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

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Message-ID: <20180906204629.GD26048@dennisz-mbp.dhcp.thefacebook.com>
Date:   Thu, 6 Sep 2018 16:46:30 -0400
From:   Dennis Zhou <dennisszhou@...il.com>
To:     Tejun Heo <tj@...nel.org>
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

On Fri, Aug 31, 2018 at 05:31:59PM -0700, Tejun Heo wrote:
> 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.
> 

Ah ok. I've made it into a call_rcu which should work.

Thanks,
Dennis