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

[Boy Wu (吳勃誼)]

On Fri, 2024-07-12 at 08:38 -1000, tj@xxxxxxxxxx wrote:
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>  Hello, Boy.
> 
> On Fri, Jul 12, 2024 at 01:39:51AM +0000, Boy Wu (吳勃誼) wrote:
> ...
> > I agree, but for multiple updaters, we not only need a spin lock
> but
> > also need u64_sync for 32bit SMP systems because u64_stats_fetch is
> not
> > protected by the spin lock blkg_stat_lock. If removing u64 sync,
> then
> > one CPU fetches data while another CPU is updating, may get a 64
> bits
> > data with only 32 bits updated, while the other 32 bits are not
> updated
> > yet. We can see that blkcg_iostats_update is protected by both
> u64_sync
> > and the spin lock blkg_stat_lock in __blkcg_rstat_flush.
> > Thus, I think we should keep the u64_sync and just add the spin
> > lock blkg_stat_lock, not replace u64_sync with the spin lock.
> 
> I don't get it. The only reader of blkg->iostat is
> blkcg_print_one_stat().
> It can just grab the same spin lock that the updaters use, right? Why
> do we
> also need u64_sync for blkg->iostat?
> 
> Thanks.
> 
> -- 
> tejun

I think I get your idea. You want to replace all the u64 sync for
iostat. However, I have one question: why use blkg_stat_lock instead of
adding a spin lock for each iostat like iostat.spinlock? We don't need
to lock between updating different iostats, but only lock when updating
the same iostat.

--
Boy.Wu