linux-kernel - Re: [PATCH v4] blk-cgroup: Replace u64 sync with spinlock for iostat

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Message-ID: <ZpmF8HJsuefjC7Xr@slm.duckdns.org>
Date: Thu, 18 Jul 2024 11:15:28 -1000
From: Tejun Heo <tj@...nel.org>
To: "boy.wu" <boy.wu@...iatek.com>
Cc: Josef Bacik <josef@...icpanda.com>, Jens Axboe <axboe@...nel.dk>,
	Matthias Brugger <matthias.bgg@...il.com>,
	AngeloGioacchino Del Regno <angelogioacchino.delregno@...labora.com>,
	Boris Burkov <boris@....io>, cgroups@...r.kernel.org,
	linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	linux-mediatek@...ts.infradead.org, iverlin.wang@...iatek.com
Subject: Re: [PATCH v4] blk-cgroup: Replace u64 sync with spinlock for iostat

Hello,

On Thu, Jul 18, 2024 at 04:41:12PM +0800, boy.wu wrote:
>  static void blkg_clear_stat(struct blkcg_gq *blkg)
>  {
>  	int cpu;
>  
> +#if BITS_PER_LONG == 32
> +	guard(raw_spinlock_irqsave)(&blkg_stat_lock);
> +#endif

Can you please collect the ifdefs into a single place? If guard can be used
for that, that's great. If not, just spin_lock/unlock wrappers are fine too,
but please collect them into a single place and add a comment explaining why
this is necessary and why u64_sync isn't being used.

Also, for blkg_clear_stat(), we're running a slight chance of clearing of
iostat_cpu racing against state updates from the hot path. Given the
circumstances - stat reset is an cgroup1-only feature which isn't used
widely and a problematic interface to begin with, I believe this is a valid
choice but it needs to be noted.

Thanks.

-- 
tejun