On Wed, Feb 12, 2020 at 11:49:58AM -0800, Linus Torvalds wrote: > I wonder if we could split up d_invalidate(). It already ends up being > two phases: first the unhashing under the d_lock, and then the > recursive shrinking of parents and children. > > The recursive shrinking of the parent isn't actually interesting for > the proc shrinking case: we just looked up one child, after all. So we > only care about the d_walk of the children. > > So if we only did the first part under the RCU lock, and just > collected the dentries (can we perhaps then re-use the hash list to > collect them to another list?) and then did the child d_walk > afterwards? What's to prevent racing with fs shutdown while you are doing the second part? We could, after all, just have them[*] on procfs-private list (anchored in task_struct) from the very beginning; evict on ->d_prune(), walk the list on exit... How do you make sure the fs instance won't go away right under you while you are doing the real work? Suppose you are looking at one of those dentries and you've found something blocking to do. You can't pin that dentry; you can pin ->s_active on its superblock (if it's already zero, you can skip it - fs shutdown already in progress will take care of the damn thing), but that will lead to quite a bit of cacheline pingpong... [*] only /proc/<pid> and /proc/*/task/<pid> dentries, obviously.
