wed, 1 Feb 2023, Marcelo Tosatti wrote:
> 
> > In preparation to switch vmstat shepherd to flush
> > per-CPU counters remotely, use a cmpxchg loop
> > instead of a pair of read/write instructions.
> 
> You are mixing full atomic cmpxchg and  per cpu atomic cmpxchg? That does
> not work.

OK, missing locked on the local functions. Can fix that.

> I thought you would only run this while the kernel is not active on the
> remote cpu? Then you dont need any cmpxchg and you can leave the function
> as is.

The remote cpu can enter kernel mode while this function executes.

There is no mode which indicates userspace cannot enter the kernel.