From mboxrd@z Thu Jan  1 00:00:00 1970
From: Tejun Heo <tj@kernel.org>
Subject: Re: [PATCH 13/15] blkcg: change blkg reference counting to use
 percpu_ref
Date: Fri, 31 Aug 2018 17:31:59 -0700
Message-ID: <20180901003159.GK1488037@devbig004.ftw2.facebook.com>
References: <20180831015356.69796-1-dennisszhou@gmail.com>
 <20180831015356.69796-14-dennisszhou@gmail.com>
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To: Dennis Zhou <dennisszhou@gmail.com>
Cc: Jens Axboe <axboe@kernel.dk>, Johannes Weiner <hannes@cmpxchg.org>, Josef Bacik <josef@toxicpanda.com>, kernel-team@fb.com, linux-block@vger.kernel.org, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org

Hello,

On Thu, Aug 30, 2018 at 09:53:54PM -0400, Dennis Zhou wrote:
> @@ -217,6 +240,10 @@ static struct blkcg_gq *blkg_create(struct blkcg *blkcg,
>  		blkg_get(blkg->parent);
>  	}
>  
> +	ret = percpu_ref_init(&blkg->refcnt, __blkg_release, 0, GFP_KERNEL);

So, while this would work in some configs, you can't depend on RCU
grace period inside percpu_ref.  blkg is now percpu-reference counted
and rcu protected object - it has to explicitly go through a rcu grace
period before release.

Thanks.

-- 
tejun