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

[Tejun Heo <tj@kernel.org>]

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?

> 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.

Thanks.

-- 
tejun