From mboxrd@z Thu Jan  1 00:00:00 1970
From: Tejun Heo <tj@kernel.org>
Subject: Re: [PATCH 11/15] blkcg: remove additional reference to the css
Date: Fri, 31 Aug 2018 17:26:03 -0700
Message-ID: <20180901002603.GI1488037@devbig004.ftw2.facebook.com>
References: <20180831015356.69796-1-dennisszhou@gmail.com>
 <20180831015356.69796-12-dennisszhou@gmail.com>
Mime-Version: 1.0
Return-path: <linux-kernel-owner@vger.kernel.org>
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed;
        d=gmail.com; s=20161025;
        h=sender:date:from:to:cc:subject:message-id:references:mime-version
         :content-disposition:in-reply-to:user-agent;
        bh=aYfyDUOdGSNfOZV/pI/zAY7JB35GXqB2P2f068rolNQ=;
        b=Z8Jenl1LUYwyuWK1agPe3HE9kQsGp3l0rCQChLVbp17znEagSbfFwtK3d0oAEsgD9A
         rOsYi/XgduU9ayspmwBgZgCkhg/XXd+rEtWSGjr7ASZo2UclSnVzYHbSY5YS9q/5WNYv
         tdGjRN2Y9BqxZaf48tshmFB8yvpnGZe1LrJdxhTtet3IlDVwFazbg7A80v9Tmd7SSfRU
         ftAglG8VG+TAK+Xd7Ru3zKbWwByME7+fEYdDcRC+KUJeFcPLeTshxkbw5ASG1ADzyul4
         0fCxkbCXUwrQQj57Hk3D0Q5TY/Ujp+iwMxgL9Ykti4U8Q7Cbvhnx+QR7FswC+P8Dv7/O
         edsg==
Content-Disposition: inline
In-Reply-To: <20180831015356.69796-12-dennisszhou@gmail.com>
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <cgroups.vger.kernel.org>
Content-Type: text/plain; charset="us-ascii"
Content-Transfer-Encoding: 7bit
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 09:53:52PM -0400, Dennis Zhou wrote:
> -	css = cgroup_get_e_css(page->mem_cgroup->css.cgroup, &io_cgrp_subsys);
>  
> -	return __bio_associate_blkg_from_css(bio, css);
> +	rcu_read_lock();
> +
> +	while (true) {
> +		css = cgroup_e_css(page->mem_cgroup->css.cgroup,
> +				   &io_cgrp_subsys);

So, while they seem similar cgroup_e_css() and cgroup_get_e_css()
behave very differently in terms of locking.  cgroup_e_css() can only
be used under cgroup_mutex because it is used during migration and has
to test cgroup_ss_mask().  The right thing to do here would be
renaming cgroup_e_css() to something else and add a new implementation
which operates in the same way as cgroup_get_e_css().

BTW, this should have triggered lockdep warning.  I'd strongly
recommend testing with lockdep enabled.

Other than that, looks good to me.

Thanks.

-- 
tejun