linux-kernel - Re: [PATCH 11/15] blkcg: remove additional reference to the css

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Message-ID: <20180901002603.GI1488037@devbig004.ftw2.facebook.com>
Date:   Fri, 31 Aug 2018 17:26:03 -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 11/15] blkcg: remove additional reference to the css

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