From mboxrd@z Thu Jan  1 00:00:00 1970
From: Tejun Heo <tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
Subject: Re: [PATCH v9 3/3] blk-cgroup: Flush stats at blkgs destruction path
Date: Fri, 4 Nov 2022 10:00:46 -1000
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To: Waiman Long <longman-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
Cc: Jens Axboe <axboe-tSWWG44O7X1aa/9Udqfwiw@public.gmane.org>, cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-block-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Ming Lei <ming.lei-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>, Andy Shevchenko <andriy.shevchenko-VuQAYsv1563Yd54FQh9/CA@public.gmane.org>, Andrew Morton <akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>, Michal =?iso-8859-1?Q?Koutn=FD?= <mkoutny-IBi9RG/b67k@public.gmane.org>, Hillf Danton <hdanton-k+cT0dCbe1g@public.gmane.org>

On Fri, Nov 04, 2022 at 02:20:50PM -0400, Waiman Long wrote:
> +/**
> + * cgroup_rstat_css_flush - flush stats for the given css and cpu
> + * @css: target css to be flush
> + * @cpu: the cpu that holds the stats to be flush
> + *
> + * A lightweight rstat flush operation for a given css and cpu.
> + * Only the cpu_lock is being held for mutual exclusion, the cgroup_rstat_lock
> + * isn't used.
> + */
> +void cgroup_rstat_css_flush(struct cgroup_subsys_state *css, int cpu)
> +{
> +	raw_spinlock_t *cpu_lock = per_cpu_ptr(&cgroup_rstat_cpu_lock, cpu);
> +
> +	raw_spin_lock_irq(cpu_lock);
> +	rcu_read_lock();
> +	css->ss->css_rstat_flush(css, cpu);
> +	rcu_read_unlock();
> +	raw_spin_unlock_irq(cpu_lock);
> +}

Would it make sense to itereate CPUs within the helper rather than asking
the caller to do it? Also, in terms of patch sequencing, this introduces a
bug and then fixes it. Prolly better to not introduce the bug in the first
place.

Thanks.

-- 
tejun