[PATCH] blk-cgroup: add spin_lock for u64_stats_update

[Markus Elfring]

> In 32bit SMP systems, if the system is stressed on the sys node
> by processes, it may cause blkcg_fill_root_iostats to have a concurrent
> problem on the seqlock in u64_stats_update, which will cause a deadlock
> on u64_stats_fetch_begin in blkcg_print_one_stat.

Would you like to mark any references to functions with parentheses?


> To prevent this problem, add spin_locks.

Another wording suggestion:
  Thus use an additional spin lock.


How do you think about to use a summary phrase like “Add a spin lock for stats update
in blkcg_fill_root_iostats()”?


…
> +++ b/block/blk-cgroup.c
> @@ -1134,9 +1134,15 @@ static void blkcg_fill_root_iostats(void)
>  				cpu_dkstats->sectors[STAT_DISCARD] << 9;
>  		}
>
> +#if BITS_PER_LONG == 32
> +		spin_lock_irq(&blkg->q->queue_lock);
> +#endif
>  		flags = u64_stats_update_begin_irqsave(&blkg->iostat.sync);
>  		blkg_iostat_set(&blkg->iostat.cur, &tmp);
>  		u64_stats_update_end_irqrestore(&blkg->iostat.sync, flags);
> +#if BITS_PER_LONG == 32
> +		spin_unlock_irq(&blkg->q->queue_lock);
> +#endif
…

Under which circumstances would you become interested to apply a statement
like “guard(spinlock_irq)(&blkg->q->queue_lock);”?
https://elixir.bootlin.com/linux/v6.10-rc6/source/include/linux/spinlock.h#L567

Regards,
Markus