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

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Date:	Fri, 17 Feb 2012 13:08:00 -0500
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Tejun Heo <tj@...nel.org>
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 09:43:04AM -0800, Tejun Heo wrote:
> Hello,
> 
> On Fri, Feb 17, 2012 at 12:28:57PM -0500, Vivek Goyal wrote:
> > I am kind of confused that what are the semantics of calling
> > blkg_lookup_create(). Given the fact that it traverses the
> > blkcg->blkg_list which is rcu protected, so either we should have
> > rcu read lock held or we should have blkcg->lock held.
> 
> Modifying blkgs require both blkcg and queue locks,

> so read access can be done holding any lock.

This is the point I am not getting. How blkg_lookup() is safe just
under queue lock. What stops freeing up blkg associated with other
queues. I thought caller needs to hold rcu_read_lock() also to
make sure it can safely compare blkg->q == q and return the blkg
belonging to the queue in question.

> > 
> > Can pre_destroy() and blkio_policy_parse_and_set() make progress in
> > parallel for same cgroup(blkcg) but different queue.
> > 
> > If yes, blkg_lookup() might be doing blkg->q == q check and pre_destroy
> > might delete that group and free it up.
> 
> And that's why __blkg_release() is RCU free'ing blkgs, no?

Yes. And you are not holding rcu_read_lock() while doing blkg_lookup()
in blkio_policy_parse_and_set(). Just queue lock will not be enough?

Thanks
Vivek
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