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

[Vivek Goyal]

On Fri, Feb 17, 2012 at 09:11:13AM -0800, Tejun Heo wrote:
> 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?

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.

So there might not be any problem, just that looking at the code
I am not very clear abou the locking sematics of blkg_lookup(). May
be some documentation will help that it should be called with 
what locks in what situation. Specifically, when should it be called
with rcu_read_lock() held.

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

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.

Thanks
Vivek
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