On Tue, Apr 09, 2019 at 09:04:15PM -0700, Eric Biggers wrote: > > What's to stop you from doing just that right now? You'd need to take > > care with barriers, but you'd need that anyway... As soon as ->i_link is set > > you'll get no more ->get_link() on that sucker, using the cached value > > from that point on. IDGI... > > 1.) The VFS won't know to drop of RCU-walk mode, so waiting an RCU grace period > before freeing the symlink target becomes mandatory. (Which I'd like to do > for fscrypt anyway, but doing it sanely appears to require implementing > .destroy_inode() for ext4, f2fs, and ubifs. I hoped I could do non-RCU mode > as a simpler first step.) You might want to check those filesystems. All three you've mentioned *have* ->destroy_inode() already. > 2.) The VFS won't know to use a read memory barrier when loading i_link. > The VFS could issue one unconditionally, but it would be unnecessary for > regular fast symlinks. Not really. All we need on the read side is READ_ONCE(); it will supply smp_read_barrier_depends() (which is a no-op except for alpha). On the write side we need smp_store_release() to set ->i_link (in addition to whatever serialization we want for actual calculation of the value to be cached, of course).
