linux-kernel - Re: [PATCH 13/15] blkcg: change blkg reference counting to use percpu

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Message-ID: <20180901003159.GK1488037@devbig004.ftw2.facebook.com>
Date:   Fri, 31 Aug 2018 17:31:59 -0700
From:   Tejun Heo <tj@...nel.org>
To:     Dennis Zhou <dennisszhou@...il.com>
Cc:     Jens Axboe <axboe@...nel.dk>, Johannes Weiner <hannes@...xchg.org>,
        Josef Bacik <josef@...icpanda.com>, kernel-team@...com,
        linux-block@...r.kernel.org, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 13/15] blkcg: change blkg reference counting to use
 percpu_ref

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