linux-kernel - Re: [PATCH v2] percpu_counter: add percpu_counter_sum

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Message-Id: <20221108172603.8d547faa8f0cf0c340fd7150@linux-foundation.org>
Date:   Tue, 8 Nov 2022 17:26:03 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Shakeel Butt <shakeelb@...gle.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Marek Szyprowski <m.szyprowski@...sung.com>
Subject: Re: [PATCH v2] percpu_counter: add percpu_counter_sum_all interface

On Wed,  9 Nov 2022 01:20:11 +0000 Shakeel Butt <shakeelb@...gle.com> 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:?

Thanks.