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    SubjectRe: [PATCH] blk-cgroup: Fix RCU correctness warning in cfq_init_queue()
    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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