linux-kernel - Re: [PATCH v5 3/3] blk-cgroup: Optimize blkcg_rstat

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Date:   Thu, 2 Jun 2022 13:26:10 -0400
From:   Waiman Long <longman@...hat.com>
To:     Tejun Heo <tj@...nel.org>
Cc:     Jens Axboe <axboe@...nel.dk>, cgroups@...r.kernel.org,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        Ming Lei <ming.lei@...hat.com>
Subject: Re: [PATCH v5 3/3] blk-cgroup: Optimize blkcg_rstat_flush()

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