Re: [PATCH -next 0/4] blk-cgroup: synchronize del_gendisk() with configuring cgroup policy

[Yu Kuai]

Hi,

在 2022/12/21 0:01, Tejun Heo 写道:
> Hello,
>
> On Tue, Dec 20, 2022 at 05:19:12PM +0800, Yu Kuai wrote:
> > Yes, that sounds good. BTW, queue_lock is also used to protect
> > pd_alloc_fn/pd_init_fn，and we found that blkcg_activate_policy() is
> > problematic:
> >
> > blkcg_activate_policy
> >   spin_lock_irq(&q->queue_lock);
> >   list_for_each_entry_reverse(blkg, &q->blkg_list
> >    pd_alloc_fn(GFP_NOWAIT | __GFP_NOWARN,...) -> failed
> >
> >    spin_unlock_irq(&q->queue_lock);
> >    // release queue_lock here is problematic, this will cause
> > pd_offline_fn called without pd_init_fn.
> >    pd_alloc_fn(__GFP_NOWARN,...)
>
> So, if a blkg is destroyed while a policy is being activated, right?
Yes, remove cgroup can race with this, for bfq null pointer deference
will be triggered in bfq_pd_offline().

> > If we are using a mutex to protect rq_qos ops, it seems the right thing
> > to do do also using the mutex to protect blkcg_policy ops, and this
> > problem can be fixed because mutex can be held to alloc memroy with
> > GFP_KERNEL. What do you think?
>
> One worry is that switching to mutex can be more headache due to destroy
> path synchronization. Another approach would be using a per-blkg flag to
> track whether a blkg has been initialized.
I think perhaps you mean per blkg_policy_data flag? per blkg flag should
not work in this case.

Thanks,
Kuai
> Thanks.