On Fri, Sep 29, 2023 at 11:20 AM Christian Brauner <brauner@xxxxxxxxxx> wrote: > > But yes, that protection would be broken by SLAB_TYPESAFE_BY_RCU, > > since then the "f_count is zero" is no longer a final thing. > > I've tried coming up with a patch that is simple enough so the pattern > is easy to follow and then converting all places to rely on a pattern > that combine lookup_fd_rcu() or similar with get_file_rcu(). The obvious > thing is that we'll force a few places to now always acquire a reference > when they don't really need one right now and that already may cause > performance issues. (Those places are probably used way less often than the hot open/fget/close paths though.) > We also can't fully get rid of plain get_file_rcu() uses itself because > of users such as mm->exe_file. They don't go from one of the rcu fdtable > lookup helpers to the struct file obviously. They rcu replace the file > pointer in their struct ofc so we could change get_file_rcu() to take a > struct file __rcu **f and then comparing that the passed in pointer > hasn't changed before we managed to do atomic_long_inc_not_zero(). Which > afaict should work for such cases. > > But overall we would introduce a fairly big and at the same time subtle > semantic change. The idea is pretty neat and it was fun to do but I'm > just not convinced we should do it given how ubiquitous struct file is > used and now to make the semanics even more special by allowing > refcounts. > > I've kept your original release_empty_file() proposal in vfs.misc which > I think is a really nice change. > > Let me know if you all passionately disagree. ;)
