Re: [PATCH v8 3/3] blk-cgroup: Optimize blkcg_rstat_flush()

[Michal Koutný]

Hello.

On Tue, Oct 04, 2022 at 11:17:48AM -0400, Waiman Long <longman@xxxxxxxxxx> wrote:
> To protect against destruction of blkg, a percpu reference is gotten
> when putting into the lockless list and put back when removed.

Just to conclude my previous remark about the loop, let me try
explaining it more precisely:

blkcg->lhead via blkg_iostat_set holds reference to blkcg_gq 
   (taken in in blk_cgroup_bio_start)

blkcg_gq holds reference to its blkcg_gq->blkcg 
   (taken in blkg_create)

The cycle has two edges, the latter is broken in __blkg_release but
that's a release callback of the involved blkcg_gq->refcnt, so it won't
be called.

The first edges is broken in blkcg_rstat_flush and that's more promising.
The current code does the final flushes -- in css_release_work_fn.
The problem is that it's the release callback of blkcg->css, i.e. it's
also referenced on the cycle, therefore this final flush won't happen
before cycle is broken.

Fortunately, any other caller of cgroup_rstat_flush comes to the rescue
-- the blkcg_rstat_flush on the stuck blkcg would decompose lhead list
and the reference cycle is broken.

In summary, I think this adds the reference cycle but its survival time
is limited to the soonest cgroup_rstat_flush call, which should not
cause practical troubles.

HTH,
Michal

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