Messages in this thread | | | Date | Sun, 25 Apr 2010 19:06:31 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH] blk-cgroup: Fix RCU correctness warning in cfq_init_queue() |
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On Mon, Apr 26, 2010 at 09:33:46AM +0800, Li Zefan wrote: > >>>>>> With RCU correctness on, We see following warning. This patch fixes it. > >>>>> This is in initialization code, so that there cannot be any concurrent > >>>>> updates, correct? If so, looks good. > >>>>> > >>>> I think theoritically two instances of cfq_init_queue() can be running > >>>> in parallel (for two different devices), and they both can call > >>>> blkiocg_add_blkio_group(). But then we use a spin lock to protect > >>>> blkio_cgroup. > >>>> > >>>> spin_lock_irqsave(&blkcg->lock, flags); > >>>> > >>>> So I guess two parallel updates should be fine. > >>> OK, in that case, would it be possible add this spinlock to the condition > >>> checked by css_id()'s rcu_dereference_check()? > >> Hi Paul, > >> > >> I think adding these spinlock to condition checked might become little > >> messy. And the reason being that this lock is subsystem (controller) > >> specific and maintained by controller. Now if any controller implements > >> a lock and we add that lock in css_id() rcu_dereference_check(), it will > >> look ugly. > >> > >> So probably a better way is to make sure that css_id() is always called > >> under rcu read lock so that we don't hit this warning? > > > > As long as holding rcu_read_lock() prevents css_id() from the usual > > problems such as access memory that was concurrently freed, yes. > > blkiocg_add_blkio_group() also calls cgroup_path(), which also needs to > be called within rcu_read_lock, so I think Vivek's patch is better than > the one you posted in another mail thread.
My apologies, Vivek! I lost track of your patch. I have now replaced my patch with yours.
Thanx, Paul
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