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    SubjectRe: [PATCH] blk-cgroup: Fix RCU correctness warning in cfq_init_queue()
    On Mon, Apr 26, 2010 at 09:39:20AM -0400, Vivek Goyal wrote:
    > On Sun, Apr 25, 2010 at 07:06:31PM -0700, Paul E. McKenney wrote:
    > > 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.
    >
    > Thanks Paul.
    >
    > I sent this patch to Jens also, thinking he will apply to his tree. Looks
    > like he has not applied it yet though.
    >
    > Jens, is it ok if this patch gets merged through paul's tree or should it
    > go through blk tree?

    I am happy for it to go either way, so just let me know!

    Thanx, Paul


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