linux-kernel - Re: [PATCH 2/9] blkcg: drop unnecessary RCU locking

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Message-ID: <20120217171113.GB26575@google.com>
Date:	Fri, 17 Feb 2012 09:11:13 -0800
From:	Tejun Heo <tj@...nel.org>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	axboe@...nel.dk, ctalbott@...gle.com, rni@...gle.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/9] blkcg: drop unnecessary RCU locking

On Fri, Feb 17, 2012 at 11:47:49AM -0500, Vivek Goyal wrote:
> So now in some cases we call blkg_lookup_create() with both queue and rcu
> read lock held (cfq_lookup_create_cfqg()) and in this case hold only queue
> lock.

So, this should be okay.  It's currently not because blkg_alloc() is
broken due to percpu allocation but other than that calling both w/
and w/o RCU read lock should be fine.

> blkg_lookup_create() calls blkg_lookup() which expects a rcu_read_lock()
> to be held and we will be travesing that list without rcu_read_lock()
> held. Isn't that a problem?

No, why would it be a problem?

> We might be examining a blkg belonging to a different queue and it
> might be being freed parallely.

How?

Thanks.

-- 
tejun
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