On Sun, 28 Jan 2024 12:53:31 -0800 Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote: > Honestly, you should just *always* do refcounting. No "free after RCU > delay" as an alternative. Just refcount it. > > Now, the RCU delay may be needed if the lookup of said structure > happens under RCU, but no, saying "I use SRCU to make sure the > lifetime is at least X" is just broken. > > The refcount is what gives the lifetime. Any form of RCU-delaying > should then be purely about non-refcounting RCU lookups that may > happen as the thing is dying (and said lookup should *look* at the > refcount and say "oh, this is dead, I'm not returning this". The deleting of the ei is done outside the VFS logic. I use SRCU to synchronize looking at the ei children in the lookup. On deletion, I grab the eventfs_mutex, set ei->is_freed and then wait for SRCU to finish before freeing. The lookup checks ei->is_freed and doesn't do anything if set, but most that logic is under the SRCU, which is what I want to make sure is finished before the ei is deleted. Hmm, I still need the logic for iput(), as dentry->d_fsdata can still access the ei. That's where I need to have the ref counters. For a lookup, I need to up the ref count when I create a new inode for the ei or its children. Then in the iput() I decrement the ei ref count. I can only free the ei if the ref count is zero. The ref count is for knowing if an ei is referenced by a dentry->d_fsdata, and the SRCU is to make sure there's no lookups accessing an ei. -- Steve
