Re: [PATCH v2] percpu_counter: add percpu_counter_sum_all interface

[Shakeel Butt]

On Tue, Nov 8, 2022 at 5:26 PM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> On Wed,  9 Nov 2022 01:20:11 +0000 Shakeel Butt <shakeelb@xxxxxxxxxx> wrote:
>
> > However there is a small race window between the online CPUs traversal
> > of percpu_counter_sum and the CPU offline callback. The offline callback
> > has to traverse all the percpu_counters on the system to flush the CPU
> > local data which can be a lot. During that time, the CPU which is going
> > offline has already been published as offline to all the readers. So, as
> > the offline callback is running, percpu_counter_sum can be called for
> > one counter which has some state on the CPU going offline. Since
> > percpu_counter_sum only traverses online CPUs, it will skip that
> > specific CPU and the offline callback might not have flushed the state
> > for that specific percpu_counter on that offlined CPU.
>
> LGTM.
>
> What are the user-visible effects of this bug?  Should we cc:stable?  If
> so, can you identify a suitable Fixes:?
>

The patch which exposed this is still in mm-unstable (converting rss
stats in mm_struct into percpu_counter) and other percpu_counter users
don't really need this new interface. So, there is no need for a
cc:stable.

thanks,
Shakeel