Re: [PATCH v2] blk-cgroup: Replace u64_sync with blkg_stat_lock for stats update

[Tejun Heo]

Hello,

On Wed, Jul 10, 2024 at 02:13:34PM +0800, boy.wu wrote:
...
> @@ -952,7 +952,6 @@ static void blkcg_fill_root_iostats(void)
>  		struct blkcg_gq *blkg = bdev->bd_disk->queue->root_blkg;
>  		struct blkg_iostat tmp;
>  		int cpu;
> -		unsigned long flags;
>  
>  		memset(&tmp, 0, sizeof(tmp));
>  		for_each_possible_cpu(cpu) {
> @@ -974,9 +973,10 @@ static void blkcg_fill_root_iostats(void)
>  				cpu_dkstats->sectors[STAT_DISCARD] << 9;
>  		}
>  
> -		flags = u64_stats_update_begin_irqsave(&blkg->iostat.sync);
> +#if BITS_PER_LONG == 32
> +		guard(raw_spinlock_irqsave)(&blkg_stat_lock);
> +#endif
>  		blkg_iostat_set(&blkg->iostat.cur, &tmp);
> -		u64_stats_update_end_irqrestore(&blkg->iostat.sync, flags);

Isn't the problem shared across other blkg->iostat.sync users too? Also,
maybe, we can just grab the spinlock without testing for 32bit. blkg->iostat
(unlike the per-cpu counterpart) isn't accessed that frequently, so keeping
it simple and consistent probably makes more sense, right?

Thanks.

-- 
tejun