On Thu, Aug 30, 2018 at 09:53:43PM -0400, Dennis Zhou wrote: > From: "Dennis Zhou (Facebook)" <dennisszhou@xxxxxxxxx> > > Currently, blkcg destruction relies on a sequence of events: > 1. Destruction starts. blkcg_css_offline() is called and blkgs > release their reference to the blkcg. This immediately destroys > the cgwbs (writeback). > 2. With blkgs giving up their reference, the blkcg ref count should > become zero and eventually call blkcg_css_free() which finally > frees the blkcg. > > Jiufei Xue reported that there is a race between blkcg_bio_issue_check() > and cgroup_rmdir(). To remedy this, blkg destruction becomes contingent > on the completion of all writeback associated with the blkcg. A count of > the number of cgwbs is maintained and once that goes to zero, blkg > destruction can follow. This should prevent premature blkg destruction. > > The new process for blkcg cleanup is as follows: > 1. Destruction starts. blkcg_css_offline() is called which offlines > writeback. Blkg destruction is delayed on the nr_cgwbs count to > avoid punting potentially large amounts of outstanding writeback > to root while maintaining any ongoing policies. > 2. When the nr_cgwbs becomes zero, blkcg_destroy_blkgs() is called and > handles destruction of blkgs. This is where the css reference held > by each blkg is released. > 3. Once the blkcg ref count goes to zero, blkcg_css_free() is called. > This finally frees the blkg. > > It seems in the past blk-throttle didn't do the most understandable > things with taking data from a blkg while associating with current. So, > the simplification and unification of what blk-throttle is doing caused > this. > So the general approach is correct, but it's sort of confusing because you are using nr_cgwbs as a reference counter, because it's set at 1 at blkg creation time regardless of wether or not there's an assocated wb cg. So instead why not just have a refcount_t ref, set it to 1 on creation and make the wb cg take a ref when it's attached, and then just do the get/put like normal and cleanup as you have below? What you are doing is a reference counter masquerading as a count of the wb cg's, just add full ref counting to the blkcg and call it a day, it'll be much less confusing. Thanks, Josef
