Re: [PATCH v5 3/3] blk-cgroup: Optimize blkcg_rstat_flush()

Waiman Long
2022-06-02T17:26:18Z

On 6/2/22 12:58, Tejun Heo wrote:
> Hello,
>
> On Thu, Jun 02, 2022 at 09:35:43AM -0400, Waiman Long wrote:
>> @@ -2011,9 +2076,16 @@ void blk_cgroup_bio_start(struct bio *bio)
>> }
>> bis->cur.ios[rwd]++;
>>
>> + if (!READ_ONCE(bis->lnode.next)) {
>> + struct llist_head *lhead = per_cpu_ptr(blkcg->lhead, cpu);
>> +
>> + llist_add(&bis->lnode, lhead);
>> + percpu_ref_get(&bis->blkg->refcnt);
> Hmm... what guarantees that more than one threads race here? llist assumes
> that there's a single writer for a given llist_node and the ref count would
> be off too, right?

The llist_add() function is atomic. It calls into llist_add_batch() in
lib/llist.c which uses cmpxchg() to make the change. There is a
non-atomic version __llist_add() which may be problematic in this case.
Note that irq is disabled in the u64_stats_update* critical section,
there shouldn't be a racing thread running in the same cpu. Other cpus
will modify their own version of lhead. Perhaps the non-atomic version
can be used here as well.

Cheers,
Longman