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

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