Hi Tejun, On 18/2/28 02:33, Tejun Heo wrote: > Hello, Joseph. > > On Sat, Feb 24, 2018 at 09:45:49AM +0800, Joseph Qi wrote: >>> IIRC, as long as the blkcg and the device are there, the blkgs aren't >>> gonna be destroyed. So, if you have a ref to the blkcg through >>> tryget, the blkg shouldn't go away. >>> >> >> Maybe we have misunderstanding here. >> >> In this case, blkg doesn't go away as we have rcu protect, but >> blkg_destroy() can be called, in which blkg_put() will put the last >> refcnt and then schedule __blkg_release_rcu(). >> >> css refcnt can't prevent blkcg css from offlining, instead it is css >> online_cnt. >> >> css_tryget() will only get a refcnt of blkcg css, but can't be >> guaranteed to fail when css is confirmed to kill. > > Ah, you're right. I was thinking we only destroy blkgs from blkcg > release path. Given that we primarily use blkcg refcnting to pin > them, I believe that's what we should do - ie. only call > pd_offline_fn() from blkcg_css_offline() path and do the rest of > destruction from blkcg_css_free(). What do you think? > In current code, I'm afraid pd_offline_fn() as well as the rest destruction have to be called together under the same blkcg->lock and q->queue_lock. For example, if we split the pd_offline_fn() and radix_tree_delete() into 2 phases, it may introduce a race between blkcg_deactivate_policy() when exit queue and blkcg_css_free(), which will result in pd_offline_fn() to be called twice.
Thanks, Joseph
