Re: [PATCH v2] blk-throttle: fix race between blkcg_bio_issue_check and cgroup_rmdir

[Tejun Heo <tj@kernel.org>]

Hello, Joseph.

On Fri, Feb 09, 2018 at 10:15:19AM +0800, Joseph Qi wrote:
> IIUC, we have to identify it is in blkcg_css_offline now which will
> blkg_put. Since percpu_ref_kill_and_confirm in kill_css will set flag
> __PERCPU_REF_DEAD, so we can use this to avoid the race. IOW, if
> __PERCPU_REF_DEAD is set now, we know blkcg css is in offline and
> continue access blkg may risk double free. Thus we choose to skip these
> ios.
> I don't get how css_tryget works since it doesn't care the flag
> __PERCPU_REF_DEAD. Also css_tryget can't prevent blkcg_css from
> offlining since the race happens blkcg_css_offline is in progress.
> Am I missing something here?

Once marked dead, the ref is in atomic mode and css_tryget() would hit
the atomic counter.  Here, we don't care about the offlining and
draining.  A draining memcg can still have a lot of memory to be
written back attached to it and we don't want punt all of them to the
root cgroup.

Thanks.

-- 
tejun