Re: [PATCH v3] blk-cgroup: Replace u64 sync with spinlock for iostat update

[Waiman Long <longman@redhat.com>]

On 7/17/24 14:24, tj@kernel.org wrote:
> Hello,
>
> On Wed, Jul 17, 2024 at 02:18:39PM -0400, Waiman Long wrote:
>> Well, it can be confusing whether we are dealing with blkg->iostat or
>> blkg->iostat_cpu. In many cases, we are dealing with iostat_cpu instead of
>> iostat like __blkcg_rstat_flush() and blkg_clear_stat(). So we can't
>> eliminate the use of u64_stats_update_begin_irqsave() in those cases.
> I mean, we need to distinguish them. For 32bits, blkg->iostat has multiple
> updaters, so we can't use u64_sync; however, blkg->iostat_cpu has only one
> updater (except blkg_clear_stat() which I don't think we need to worry too
> much about), so u64_sync is fine.

I was wrong about __blkcg_rstat_flush(). Right, the main updater of 
iostat_cpu is  blk_cgroup_bio_start(). We do need to drop down some 
comment on what is protected by u64_sync and what is by blkg_stat_lock 
though. It can be confusing.

Cheers,
Longman