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

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Message-ID: <ZqlD5WXx39E8xVnA@slm.duckdns.org>
Date: Tue, 30 Jul 2024 09:49:57 -1000
From: "tj@...nel.org" <tj@...nel.org>
To: Boy Wu (吳勃誼) <Boy.Wu@...iatek.com>
Cc: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"boris@....io" <boris@....io>,
	"linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
	"linux-mediatek@...ts.infradead.org" <linux-mediatek@...ts.infradead.org>,
	"cgroups@...r.kernel.org" <cgroups@...r.kernel.org>,
	"axboe@...nel.dk" <axboe@...nel.dk>,
	Iverlin Wang (王苳霖) <Iverlin.Wang@...iatek.com>,
	"josef@...icpanda.com" <josef@...icpanda.com>,
	"linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>,
	"matthias.bgg@...il.com" <matthias.bgg@...il.com>,
	"angelogioacchino.delregno@...labora.com" <angelogioacchino.delregno@...labora.com>
Subject: Re: [PATCH v4] blk-cgroup: Replace u64 sync with spinlock for iostat

Hello, Boy.

On Fri, Jul 26, 2024 at 03:43:27AM +0000, Boy Wu (吳勃誼) wrote:
...
> The use of a spinlock with u64 sync is suggested in
> include/linux/u64_stats_sync.h:33.
> 
>  * Usage :
>  *
>  * Stats producer (writer) should use following template granted it
> already got
>  * an exclusive access to counters (a lock is already taken, or per cpu
>  * data is used [in a non preemptable context])
>  *
>  *   spin_lock_bh(...) or other synchronization to get exclusive access
>  *   ...
>  *   u64_stats_update_begin(&stats->syncp);
>  *   u64_stats_add(&stats->bytes64, len); // non atomic operation
>  *   u64_stats_inc(&stats->packets64);    // non atomic operation
>  *   u64_stats_update_end(&stats->syncp);
> 
> Is this a incorrect statment?

That's not incorrect and it'd make sense if we really want to use u64_sync -
e.g. the reader is hot path. Here, just a spinlock would be simpler and do
fine.

Thanks.

-- 
tejun