On Wed, May 24, 2023 at 12:10:55AM -0400, Waiman Long wrote: > > On 5/23/23 22:37, Ming Lei wrote: > > Hi Yosry, > > > > On Tue, May 23, 2023 at 07:06:38PM -0700, Yosry Ahmed wrote: > > > Hi Ming, > > > > > > On Tue, May 23, 2023 at 6:21 PM Ming Lei <ming.lei@xxxxxxxxxx> wrote: > > > > As noted by Michal, the blkg_iostat_set's in the lockless list > > > > hold reference to blkg's to protect against their removal. Those > > > > blkg's hold reference to blkcg. When a cgroup is being destroyed, > > > > cgroup_rstat_flush() is only called at css_release_work_fn() which > > > > is called when the blkcg reference count reaches 0. This circular > > > > dependency will prevent blkcg and some blkgs from being freed after > > > > they are made offline. > > > I am not at all familiar with blkcg, but does calling > > > cgroup_rstat_flush() in offline_css() fix the problem? > > Except for offline, this list needs to be flushed after the associated disk > > is deleted. > > > > > or can items be > > > added to the lockless list(s) after the blkcg is offlined? > > Yeah. > > > > percpu_ref_*get(&blkg->refcnt) still can succeed after the percpu refcnt > > is killed in blkg_destroy() which is called from both offline css and > > removing disk. > > As suggested by Tejun, we can use percpu_ref_tryget(&blkg->refcnt) to make > sure that we can only take a reference when the blkg is online. I think it > is a bit safer to take a percpu refcnt to avoid use after free. My other blkg_release() does guarantee that no new stat associated with this blkg can be added any more, so what is the use-after-free? > concern about your patch is that the per cpu list iterations will be done > multiple times when a blkcg is destroyed if many blkgs are attached to the Yeah, but is it one big deal? The percpu list should be flushed just in one of blkg's release handler. > blkcg. I still prefer to do it once in blkcg_destroy_blkgs(). I am going to Problem is that new stat still may be added to this list after percpu_ref_kill() returns. To be honest, I really hate to grab/put blkg refcnt for adding/consuming state, and this way is too fragile. Thanks, Ming
